What If Life Did Not Originate on Earth? | Hacker News Reader

What If Life Did Not Originate on Earth?

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300 pointslaurex233 comments

An interesting phenomenon I rarely see brought up in support of panspermia is the apparent universality of the genetic code. The fact that all known life shares a common genetic structure implies a common ancestor. The lack of competing genetic codes could be interpreted to imply that life only started on earth once, or that it only made it to earth once. Natural selection is a constant force, presumably new genetic codes could offer an advantage, e.g., robustness against damaging mutation, resulting in multiple competing genetic codes.

The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Or the fact that convergent evolution "reinvents" similar species in response to similar selective pressures. Selective pressures constantly "push" life in the same directions, so it would seem that whatever forces gave rise to life would have happened so repeatedly, but there doesn't seem to be any evidence for that.

A clean explanation for this would be that life didn't start on earth, and earth may have never had the right conditions to originate life, hence no competing genetic codes. But it was still hospitable enough to maintain life, so one universal genetic code flourishes with no emergent competition.

Yeah, it seems to have been incredibly unlikely. It took a long time.

Or life started multiple times but the results of one of those times outcompeted all the others.

This is true, but if that did happen presumably we could find some evidence for it, since it would be a relatively common occurrence during the period where earth was capable of abiogenesis. Because if it occurred at least twice, it probably happened many more times.

The counterpoint is that the fossil record is very incomplete when going that far back, seeing as tectonic subduction literally wipes the slate clean.

When lifeforms out compete each other, the winner eats the loser.

Or that the self replicating systems of molecules merged into one (organic life on earth is base 4, containing two sets of two complimentary pairs - it potentially could've done it with just one of those sets),

Or that the same self replicating systems were the most (or only) stable systems that could be forged from the locally available inorganic molecules.

Or there might have been a catastrophe that killed off almost everything. In which case the organisms that survived might not have been very widespread before the catastrophe, or typical of what existed before the catastrophe.

If a large asteroid hit the Earth today it might be only a few subterranean extremophile microorganisms that survive. Asteroids were more frequent during the early history of the solar system.

Doesn't that just beg the question? If life originated on some other planet and made it to Earth, why did only the DNA-based lifeform(s) make it here?

If we assume that different kinds of life based on different nucleic acids can co-exist, then it seems unlikely that a single kind would have happened to be carried to Earth by a natural process.

If we assume DNA has some inherent fitness advantage (so that only DNA-based life survived the journey), than we don't need the additional assumption of panspermia: perhaps only the DNA-based life survived some early period in Earth's history.

Also, for abiogenesis, given that we still can't reliably reproduce something like that, and that we have never seen new kinds of life spontaneously appearing on Earth, we do have some circumstantial evidence for the possibility that it is an incredibly unlikely event, such that it might have only happened once in Earth's history.

Edit: There is also another simple possibility: perhaps given the resources available on Earth, either forever or in the period where abiogenesis was possible, perhaps DNA(+RNA) is the only molecule with the right properties to act as genetic material. Perhaps there is no common ancestor to all life on Earth, but a "forest" of inter-breeding, spotnaneously arisen original organisms, but they were all DNA/RNA-based simply because of the available resources.

Another possibility often overlooked is that there's life out there on completely different timescales than we can recognize. We're only likely to notice those within a few orders of magnitude to our own.

Maybe DNA is simply geared well compared to other mechanisms for the timescales we're interested in.

This is an interesting thought. Plants are actually very animated at their own timescale, adapting to their environment through phenotypic plasticity. However this happens far too slow for humans to notice without a time lapse.

This is why it takes so long to hold a meeting of the Ents.

That's a moot point.

But plants are really obviously complex life forms when we examine them at human timescale. I don't see how a complex life from escapes our noticing it by moving really slowly.

Sundiver is a recommended read in that regard ;)

>why did only the DNA-based lifeform(s) make it here?

Possibly genetic drift. The odds of life making a successful journey to earth seem very poor, so maybe it was a small population that only made it here once, and then began to speciate.

> perhaps only the DNA-based life survived some early period in Earth's history.

My thoughts exactly.

Also, the emergence of an intelligent, technologically capable species (i.e. humans) is a relatively very recent occurrence in Earth's history. It's as if it just suddenly happened, even though there was a long history of dynamic evolution for many hundreds of millions of years. But the sudden emergence of humans doesn't mean there had to have been an external cause.

> the emergence of an intelligent, technologically capable species (i.e. humans) is a relatively very recent occurrence in Earth's history.

Citaton needed.

"History of Earth"

https://en.m.wikipedia.org/wiki/History_of_Earth

That's what we currently know based on what we discovered so far. You can't really rule out the existence of a relatively advanced civilization - although it might advance in a completely different way than ours - whose remains lie buried somewhere under the Mariana Trench for example.

https://en.wikipedia.org/wiki/Russell's_teapot

Sure, but we aren't trying to prove or disprove anything here, we're speculating for enjoyment.

Yes, this has been investigated before - e.g.

https://arxiv.org/abs/1804.03748

https://www.theatlantic.com/science/archive/2018/04/are-we-e...

But the evidence we have (as per prior comments) is that a human-level intelligence civilisation has arisen only once on Earth.

There are actually an infinite number of possible things that could have happened which we can't rule out. Let me preface what I'm going to say with I don't mind what anyone believes. If it makes you happy to believe one thing over another thing, then I think that's great.

With that out of the way, when we're having these kinds of discussions, it's not really productive to bring up an infinite number of possibilities that we can't rule out. Which one should we choose to support? Without some criteria, we will just talk about it forever -- which might be fun, but it's not productive.

This is where science comes in. I'm going to say something a bit controversial. Science doesn't really talk about absolute truth. We can't know what that is -- because there are an infinite number of possibilities that we can't rule out. Instead it talks about things that are "productive" (my term).

In a very hand-wavy way, science is about making observations and building a model that explains those observation. There are a couple of other important things. The model should also make predictions about thing we haven't observed that we can test. We try it and if it also fulfills those predictions, we thing it's a "productive" model (again, my term). Why is it productive? Because it allows us to reason about things and make conclusions that are likely to work out for us. However, we shouldn't mistake that for the absolute truth. There really is a possibility that things appear one way to us, but are very different in ways that we can't see. The key is that from the perspective of science that's fine -- as long as we can keep making predictions and they keep working out for use, we can use our model.

But what if we have 2 models that are both equivalent? For example, let's say they both explain the observations we see and they make predictions that work out. Which should we pick? By convention, we pick the simplest one. In truth, the simplest one might not be correct, but it's always easier to work with a simpler model... because it's simpler ;-)

So while there may truly be advanced civilizations out there -- we can't rule it out, there is no test we can currently do that would show that advanced civilizations exist. Our models without advanced civilizations (except our own) explain everything we can currently observe and everything we can currently test. So we choose the model with no other advanced civilizations.

You really don't have to believe that, but when discussing with others we have to set some rules. If we keep in mind other advanced civilizations, it's just as reasonable to keep in mind an infinite number of other possibilities -- some of which would be truly bizarre. In order to keep the conversation on track, we have to choose somehow and the scientific method is a good, proven way to narrow the discussion down to something that is productive. However, if you find people who want to talk about the kinds of things you want to talk about, I think there is nothing at all wrong with that. You should do what makes you happy.

I hope that makes things a bit clearer. I often meet people who make comments like you did and I think it's really mystifying to those people why science oriented people act this way.

Yes, it's very clear, thank you.

On the other hand, I believe the very core of science is to use our imagination and creativity to look for things that don't fit any models, that are not accounted for by any of them, for higher-order structures that might explain the current models better and so on.

For example, our civilization is more or less built on energy, with the electric current as its main medium, so we are totally dependent on external energy sources - and that's currently the main problem of our civilization. It's not impossible other civilizations might exist (in other space-time environments) that somewhat learned to achieve similar results we do (transportation, healing, distant communication) without using external energy sources. This would at least account for lack of artefacts we currently identify an advanced civilization with.

I think the emphasis was, tongue-in-cheekily, on "intelligent".

What is surprising to me is not just that it's always DNA, but that it's always translated in the same way (codons to aminoacids, and the mapping function is fixed). Is there any simple reason for that?

Changing it makes the organism non-viable, so a change there never enters a population. In contract DNA/RNA-mutations can be viable so happen all the time.

There's a tiny bit of variation across the tree-of-life, see https://en.wikipedia.org/wiki/Genetic_code#Alternative_genet...

But the variation is in the one place you could possibly have it - by changing the interpretation of one or two of the three available STOP codons.

The simplest reason is that the ribosome evolved very early on (and is probably the best preserved remnant of whatever the earlier "RNA world" looked like) and it was always easier to leverage that machinery than to evolve another whole system from scratch. Once you have a world populated by any form of proto-life that can self-reproduce, even with crappy fidelity, it's basically game-over for any of the other semi-closed metabolic pathways that might have been competitors up until that point - they're now just food. There's no opportunity for another system to arise because all the raw materials are being scooped up by the new superpredator on the scene - the ribosome & whatever associated machinery it drags along. The competition now heats up between different ribosome-equipped complexes - improve metabolism, increase independence, increase reproduction rate, improve reproductive fidelity, etc.

It'll be easier to send a DNA without a life-form. A life-form is very expensive (need to eat and breath) and not necessarily adaptable to earth conditions.

A very well encapsulated and protected DNA will land on earth and create the most adaptable life-form there through mutation and competition. All of these life-forms have the trait of wanting to pass their genes, multiply as much as possible and then spread around.

So the engineers probably had that in mind:

1- Extremely small: Probably so that they can propel it through space at very high speed + propel lots of it.

2- No maintenance required.

3- Will create a life-form if possible that is adaptable to the target country.

4- The life-form has the property of multiplication, and spreading.

5- The new superior life-form will propel more of these DNA.

>A life-form is very expensive (need to eat and breath)

Not necessarily, many organisms can undergo cryptobiosis, where all metabolic activity ceases, and then restarts when conditions are appropriate (sometimes decades later).

Fungi & bacteria can turn into spores & travel through space. There was also recent article about fungus growing on the outside (!) of the space station.

There are also things like tardigrades [1] which are pretty much the epoch of survival evolution. And this is all just stuff on our little planet. It's hard to say anything about life on a large scale, since we are basing everything we know on a sample of one. But it's probably safe to say that we have not even scratched the surface of what's possibility through just plain old evolution alone.

[1] - https://en.wikipedia.org/wiki/Tardigrade

There are still a few billion years left for earth to shine.

> There are also things like tardigrades [1] which are pretty much the epoch of survival evolution

“the epoch”? Apex, maybe?

Nematode worms have been brought back to life after having been frozen in tundra for 40,000 years.

> It'll be easier to send a DNA without a life-form.

DNA is useless without the cellular machinery to translate it into proteins and new cells. To do that, you basically need cells.

Would a virus meet this project's specifications?

> A very well encapsulated and protected DNA will land on earth and create the most adaptable life-form there through mutation and competition.

No, it won't. DNA alone without cellular machinery (that is, without a “life form”) doesn't produce anything. That's why viruses need hosts.

> If life originated on some other planet and made it to Earth, why did only the DNA-based lifeform(s) make it here?

The problem is that we can't see through survivor bias. Maybe life has lots of possible configurations and only DNA-based life was highly successful on Earth. It's also possible that non-DNA-based life exists, we just haven't noticed it because it is too small, too slow, or too something-else that we missed it.

I think the most likely reason is simply that they were first. And whatever managed to close some metabolic or reproductive cycle first got, and I mean this quite literally, an exponential boost over everything else. So think of early proto-life on Earth not as some single thread doggedly proceeding up an evolutionary ladder, but rather a series of epochs where there's great diversity until BOOM some set of molecules manage to close some metabolic or reproductive loop and then outcompetes everything else in essentially a blink of an eye.

Couldn't that be the same explanation in the case where it originated here? It was first and dominated?

That's what I was trying to say; I'm in no way a panspermia fan. I was responding primarily to the line "perhaps DNA(+RNA) is the only molecule with the right properties to act as genetic material" with the notion that it was probably more special because it was first rather than something intrinsic about the DNA->RNA->ribosome system that made it inevitable. If we could roll the clock back 4.mumble billion years, we'd probably still get life that had a separation between code and structure, but what that code was and what that structure looked like might have been pretty different.

I'm not sure there is anything else with the flexibility of D/RNA. Its self-replicating property is fundamental. Is there something else known to work anything like this?

Some possible alternatives are covered here: https://en.wikipedia.org/wiki/Nucleic_acid_analogue

And I agree that RNA and DNA are pretty good molecules and probably close to some notion of 'optimal'. The ribosome, however, is big and complicated and there are probably millions of different ways something like that could have come about and that would affect the 3-base codon, protein structures, etc.

Also, look at the contortions DNA has to go through to copy what's called the "lagging strand". Clearly duplicating double-stranded DNA did not come early or easily; it's pretty obvious a system bolted on top of some other, older system. https://en.wikipedia.org/wiki/DNA_replication#Lagging_strand

But that doesn't explain why DNA/RNA is the only. If DNA got a large boost I'll believe it is dominate, but why isn't there anything else somewhere still struggling along.

There is a very (very!) small chance that there are isolated biomes that split from life's tree before what we call the last universal common ancestor (LUCA) and are still plugging along. But it's pretty much impossible that some sister-metabolism from the pre-DNA days could just 'struggle along' side-by-side what we consider 'biology'. When something out-competes another by factors of 1000x or 1000000x, extinction is the only outcome. It's not like elephants and ants finding different niches for billions of years; we're talking about systems of molecules where most of the building blocks are at most a dozen or two atoms and most of the feedstocks are less than 5 (CH4, NH2, O2, H2S, CO, CO2, etc). What's this poor, struggling cousin that we outcompete by a factor of a million going to eat that we can't eat better? Similarly, how does it adapt and evolve against its peers when 999999/1000000 interactions aren't against peers but against us and our self-contained, portable, self-replicating, weaponized Krebs cycle machines?

Re: When something out-competes another by factors of 1000x or 1000000x, extinction is the only outcome.

Maybe it's around, but too small and isolated a population to be discovered.

Or not alive, but fossilized. Speculation gets us nowhere - unless you happen to construct outside alternative life with modern nanotechnology and chemistry, it is bootless to even argue for other forms.

The closest we got is with Threose Nucleic Acids. Managed to get a working hemoglobin binder with this. But it is a relatively obvious analog.

I don't see why 100000x better means extinction must happen. There are "fish" that have copper blood even though iron is significantly better, they are out competed, but that isn't enough to make them extinct: there is still enough oxygen for them to compete.

> Doesn't that just beg the question? If life originated on some other planet and made it to Earth, why did only the DNA-based lifeform(s) make it here?

Maybe Earth is just a big petri-dish for a very advanced alien species trying to reconstruct their own evolutionary history?

It's something I've thought about since I've been a kid (I've been a weird kid).

The time-scales make this kinda implausible, but there's still Clarke's third law, so who knows if properly advanced technology might even allow for artificial time dilation to facilitate such an experiment in a useful way.

> Maybe Earth is just a big petri-dish for a very advanced alien species trying to reconstruct their own evolutionary history?

When I was in like 3rd or 4th grade we were each given a petri dish and told to swab different surfaces (bathroom door handle, water fountain spout, etc.) and then we would learn about bacteria while we watched ours grow. Well the teacher told us our petri dish would grow “it’s own little world” and I (fellow weird kid) had just seen Men In Black so I was fascinated by the idea of an entire world so tiny that it could exist on a cat’s collar. Of course I asked what would happen to our worlds after the experiment (I thought we’d get to keep them!) and when I found out it would go in the trash I refused to participate. (Weird and stubborn - my poor parents.)

And I had completely forgotten about that for 20+ years until today, when you’ve got me laughing at the thought that we could be the petri dish of some clueless 4th grader of an advanced alien species.

I used to joke with my friends that earth was the equivalent of a baking soda volcano at a science fair. Some alien made us and we are completely uninspiring common.

> perhaps only the DNA-based life survived some early period in Earth's history.

There is a theory that early life was RNA based. The mechanims are simpler, and it's easier to create "spontaneously appearing" RNA "life".

But as soon as this RNA life evolved and produced DNA based encoding system, the DNA based life evolved to wiped them out. Right now earth is full of RNAses, which are proteins that destroy RNA, made by DNA based life.

Evolution works in non-intuitive (mysterious) ways.

This is a very plausible theory. An analogous phenomenon happens in the world of memes, where some ideas produce "phenotypes" that wipe out other, competing ideas. c.f. Tienanmen Square in China or the Nag Hamadi scrolls.

Can you elaborate on your examples?

Yes, but have you read "The Selfish Gene"? If not, you need to do that first.

I have, I'm interested in memes as Dawkins defined them then. I've also read many translations of Nag Hamadi scrolls, so your example sparked my curiosity. Perhaps I need to re-read The Selfish Gene, it's been more than a decade now.

Edit: with reflection I think I understand what you were getting at, that the memes contained in the Nag Hamadi corpus had been successfully suppressed by their memetic cousins which for whatever reason came to be the dominant 'version' of that information, until an accident of history reintroduced the different versions that had been all but extincted in the memepool.

Exactly right.

> Also, for abiogenesis, given that we still can't reliably reproduce something like that, and that we have never seen new kinds of life spontaneously appearing on Earth, we do have some circumstantial evidence for the possibility that it is an incredibly unlikely event,

We have evidence that it is extremely unlikely under current conditions, but then we know that current conditions are unlike the conditions when life emerged on Earth (whatever it's origin), in part due to effects of life itself (such as the Oxygen Catastrophe).

Abiogenesis could be very likely under a combination of conditions that existed in the Hadean or Eoarchean Earth.

Probably, although a not-widely-accepted but interesting theory is that there might be microorganisms that we can't detect because we're not looking for the right things (because the things we look for are based on macroscopic life and things that interact biologically with it).

https://en.wikipedia.org/wiki/Shadow_life

xkcd just ran a strip with a joke about this theory last week

https://xkcd.com/2171/

I think it's unlikely because it seems hard to imagine no interaction between the kinds of life ever after billions of years of coevolution. On the other hand, there are tons of microorganisms that do share our biochemistry that we haven't noticed or identified yet, so maybe it's not impossible to imagine some that don't that we also haven't noticed or identified (since it would be more difficult for us to recognize them).

It is interesting, but following Popper's epistemology rules, this hypothesis is not scientific. So saying it is a not-widely-accepted theory is right, but with reasons: this is not a theory.

Not that just because a hypothesis is not scientific, it doesn't mean that people can not research or fine-tune it until it become a scientific theory, but in this state, spending public research credit on it is not a very good choice (especially in the current state of funding for biology).

(it can be read wrong, my command of english is not great: i'm just adding thing to your comment, this is not a critisism of you or what you said)

Popper's view is neither the last word on what science is, nor the final arbiter as to whether something is scientific: what he did was to add some powerful insights to a description of how science works.

Having said that, it is neither unscientific nor against Popper to postulate something for which there is currently no evidence (the search for exoplanets was started before there was evidence of them) - you would have to postulate something that cannot, in principle, be empirically determined (such as p-zombies) to violate the falsifiability principle (if that's what your specific issue is here.) Such hypotheses are not outside of science; they are an essential part of it, and some unspecified part of scientific spending is effectively spent on generating them.

On the other hand, as Hossenfelder has pointed out [1], falsifiability is a low bar. In the case of shadow life, it is reasonable to ask both how it could remain isolated and why have we not noticed it yet.

[1] https://backreaction.blogspot.com/2019/01/just-because-its-f...

An alternative explanation (s) is that "there's only one way to do life" or that the origin of life is extremely rare/unlikely. But panspermia too.

Interesting but your thought experiment only "proves" that the likelihood for something that can replicate itself to emerge is far smaller than the likelihood that that something eventually evolves into something extremely complex. Nothing more imho.

> The fact that all known life shares a common genetic > structure implies a common ancestor.

Or a (local) optimum.

Prions have a more optimal optimum, but they are not exactly "life" from what I have read.

You can't quantify the complexity of something like an eye. The eye could actually be very very simple to form, due to circumstances and variables outside of our knowledge.

Scientists have discovered that the animal with the most genes--about 31,000--is the near-microscopic freshwater crustacean Daphnia pulex, or water flea. By comparison, humans have about 23,000 genes.

One of the most astonishing features of the D. pulex genome is its compactness: despite being around 200 Mb in size (around 16-fold smaller than the human genome which is 3,200 Mb in size); its 12 chromosomes contain a minimum set of 30,907 predicted protein-coding genes, more than the 20,000–25,000 contained in the human counterpart.

The unexpected complexity of the water flea genome compared with a human genome illustrates unknown dimensions of complexity that we cannot fully comprehend.

What this means is this. You do not know how complicated the original template of life is... the complexity for the first genome to produce instructions that allow for self replication to kick off evolution may be so complex that it could only happen once on our planet, once in this galaxy or once in the universe.

We simply do not have the knowledge to know the probability space of life on earth or life as we know it.

Why are there no other human species (anymore)? Because there was no room for them anymore and they could not compete. In the same way, it it very well possible that life utilizing other genetic code got stamped out (early on).

well, if you look on viruses and bacteria situation stops looking that much uniform, there're some odd bugs there https://upload.wikimedia.org/wikipedia/commons/9/98/A_Novel_...

OK, I'm not any kind of expert in this area, but is RNA not considered an early form of genetic coding? And DNA evolved from it? Also, don't viruses replicate by "borrowing" the mechanism of their hosts?

My understanding is that life visible to the naked eye is a minority of life forms on the planet. Maybe other forms of genetic coding than DNA are more prevalent than we anthropocentrists think.

Yes. DNA probably "evolved" from RNA. Keep im mind that the time scale is huge and that at the time, the earth was really hot and pressurized, so we can only have speculations.

A posituve thing is that in Miller experiment, he found out that those conditions create aminate acids. That also indicate that panspermia theory is unlikely (not impossible, but not Occam's razor choice at least).

My own tin-hat theory on that is DNA probably co-evolved with RNA pretty quickly. The main difference between RNA and DNA is that a single RNA strand can fold up and bond with itself to make interesting() 3d shapes while DNA can't self-fold. I think single-stranded DNA initially served as a durable, robust structure for RNA to assemble against before it went off and folded up and did whatever it did chemically. This might help explain why RNA polymerase only runs 3'->5'; in fact one of the early functions of one of the RNA polymerase components was maybe to keep the single DNA strand "tidy" and free of random base-pairings. Also, it might help explain why the 3'->5' DNA strand has to jump through so many extra hoops to get replicated - double-stranded DNA came later. It might also explain the reason reverse transcriptase exists at all. Again, I am probably provably a crank on this.

() This is the whole appeal of the "RNA world" hypothesis, you have one molecule that can serve both as "code" and catalyst for other chemical reactions.

On the contrary. If life started multiple times on earth competitive exclusion probably took care of all other primitive forms rapidly. Even small differences, energy efficiency and reproduction lead to situation where only one form of primitive life survives.

Once the life really started going, it changed the chemistry of the earth rapidly. Similar chemical evolution starting from scratch was not possible once the life started. Eventually when photosynthesis started, biologically induced molecular oxygen caused so called Oxygen Catastrophe where Earth's chemistry turned from reducing into oxidizing. Oxygen also changed the chemistry in the seas and rocks. Molecular oxygen is great disinfectant.

How about different chirality? Different chiral forms would have the same advantages.

Most precursor molecules (e.g. CH4, NH2, etc) aren't chiral, so differently-chiral'd proto-life would still compete for the basic building blocks. Each time one set of complexes managed to get more efficient at something, it probably drove more than half of its competitors to extinction. Repeat a few times and you wind up with essentially a single global consensus and the arms-race moves on to higher levels of abstraction.

> Most precursor molecules (e.g. CH4, NH2, etc) aren't chiral, so differently-chiral'd proto-life would still compete for the basic building blocks.

Yes, but horses and cows also compete for the same building blocks. And yet, we have horses and cows.

Your argument could be right if there wasn't an abundance of these building blocks.

And yet we have no more mastodons, dinosaurs, trilobites, ... Stuff still goes extinct, it's just a lot slower with cows than with molecules.

I think you're just failing to grasp the sheer numbers involved here: there are probably around 10^9 cow-like critters and they interact with other, competing critters every, let's say, 10^4 seconds, with the odds of any one of these interactions being fatal being, again, let's say 1 in 10^4. By comparison, there were probably something like 10^37 (if I counted my orders of magnitude right) biomolecules interacting every 10^-6 seconds where every time some new chemical "A-ha!" moment occurred, that reaction now became 1000x more favorable than any other competing reaction. That's like, what, 41 orders of magnitude; even if I'm off by 10+ orders of magnitude, there was just a whole lot more selective pressure going on during the pre-biotic era. Less favorable chemical pathways were outcompeted on scales so vast you'd need trillions of cow-years to approximate how de-de-de-deselected-for they were.

When life forms of opposite chiralities exist in the same place, they compete against each other s by turning precursor hydrocarbons into forms that their mirror can't use. There is also the fact that while polymerization of alternating left and right handed nucleotides is relatively rare, it happens in molecules like siderophores.

Btw. Mirror chiralities are not exactly symmetric. There are tiny asymmetries between chirality due to weak interactions and electron interactions. Weak interactions might affect long hydrocarbon chains. Electrons from beta decay interact with molecules of different centrality with slightly different speeds (difference is something like 0.01%).

https://www.nature.com/news/force-of-nature-gave-life-its-as...

Nature might be slightly biased due to the asymmetry of the weak force.

On the contrary. Life diverges to occupy niches. You will find multiple examples of less efficient processes and body plans living in niches not far from more efficient ones. You can still find all 3 domains of life on earth (Archaea, Eukarya, Bacteria). You can find less efficient photosynthetic pathways dominating their environment (see c4 rice project).

Even the most primitive current life forms are highly optimized and adapted. I don't think the evolution and adaptability of primitive earliest life forms was similar as it is today. Earliest cells were very unstable shaky chemical assemblies and would be eaten by any organism that exists today.

> You can find less efficient photosynthetic pathways dominating their environment (see c4 rice project).

It's not absolute advantage. c4 has advantage in warm and low CO2, low water conditions. In colder climates and with more CO2 and water c3 plants gain more carbon.

> The fact that all known life shares a common genetic structure implies a common ancestor. The lack of competing genetic codes could be interpreted to imply that life only started on earth once, or that it only made it to earth once.

The more I read about DNA and RNA, the more it seems to me they're somewhat fundamentally grounded in chemistry - you can't just make codons mean arbitrary things by messing with the replication mechanisms. There's less path dependence, and more of "one optimal way of doing things".

That said, I think it's reasonable to assume we all have common lineage[0] - it's arguably a winner-takes-all scenario, as there's a huge first-mover advantage for the first self-replicating family of things that got loose on the world.

[0] - causally, not genetically; the history of life is not a tree structure due to constant lateral gene transfer that bacteria and archaea engage in.

The genetic code isn't quite as universal as you might think. Here's an interesting article on the peculiarities of encoding in the mitochrondial genome, and what it tells us about evolution. https://whyevolutionistrue.wordpress.com/2014/09/28/guest-po...

Also, https://en.wikipedia.org/wiki/Genetic_code#Alternative_genet...

Although most (all?) of the variability is in the one place you can make it without almost-certainly killing your host organism - in repurposing the STOP codons. There are usually three (and likely UGG was a fourth before it got hijacked by Tryptophan in the before time) and all you need is one.

Perhaps genetic replication did emerge multiple times on Earth, but always to the same solution which had the highest probability?

Drew Endy gives a pretty fun talk along the same lines:

https://www.youtube.com/watch?v=GY28A0tBNPU

The panspermania hypothesis has one very large hurdle to get over: the half-life of DNA is ~500 years [0].

If life came from outside our planet via an asteroid or something, then it had to be a LOT of DNA that came over. More exotic theories can be concieved, but the jist is that it is really really hard for DNA to travel the stars and between planets intact.

The simplier explination is that it started here.

[0] https://royalsocietypublishing.org/doi/full/10.1098/rspb.201...

I get the impression that this article is mostly about dating bones.

When people talk about DNA-based-panspermia, I've always assumed that they meant either a living or dormant organism (with the potential for DNA replication/error-correction), rather than a completely inert blob of DNA.

I always figured that the extremes of interstellar space automatically precluded anything that was even remotely alive. The temperature swings, vacuum, radiation, and ejection and re-entry violence always seemed to me to be so crazy that I assumed that nothing could survive it; just bits of genetric material, a dead ameoboid carcass, etc.

I'll have to ponder on the idea that anything could get past all those 'gates'. If it did, woah, what a hardy little thing!

There's one possibility that explains it: that the probability of a genetic code evolving naturally is really, really, really low. Like, less likely to happen than once per universe. Then, once this barrier is attained, other forms of evolution occur more readily. As we don't know the mechanisms for how it happens, it's a possibility. The anthropic principle allows for it, but it likely requires multiverse theory and/or infinite universes coming and going over time. There's just no consciousness in all the infinite universes where it never occurred, so there's no one there wondering why there's no life.

> The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times.

Not quite. Simple life forms probably existed for far longer than DNA. The fact that we see convergent evolution in complex life forms, which emerged very late in the history of life, only suggests that there are recurring patterns in the system that enabled the formation of higher life. It doesn't suggest anything about the origin of cellular life itself, which existed for hundreds of millions of years before eyes or lungs.

> The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times.

Evolution of eyes and abiogenesis aren't obviously similar enough for this analogy to be compelling without some narrative as to why what occurred with one should shape expectations of the other. The conditions in which life first emerged in either the Hadean or Eoarchean eons weren't around for much more of the history of the Earth; if the average expected frequency of abiogenesis under the conditions it actually emerged was once every quarter billion years or so, then—no matter which point in the possible range of time it first occurred—conditions wouldn't have been right for more than one to a very few instances, and if there were more than one, still one would likely be much more established by time available to spread alone, and then there were be massive environmental changes likely producing extinctions. Only one lineage surviving that bottleneck isn't surprising even if there were more before, and while we have evidence of early life it's far from complete either in coverage or ability to determine biochemistry and lineage.

It's really not necessary to invoke extraterrestrial origin to explain only one lineage on Earth (and it actually may make the problem worse: because then you have to explain why we haven't gotten alternate lineages from the same or other extraplanetary sources.)

>Natural selection is a constant force, presumably new genetic codes could offer an advantage, e.g., robustness against damaging mutation, resulting in multiple competing genetic codes.

But what would the advantage look like? Isn't successfully surviving and propagating for billions of years, the ultimate advantage? It would seem like our current state is the best one then..

Methane is not nearly as good a signal to look for life as larger organic molecules are. Methane can easily be created by abiotic processes and remain stable when not directly exposed to radiation or oxidizers indefinitely: this happens on earth, and we do have oxidizers everywhere. Larger organic molecules are less likely to have abiotic origins by any known process and are almost always much less stable.

On earth you can test whether methane has organic or abiotic origins by testing its isotopes. We can do this because we understand decently well how the geologic past of earth unfolded. We could probably do a similar thing for mars

>The Earth is 4.5 billion years old. And the universe, at least based on estimates from the Big Bang, is something like fourteen billion years. So, if life evolved somewhere else, that buys you about ten billion years of time.

Obviously I'm no expert, but you have to keep in mind, it took a decent amount of time for non-violent stars to become the norm / stellar neighborhoods to calm down and for overall metallicity to become significant enough to allow for the formation of terrestrial planets.

Carbon dating depends on the details of the biosphere. C14 is produced in the upper atmosphere by cosmic rays, and then (once it's fixed into a tree) it decays with a half-life of 5700 years.

So if a chair is made of wood with a C14 fraction that's 1/4 of the atmospheric fraction, you can say it's 11400 years old (as a first-order approximation -- the actual science calibrates out a lot of things.)

But it's not clear that the presence of life would affect the C14 ratio in the upper atmosphere. The fraction should be where the production rate (proportional to the cosmic ray flux and percentage of nitrogen) matches the decay rate (1/5700 / year)

But Mars also has an atmosphere (although thin) of CO2 exposed to cosmic rays, no? So we could determine if there is some sort of CO2->methane cycle by seeing if there is any appreciable amount of C14 in the methane we find on Mars?

Apologies if that's a stupid question, I'm out of my element here

C14 is actually produced from cosmic rays hitting nitrogen, not carbon. The impact turns a proton into a neutron. Both CO2 and CH4, once the chemical bond is made, only decrease in C14 fraction as it spontaneously decays.

Astronomers are pretty good at deducing things from the few, imprecise measurements they can make from millions of miles away, so you should never rule out them being able to figure out something. But it'll depend on a lot of assumptions.

> I'm out of my element here

Nice

>> and for overall metallicity to become significant enough to allow for the formation of terrestrial planets.

You don't need much metallicity before terrestrial planets start forming. They would certainly be rarer in the early universe, but stars filter material very quickly (pushing light stuff further away, stripping new planets to their dense cores). So even with 10% of today's metallicity, there would still be plenty of terrestrial planets.

(lol. My spellcheck corrects "metallicity" to "Metallica".)

So even with 10% of today's Metallica, there would still be plenty of terrestrial planets.

Pardon my ignorance, but the Earth is 1/3 the age of the Universe?

As far as our current understanding goes, yes. But it looked very different in the first billion years.

About that.

The first stars appeared very early, and lasted for a very short time, so this may be a huge timespan.

They would have produced metals pretty quickly given that most of them would be massive and emit stellar wind and/or supernova?

That's what I understand, but I'm not an astronomer.

Yes, but it is projected to last for < ~0.1% of the overall lifespan.

Yep, time is the universes only dimension the earth is not a tiny and insignificant blip on, so far anyway.

Yeah this exactly. Today my perspective of the universe changed a little.

What did you think it's age was?

I can't answer for them, but it's completely believable they knew the age of the earth and the age of the universe in the abstract sense and never really connected both ludicrously large numbers together. That's why we have tools like Sagan's squeezing it into a year calendar just so humans can understand it in relatable terms.

Of course, not. Universe is infinite in size, time, and scale.

The farthest object we can see in _visible_ light is at least 32bly away, so it is at least 32by old, or just 13.4by old by mainstream theory called «Big Bang».

You're getting down voted for a fair assumption, and no explanation.

We can see more distance than the age of the universe Because Expansion of the universe

Here is a complete explanation

https://medium.com/starts-with-a-bang/how-is-the-universe-bi...

This explanation has lot of weakness. Some scientists disagree with it.

For example, if we heat up a metallic sphere, then it will expand in all directions. If we project this expansion backward, then we will see that metallic sphere was created just few hours ago. Do you agree with that?

Moreover, why I should accept that Universe is expanded in single whoooping event? IMHO, constant vibration (noise) of gravitation field explains same effect without need for this single event. Vacuum just contracts and expands by tiny bit all the time, because something happens in Universe all the time, so light ages as it travels.

I won't pick out all of your arguments, but there is an abundance of evidence for the long-term expansion of the universe as measured by redshifted cosmological objects at numerous distances.

Stephen Hawking's The Universe in a Nutshell does a nice job of introducing many of these concepts for those interested.

Stephen Hawking's book was published in 2001. Please, look at data from 2019: https://en.wikipedia.org/wiki/Hubble%27s_law#Measured_values...

We aren't projecting anything, we're observing this expansion backwards as far as we can see, right back to around the CMB.

To take your analogy, we witnessed the expansion of the metallic sphere from a point too small for us to see, there may be more going on that we don't understand, but it certainly looks like it expanded out of nothing.

All that is speculation, which was based on two premises: 1) Hubble constant has value about 70(km/s)/Mpc; 2) with distance, we see "younger" (bluish) galaxies.

(1) - wrong, Hubble "constant" has different values depending on frequency of light or radiowaves used for measurement.

(2) - wrong, but papers about this started from 2008 only. In short, increasing diameter of telescope makes galaxies look "older".

Yes. The universe is approximately 9000 years old.

Dude. How do people know how big or old the universe is? That’s just the VISIBLE universe. I bet they will change their estimates in 100 years again

There's a great Imax film called Dark Universe were Neil deGrasse Tyson explains how it's likely that if an observer in the vicinity of Earth were to travel to the furthest point in space that we can observe, they would see the exact same thing in all directions that we see from Earth. They would see beyond our Earthly visible region of the universe and see more universe beyond it. If the observer travelled further again, they would again see the same thing in all directions. Basically we can only see so far, but it's likely no matter how far you travel in the universe, you'd just see more expansive universe in all directions. None of this is proven of course but it's an interesting and plausible idea.

It possible that we will see much denser region of Universe in one direction at some point, while we will see less dense regions until clean vacuum in other directions. It's hard to predict which outcome is more plausible at this point of science research. My own research predicts that Universe has ceiling for black hole size, which means that first variant is much more likely than second.

Assuming the universe is homogeneous (ie we are not in some special place) you only need to see a small portion of the universe to date it.

https://en.m.wikipedia.org/wiki/Age_of_the_universe

Even though we have only one datapoint, I wonder how much can be inferred from this planet's life characteristics.

First, life came about rather shortly after the planet was able to sustain it. So this timing is either very unlikely, or biogenesis has a high probability of happening, or the initial life got seeded from off planet (from a very rare initial phenomenon).

Second, all the biosignatures from all forms of life are similar. For example, corality (left handed versus right handed molecules) is the same in all life examples we had. So that means that biogenesis happened only once. Otherwise we would have multiple unrelated examples (yes, there is the possibility that one form "ate everything else", but we have all kinds of variety instead of only one organism becoming dominate). So if biogenesis happened only once (for all of our bio examples), then it is either a rare event that just happened to occur very shortly after the planet cooled, or it happened sometime before Earth was hospitable for life and elsewhere in the universe.

Does any of this make sense? Or am I reading too much into the tea leaves?

>Even though we have only one datapoint

I think we can wrangle a few more datapoints if we are creative with perspective, for example, we have extremely good evidence to suggest that the planet Mars is currently inhabited by alien robots.

Your tea leaves are sound.

I think this is why those of us interested in this are very much hanging out for results from our explorations of Mars. It's pretty much our only opportunity to see if life emerged twice in the same planetary system under similar conditions.

What are the chances?

On the positive side Mars had pretty much all the ingredients for life (chemically) and had a long enough period for at least something intersting to develop. On the negative side, the environment was still different to ours (gravity, climate, whether, etc).

We still aren't clear on what conditions need to met for like to get going. This is why both positive or negative results on Mars are exciting.

My favourite theory is that life actually takes ages to get started so much so that it started on Mars and finished on Earth (from Mars being impacted and debris making it to Earth). Kind of like a passing of the torch.

Mars: "Crap. I'm dying. Hey Earth, can you take this thing I've been working on?"

Earth: "Yep. No worries. I got this. Will try to send some back in a few billions years. Take care."

>It's pretty much our only opportunity to see if life emerged twice in the same planetary system under similar conditions.

Well there's also Titan, Enceladus, and Europa right? Titan is by far the most exotic but it could serve as an example of what happens when life evolves under a radically different environment. It has some complex organic molecules there and a very interesting/varied geology and topology. Enceladus and Europa could exhibit very similar conditions to those of the deep sea on Earth as well, which could very well be where life originated. Also at one point even Earth could have became a "snowball" planet and still able to sustain life: https://en.wikipedia.org/wiki/Snowball_Earth

>It's pretty much our only opportunity to see if life emerged twice in the same planetary system under similar conditions.

You mean carbon based life. Methane based life may be a possibility, and you wouldn't have found it on Mars. You might find it on Titan.

Methane is an organic (carbon-chain) molecule: CH4.

Titan would have a low-temperature carbon-based chemistry, but it would remain carbon-based if focused on methane.

Alternative hypothetical biochemistries usually focus on silicon.

True. But a methane based lifeform would be radically different than the carbon based lifeforms found on Earth, and would have emerged separately.

Oddly, that wasn't your claim.

It was. Read the OP's claims about Mars.

Mars alone does not provide the answer about whether or not life evolved independently in our solar system.

Can you elaborate?

They would not have cell membranes with phospholipids for one thing.

Why not? Methane is a hydrocarbon. We are made of hydrocarbons.

I just can't understand what the chemical difference is. Are these differences because of oxygen? I.e. life forms of C+H vs lifeforms of C+H+O?

Yes. Phospholipids are not viable outside of water; they would not be strong enough to perform well in liquid methane.

So the suggestion is carbon based biology at very reducing conditions?

What temperatures are we considering?

Cold. Methane becomes liquid at -161.6C.

It is not my suggestion. It's suggested by scientists that Titan may host methane based life.

Do you have any references handy?

https://advances.sciencemag.org/content/1/1/e1400067

Unfortunately, we cannot be certain life on other planets in our solar system did not originate on our own. It has been determined that meteor that killed the dinosaurs possibly seeded planets and mas far out as Saturn with microbes.

Wasn't Venus at one time more Earth like than Mars? Why is Mars the only chance?

Because Venus’ atmosphere is full of Sulphuric Acid and various greenhouse gases which wouldn’t allow life as we know it to exist.

Our atmosphere is full of Oxygen, which is very poisonous gas, which killed 99% of live organisms on Earth. See https://en.wikipedia.org/wiki/Great_Oxidation_Event

Life adapts.

There's some speculation that Venus may have been far more earthlike for some, possibly much, of its history.

Similarly, Earth wasn't particularly earthlike, in contemporary terms, for much of its history -- sometimes far hotter, frequently far colder.

> So that means that biogenesis happened only once.

No, it just means that only one lineage did well enough in competition that we can find examples to study its biomarkers.

Or that there are factors we don't yet understand that favor similar biomarkers in independent lineages.

Or...luck.

> if biogenesis happened only once (for all of our bio examples), then it is either a rare event that just happened to occur very shortly after the planet cooled, or it happened sometime before Earth was hospitable for life and elsewhere in the universe.

Or biogenesis itself changed the environment in a way which made it no longer conducive to biogenesis.

A very minor note on your fun comment- the word is spelled "chirality" not "corality" :).

The new life may have spread very rapidly and ended the conditions that abiogenesis required, in a sense it had a massive "leg up" on the "evolution" of non-living compounds, and we already know that species that get out competed are quickly driven to extinction.

Consider for example that abiogenesis is probably impossible in an atmosphere containing oxygen, because everything gets oxidized too fast, although free oxygen in the air happened quite a long time after life arose.

To abiogenicly form long chain carbons you need a free oxygenless reducing environment, oxygen from the new microbes could have easily poisoned all the viable sites for abiogenesis.

I like the thoughts you're exploring here.

But I would be cautious calling a billion years or so "shortly" even in the context of a four-something billion year old planet. A billion is a very big number, especially when you multiply it by how many trillions of little potential multi-dimensional petri dish situations were around for each second of each year of those billion years.

And it could have happened only once because once established, it tends, on a timescale orders of magnitude shorter than a billion years, to squelch the appearance of other forms, at a minimum by guiding them along with its template, or alternatively by smothering them.

But this all says nothing about whether it happened on Earth or elsewhere. Either one could still be true.

I thought it was more like 400 million years between water and the first life -- and that is the oldest life we have evidence of. It could conceivably take a couple hundred million years for life to go from the initial spark, to having evolved enough (and have a sufficient population) to leave detectable fossils.

> So this timing is either very unlikely, or biogenesis has a high probability of happening, or the initial life got seeded from off planet

Or life can only appear on young planets (maybe while it's hot), or some astronomical event triggered it (Moon?), or there are enough hard steps on the path to intelligence that it couldn't appear if life took too long to form. It's a good question anyway.

> So that means that biogenesis happened only once.

Life has repeatedly shown a tendency of consuming all available raw matter and populating every niche. Biogenesis happening only once is completely non-surprising.

There is a rather intriguing BBC documentary "Do we really need the moon?" about the topic on YouTube. The argument being made was that the moon propelled the chemical clockworks to develop life in a time when the moon was much closer, hence its tidal forces much stronger. To me that would mean that, Mars has, for a lack of such history, a weaker case for being dubbed the origin of life, unless research came across really strong evidence. Like, it would have to be from a time before Earth cooled down enough to even remotely harbor life.

How long did it take for life to exist after there were oceans?

The question of where life originated will have no good answer until we figure out how to create life from basic components. We still don't know how to do that yet.

Yes, Urey and Miller showed that you could get simple organic molecules by passing an electric charge through gasses thought to be present around the early Earth.

But going from low molecular weight inputs to life is a vastly different problem. You could think of it as the most complicated bootstrapping problem in the universe.

Nobody has managed to do it in the lab, either. What can't be built from scratch can't be understood very well, and therein lies the problem.

If we had a stepwise procedure for building a self-replicating, self-feeding organism of any level of complexity from base components, we would know exactly what to look for.

Until then, the idea of sending a DNA amplifier to Mars isn't a bad fallback position.

If we succeed, it would be the most important breakthrough in the history of humanity, and would have very deep existential implications. It wouldn't definitely rule out a supreme being as the creator of the universe we live in, but it would make the gap between us and that being much smaller than millions of people on earth believe now.

> would have very deep existential implications.

I doubt that.

> It wouldn't definitely rule out a supreme being as the creator of the universe we live in

Yes, that's exactly what the believers will tell you even then.

Think about it: the science knows today that every atom in our bodies is either produced 13.8 billion years ago (is it is H2) or in the explosion of some star. It's true for every element that you learn in the chemistry class. We know that the Earth is 4.7 billion years old. And we already know that humans share 99% percent of the DNA with chimps. And that 0.006 billion years ago on the Earth neither humans nor chimps existed, but their common ancestor.

Do people who believe in a "miracle" divine intervention that supposedly happened only some 0.000002 billion years ago (or even only 0.0000014 billion years ago) when only at that point their deity became involved with the believers, do these people in anyway feel affected by all that? No.

Compared to what we know today, their religious "messages from the deity" obviously prove that they were written by plain humans of older times, because these messages don't contain any knowledge we have today, but instead the "eternal" truths like "at the end of the day the Sun sets in the muddy pond" or that the Earth's sky is a solid dome: https://en.wikipedia.org/wiki/Firmament#Biblical_use

Do the believers feel affected by that? No.

Um. That's been done? In several ways? Replacing parts of existing life with different elements (arsenic?). Hand-crafting a genetic code from parts of existing creatures, to create entirely new and different creatures. There's even a company in the town next to me (IDT) that delivers to-order DNA samples via fedex anywhere in the world.

To me that's more like splicing PC components into a Macbook Pro. It doesn't prove or explain how they both evolved from a Tandy computer.

Its like, building your own computer entirely from parts. One that never existed before.

And anyway, I call No True Scotsman. When is it 'really, really creating life'? If making something that lives, eats and reproduces from parts is not 'making life' then what?

I just meant to point out, the old tired philosophy issue of 'Until we can create life, God still has primacy' is out of date.

There has been progress within the theory of non equilibrium thermodynamics which hint that local entropy decrease is baked into the physics. That is to say life just kind of falls out of the physics of thermodynamic systems of a certain type. That certain type being anything that dissipates a source of energy. Personally I think abiogenesis is relatively easy and there is large scale panspermia going on. The universe is like a yoghurt, bacteria everywhere and novel abiogenic origins occurring all over the place, although there are probably dominant kinds of life. DNA based being one.

> The reason this news registered among scientists is that methane is often a sign of life; although the gas can be produced by various chemical reactions, most of it comes from animate beings.

This doesn't sound right at all. There are literally lakes of methane on Titan. The atmospheres of Jupiter, Saturn, Uranus, and Neptune all contain significant methane--it's what makes Uranus and Neptune blue. Pluto has methane ice.

IIRC from the original reporting, some recent research suggested that there's more methane than contemporary geological models of Mars predict. Which likely says more about the [non-biological] deficiencies in those models than it does the likelihood of life.

I'm not an astro-anything, so feel free to correct me. But it's telling that the interviewee never comes close to confirming the reporter's claims; they're simply quoted as saying, "I think probably many people would like the idea of methanogens on Mars", and "the idea that they might be related to methanogens on Earth is not crazy." Isaac Chotiner is a name I'll try to remember so I can avoid his articles.

IIRC, methane should not persist on Mars, and theres not been a global phenomenon detected that can explain its occasional detection. Therefore, transient detections are considered indicators of possible life, since nothing else seems to explain it.

It's not that it's rare everywhere, it's that it's rare on Mars.

There is ancient methane in those places. But something on Mars is producing methane.

Methane breaks down pretty fast in Mars' atmosphere, and yet it reappears there occasionally. That can certainly have non life explanations. But they're fairly exotic.

The author (Chotiner) has indeed made a mis-statement about the state of knowledge on planetary CH4, but that doesn't pertain directly to most of the article's content, which is about DNA/RNA.

As you point out, the state of Mars CH4 is quite complex and extremely controversial, with 3 different measurements (Curiosity + 2 orbiters), low concentrations observed (~10 ppb) and multiple physical processes going on. Here's a pretty good recent summary:

https://www.abc.net.au/news/science/2019-04-02/mars-express-...

The DNA-sensing instrument that's being prototyped is here: http://setg.mit.edu

Minor fun fact: Fred Hoyle, possibly the most public proponent of Panspermia in the last few decades, also coined the phrase "Big Bang theory" to dismiss an alternative to his preferred Steady State theory of the universe.

Sometime champion of arch positions, he was once quoted as saying "it is better to be interesting and wrong than boring and right".

https://en.m.wikipedia.org/wiki/Fred_Hoyle

It is a pretty out there theory, but at one point the universe would have cooled enough from the very hot big bang origins to allow liquid water to exist everywhere, but not cooled sufficiently to cause everything to freeze.

If any rocky planets existed at the time, maybe that was the origin of life which was much more able to spread widely due to the small size of the early universe.

https://arxiv.org/abs/1312.0613

This just sounds like a cool sci-fi setting/backstory.

Hoyle was great man, who said what he thought. His outspoken character probably cost him a Nobel Prize, though.

The idea life emerged long before Earth, and may be widespread in the universe, is fairly old:

https://en.wikipedia.org/wiki/Panspermia

I suspect that a formal, anonymous survey of astrobiologists would find that the idea is far more prevalent among experts than Ruvkin's estimate that "one per cent would buy into the idea of life spreading the way I’m sort of promoting it".

It's just risky to express, for cultural reasons, until there's tangible proof. Many scientists are temperamentally reluctant to speculate without conclusive evidence, especially given the penchant of the press to sensationalize any such theories. It'd raise difficult questions about our place in the universe, and possibly popular fears.

Thinking science had already settled on a consensus that simple life is everywhere might even paradoxically reduce funding for new missions to test the idea. After all, there have been tantalizing hints of life-processes on Mars going back to the Viking lander experiments of the 1970s – and yet we still can't seem to send a lab package that'd definitively answer the question! But the hope of figuring it out motivates new missions.

(As should be clear, I'm strongly in Ruvkin's camp – and regularly comment to that effect here on HN: https://hn.algolia.com/?query=gojomo%20panspermia&sort=byDat... )

There was this interesting article posted here some time ago that plotted the complexity of life found on earth vs time, and by using the right methodology and logarithmic scales, it arrived at a nice linear line over many data points and several billions of years. And the line converged to a 0 point, but that point was a few billion years before earth was created 4.5 billion years ago, adding more credibility to a panspermia theory. Can anybody remember that article and its url?

Update: found it. Apparently the theory is contentious, with accusations that they cherry-picked their data points. Discussion: https://news.ycombinator.com/item?id=5580334

Is this it? https://www.technologyreview.com/s/513781/moores-law-and-the...

(I found it elsewhere among these comments)

What if life is a fundamental property of a sufficiently complex universe and a stable energy gradient?

That's where I'd put my money.

This right here. Provided a body that can retain and emit heat from a star, it intuitively makes sense that given correct conditions the warm body would trend towards more complex head dissipation mechanisms.

Life is just a more complex — and sustainable — exothermic reaction than standard oxidative mechanisms (fire).

By the way: this isn't a new concept. Jeremy England at MIT has been espousing the concept for a while [0].

[0] https://www.quantamagazine.org/first-support-for-a-physics-t...

Hm. Also, money could be a fundamental property of a sufficiently complex life.

I understand the downvote, but I also agree with the statement.

IMHO money, markets, even democracy seem to me to follow directly from the challenges of living in a complex society which follows from the benefits of working at large scales.

They even seem to me to be simply abstracted versions of more naturally evolved systems baked into life on Earth a billion years ago, i.e. ATP metabolism and multi-cellular life

The real question is - are iPhones a fundamental property of sufficiently complex life?

Will the first alien culture we encounter have a 2D vision & touch-based thin interface to their global (galatic) network?

Seems like a bad bet given that from the evidence we have so far, life is an unbelievable statistical anomaly. There's no reason why life had to have evolve on Earth or survive long-term; it is sheer luck that we haven't been hit by gamma ray bursts or a supernova or any one of the other nasty, super-deadly things the cosmos could throw at us.

Don't confuse the delicate dynamic and charismatic oxygen breathing multi-cellular life on land, sea, and air, for life its self at all. Consider extremeophiles, namely endoliths that live two miles below the earth's crust. They're not going to be bothered by most the universe has to throw their way. And once things like mushroom spores hit intersystem space it's all over.

Also, don't fall for innumeracy. Considering the amount of material on Earth, let alone the universe, the spontaneous ease of amino acids, the inherent self construction of simple RNA strands. We only need Generation III stars for Sulfur which is the heaviest atom in Animo acids. So, chances are there was the potential for life 400 million years after the big bang, 13.4 billion years ago.

Fair points through and through; big picture, it would be surprising to me if something else we'd recognize as "life" didn't exist elsewhere in the universe. I just wouldn't go so far as to put money on it being a "fundamental property" of the universe from a strictly objective, statistical view based on what we know for certain in the here and now (which arguably is a needlessly pedantic stance, but eh...).

I concede, "fundamental property" is a bit more poetic than the strictest version of that term. I'll stand by "inevitable" though, by sheer numbers it just HAS to happen by sheer chance alone (or so says my deepest heart of hearts backed up by my brain reading up on state of the art cosmology, biology, and material science).

I have wondered if panspermia could have occurred in the Sun's birth cluster. This cluster would have had a very high density of stars, with gas around them to slow and capture material ejected from their neighbor systems. And the asteroids in orbit around these stars would have still be warm enough to have liquid water in them, so life could potentially have been seeded in a wide variety of environments.

This possibility, if true, means SETI might be worthwhile even if the actual origin of life is astronomically rare, much less than once per observable volume of the universe. SETI would look at the other stars that were in that cluster. Even if the rest of the universe were devoid of life, they (like the continents of Earth, or planets in our solar system) could share the same origin event.

This scenario might also explain why life apparently originated so early in the history of our solar system: this sort of natal panspermia would statistically amplify origin-of-life events that happened early on, by multiplying the number of systems they'd affect. This amplification would occur even if OoL was extremely rare.

Based on nothing but faith and an irreducible cynicism, my money says we end up going full circle, from "Earth is alone in the universe in regards to life" to "Life is a fungus, found anywhere the most basic of conditions are met" (at least over periods of millions of years at stellar scales)

People forget that this discussion is not necessarily related to life. When various European explorers went out, in many cases they didn't know what to expect. When they met strange peoples, most of them refused to believe these were actually human.

It takes a good long while for humans to recognize other lifeforms like us. We naturally think that we're unique, that wherever we call home is special and the center of things. Combine that with galactic distances and it might be a good while before we realize that the universe is actually full of aliens that look just like humans only with bumpy heads (Obligatory Star Trek joke)

Maybe we could test this someday with petri dishes in orbit. Outfit satellites with catch basins that funnel into various potentially habitable pods and see what grows. Park them in a lagrange point for better isolation.

But if panspermia is true such critters in the cosmic wind would be cousins of life on earth, and it would be difficult (impossible?) to distinguish an alien microbe from domestic contamination. If it's false we see nothing, and wonder if it's because space is sterile or we're just not yet offering it the right primeval soup.

Panspermia doesn’t only work (if it works) by microbes floating in space. Rather the idea is that microbes are embedded and somewhat protected in asteroids which then land somewhere habitable. The occasional close brushes between stars and their asteroid clouds provide the medium where microbes could be transferred between stars quickly enough to launch and land before the radiation destroys any hope of survival (like 20 thousand years.)

Cosmic rays sterilize everything in time, so it seems unlikely we would find anything.

My biology knowledge is admittedly weak, but a question for those who might know: a lot of the discussion here is about how we only know of "life" based on nucleic acids (DNA). However, prions seem to replicate themselves without the help of nucleic acids. Could this in some way be seen as a competing form of "life"?

Prions don't synthesize themselves, they just self-assemble. RNA can encode it's own information while also catalyzing self-replication.

It always seemed to me that the biggest argument against panspermia is the relative hostility to "life" of space versus a planet like Earth. The heliosphere protects the solar system from a significant amount of interstellar cosmic rays and x-rays, Earth's magnetosphere offers additional protection, and our atmosphere, particularly the ozone layer, offers additional protection while also protecting against UV and high energy particles from the Sun. Ionizing radiation and short wavelength EM radiation are destructive to life and really the persistence of any large complex molecules. Combine that with the vacuum of space and temperatures barely above absolute zero and life originating via panspermia seems significantly less likely than originating natively on Earth.

Not to mention atmospheric entry and asteroid impact.

I'd say the biggest argument against panspermia is that the organisms responsible for it need to be successful locally and universally, at the same time. Reproduction is expensive and intrastellar reproduction is incredibly low yield. So organisms would need to have evolved to both capture an entire ecosystem AND waste a bunch of energy blowing their seed into the solar system.

The idea just doesn't jibe with my understanding of evolution.

What if we began a 'seeding' experiment, suppose there could be more life but isn't because life doesn't spread well in the vacuum of space. What if we began sending bacteria and other micro-organisms to potentially habitable planets via tiny spacecrafts. If we were to seed as many planets as we could possibly find, then eventually (in a million years or so), this entire galaxy could be teaming with life and the fermi paradox will be refuted as there will be no way not to see the life that is out there. We may still not be able to traverse the stars by that time at least not without generation ships but our descendents could at least wave from afar and maybe communicate across vast distances with intelligent life that we today helped seed/create.

I'm not really on board with the idea of just blindly infecting all habitable planets with Earth life. We should look but don't touch until we have some clue what the heck we are doing and why.

What I find more interesting is the potential frequency with which new life from origins other than earth has been mixed together with what is here. A lot of stuff falls into our atmosphere and I doubt we know where all of it comes from.

I commented today on a bit of dermatological discussion where they claim our skin has a biome of bacteria. This is in addition to the gut biome that is starting to look like it contributes to a wide array of human health issues. Part of me wonders how much of those biomes (e.g. how many bacteria within that biome) are from sources outside our planet.

Well, if they did, then the fact that the basic DNA machinery is the same implies that we and the alien bacteria had a common ancestor somewhere back there.

> implies that we and the alien bacteria had a common ancestor

My intuition suggests that wouldn't be necessary. For example, if I saw a snowflake like structure on another planet I wouldn't be surprised since that kind of structure is a result of the physical properties of the elements involved. It could be the case DNA is very similar in that respect, a mathematical necessity given the properties of hydro-carbons. I suspect the science on that issue isn't settled.

I'm thinking someone may have proved by construction that DNA isn't the only possible code. Didn't someone make "artificial" DNA by substituting something chemically for some of it?

That sounds slightly different than what I mean.

You appear to be saying: "It is possible something DNA-like could be constructed from different materials in different codes and it would still be viable".

I was saying: "It is possible hydrogen, carbon, oxygen, etc. will form into DNA as long as suitable conditions exist and this is common enough occurrence that multiple sources of DNA independently developed across the galaxy/universe". I only say this to assert that the appearance of DNA from a source other than Earth should not necessarily imply a common ancestor.

Maybe so, but it would be surprising if it settled on the same instruction set https://en.wikipedia.org/wiki/DNA_codon_table

Not sure if this is what you're referring to, but we have created DNA with additional "letters" which don't appear in nature.

https://www.livescience.com/64829-hachimoji-dna.html

Not a biologist, but my impression is that the concepts involved in DNA/RNA, and especially the machinery around it, are so complicated and have so many degrees of freedom, that snowflakes, which are rather simple crystallizations of only water, may not be an apt comparison.

Maybe it's closer to, say, someone having just learned the basics of analogue electronic circuits (but nothing beyond that), going into a basement, and coming out with a fully formed IBM-compatible PC with USB ports.

Again, not my field, and I'm happy to be taught otherwise on that one.

If you're saying that DNA is too complex to have evolved on earth, where else would it evolve? Panspermia just puts the origin of life in another location in the universe. It doesn't answer any questions about how that complexity would evolve.

Thats not what is being said, hes saying that if life evolved twice, it would be almost certainly completely different and "incompatable" at a molecular level.

Consider the design of processor architectures and operating systems, similar design goals but typically one cant run code from the other.

The RNA to protein encoding specifically is completely free and arbitrary.

I wish he had posited a theory for how life from Mars would have even gotten here. Some comments have said an asteroid hit Mars and then redirected to Earth with early life on it? Is that possible/likely?

Yes, it happens. There are about 200 meteorites found on Earth that are known to have come from Mars: https://en.wikipedia.org/wiki/Martian_meteorite

Don't know if anyone has ever considered this (ignoring gross simplifications), but as depictions of aliens are typically conceptualized from creatures we see on earth. Is it possible that if we are the only living creatures that have (yet) gained heightened consciousness, then spreading the DNA of all life on earth across the universe in a similar fashion to how our microbial life arrived on earth is how we ensure that life truly goes on if we fail?

“...what if it started here and then spread elsewhere? And so we found some sort of DNA on Mars, but that’s because it spread there from Earth. Is that possible?

Yeah, but that sort of places us at the center of the universe, and all the force of history is to say, “Don’t think of us as the center. We’re nothing.””

What kind of argument is that? Sounds like some kind of anti-geocentric dogma, not an actual argument.

And what if life did originate on Earth? That's one of those hypothetical questions where the answer wouldn't make any practical difference, if we even had an answer. If and when we find extraterrestrial life, maybe we can come up with questions where the answers would be of practical value.

James Tour: The Mystery of the Origin of Life https://www.youtube.com/watch?v=zU7Lww-sBPg

>It just seems like, once it evolves, it spreads.

Hence why I draw exactly the opposite conclusion. It's my thinking that if life existed throughout our 13 billion year old galaxy it would have evolved to saoience and consumed all available resources long before the rise of humanity. The fact our planet hasn't been strip mined or colonized is, absent hard data, good reason to believe we're alone.

There's a big difference between microbial life and intelligent life.

Bostrom and others have argued that, based on how long evolutionary milestones took on Earth, the "difficult step" is actually complex multicellular life.

The timeline implies that but experimentation indicates it may not have been that hard.

https://www.nature.com/articles/s41598-019-39558-8

https://www.sciencemag.org/news/2018/06/momentous-transition...

That's interesting.

I agree with your conclusion that Fermi's paradox simply means there is no other intelligent life, though I think the jury's still out as to where the filter is.

Indeed. There's quite a lot of debate surrounding it and a couple of mathematicians arguing we haven't hit the filter yet, but I maintain hope we're past it. It wounds my sci-fi loving heart to say this but humanity is probably better off alone.

An interesting discussion of that here: https://www.technologyreview.com/s/513781/moores-law-and-the...

Why would aliens need to stripmine earth? What do we have that they need so much that isn't abundant elsewhere? There's an asteroid out there alone between Mars/Jupiter with possibly more Gold than all the gold on earth combined. Likely there are billions and trillions of these things. Advanced races probably can create just about anything from base components and minerals easy enough so they'd just need to mine whatever is nearby.

Now, they could possibly need stars and galaxies (i.e. all stars in a galaxy) to capture as energy for large enterprises, but pretty sure there's not kardashev 2-3 races in our vicinity --we'd surely know it if there were. Energy is probably the only resource that aliens might go looking for, everything else is presumably abundant.

I think we should really begin colonizing and expanding our biome to other planets asap so we can possibly beat other civilizations to it. I'd trust a civ that grew from our basic microbes over one from silicon -because we know how we turned out at least, we can't be so sure about them.

I'd be interested to see (if I could live long enough) what would happen if we sent microbe probes to every semi-habitable planet we find, then wait 1 million years to see what life progresses from each planet.

Did you mean to say the asteroid has more unmined gold in it than all the mined and refined gold on Earth? Because the core of the Earth likely has more mass of gold in it than all the asteroids, moons, and dwarf planets in the solar system put together.

We just find that particular gold more difficult to bring to market than gold found outside our gravity well.

To extra-terrestrial aliens, strip-mining would be grabbing all the loose asteroids and comets first, at the "surface", before venturing down into the deeper holes represented by the gravity wells. Stripping larger planets is metaphorically deep-shaft mining, from a space perspective.

I doubt sapient extraterrestrial life is going to bother with the hassle of shipping resources up from a gravity well, when those same resources are already abundantly accessible everywhere else.

I've wondered for years if maybe the idea of life originating on Earth might be the final geocentrism.

We haven't yet collected evidence that says terrestrial life originating on Earth is impossible or even unlikely, nor has anyone produced an alternative explanation that doesn't involve introducing additional external factors. It may indeed turn out to be incorrect (although I wouldn't be on it), but it's in no way comparable to geocentrism which simply couldn't be reconciled with experimental observation.

If you lived on Long Island, and didn't know where humans came from, but you knew about how big the earth was, would it be rational or in accordance with Occam's razor to assume homo sapiens originated in your vicinity? Most people would in fact be wrong if they even assumed humans originated on their continent.

I feel like people get sidetracked into talking about evidence, when the issue is that life originating on Earth should not be the default assumption. I feel like someone more lucid than me could make a good Bayesian sort of argument.

It's not exactly the default assumption though. Historically the default is that life was magicked into existence or came here from a previous world or some other creation story. The current abiogenisis story of molecules->proteins->cells->Life is relatively recent.

The other thing is that because we have a fairly... ?linear record of evolution: if life did originate somewhere else it got to Earth in something like prokaryotic single cell form 3.5 billion years ago. Which means we can't use the idea of panspermia to tell us anything. It's neither predictive or explanatory. It tells us nothing of how or where life did originate or how life got started on Earth or what or where we should look for or expect in the rest of the Universe.

The current theory suggests life could pop up extremely quickly, which itself already implies life is likely to exist in the rest of universe on some level so panspermia doesn't even mean anything with regards to how likely life is anywhere else.

The difference is that you've never seen evidence of humans living anywhere outside of Long Island.

Clearly if you see humans living all over the globe they almost certainly originated elsewhere. But if humans only live on LI, then it's reasonable to conclude they originated on LI.

Given the gaps in our knowledge of early life, a definite statement in either direction is unlikely to be in the offing. However, terrestrial life is decidedly unlikely. Life is decidedly unlikely, and no one has so far suggested an even remotely plausible mechanism for abiogenesis on Earth. This means if you are betting, an extraterrestrial origin is the safe bet; if the odds of abiogenesis are so slim, while life once formed is tenacious and hardy enough to travel through space, it is more likely life traveled here.

It's not clear to me how much panspermia really helps with the abiogenesis problem.

Panspermia might increase the window of time and set of planets where the abiogenesis of Earth's life could have occurred. But are there any grounded estimates of the factor by which it expands those windows? For example, is it safe to assume that if abiogenesis occurred on any planet in the Milky Way before the Earth formed, then samples of that life would have reached Earth within say 200M years after the Earth formed? That would require estimating the rates at which samples leave the planetary gravity well, how likely those samples are to reach Earth's gravity well, how likely the samples are to survive the trip, etc. I have not yet read any detailed estimates of this kind (but I'd like to!).

Another problem ... if the answer to the previous question is "yes", then that might expand the abiogenesis window by, say, 10^13 (say 200B planets and a 50-fold increase in the available time window, ignoring other galaxies). Is that enough to flip abiogenesis from "not likely" to "likely"? 10^13 is a big number but if abiogenesis is extremely unlikely then we're already dealing with very low probabilities. In other words, panspermia only helps if the probability of abiogenesis occurring on a planet in a given time window falls in a certain range, and in context that range might be quite narrow.

It's an intuition or speculation at this point. The only major evidence we have is that a logarithmic graph of information vs time in evolution seems to cut off abruptly at the age of the Earth, suggesting that the process started before Earth if a constant rate is assumed.

https://dailygalaxy.com/2018/09/moores-law-life-is-older-tha...

This is definitely not conclusive as it could also mean that some other process with a different rate or probability structure started life. You can't always just do a regression and draw a line and assume that it holds for all time, as many startups discover. ;)

We also know that organic molecules are ubiquitous in the universe, so there's that too.

We have a seemingly pretty firm age for the universe of 13.7 billion years; earth is 4.5 billion years old, and life on earth is at least 3.5 billion years old.

So I think the step of going from nothing to the first single celled life is implausibly large to happen in a few million to a billion years, and to me, Occam's razor would imply that something necessary was developing during the previous 9 billion years, such as the development of chemotrophs underground in deep space.

What if this entire universe is an organism?

Given the complexity of DNA I think it is near obvious life did not originate on earth. If you assume it did not originate on earth that gives you at least another 10 billion years to work with.

No, it really doesn't; it took a few generations of stars exploding and slamming into each other to make all those heavy elements we kinda consider important. You might be able to move it back another 3-4 billion years, which is admittedly a good long time.

But really there's nothing yet that even remotely necessitates an extra-terrestrial origin for life. The problem is that, each time there was a "winner" in the very earliest more-chemistry-than-life arms-races, it essentially obliterated all record of what the competition looked like. It's not until pretty late in the game that no one new strategy obliterated all its peers - at that point we got the biological domains (prokaryotes and eukaryotes (which subsumed other prokaryotes as mitochondria and chloroplasts)) and viruses. From then on, the arms-races have been at higher levels of abstraction and the fight over the basics of biochemistry is 'settled'.

Wasn't there a movie about this?

"It could be that I’ve never had a religious thought in my life."

The very idea that life has to originate from somewhere/somewhat and cannot just happen sounds like a secular reformulation of a typically religious statement.

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