Comment by ThePhysicist | Hacker News Reader

Comment by ThePhysicist | Hacker News Reader

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I think what will really help us to get at a better estimate is to answer the question whether there exists life in our solar system that evolved independently of that on Earth. If we should find traces of bacteria on Mars or another planet, that would immediately take the corresponding probability factor in the Drake equation to a number close to 1 (i.e. most solar systems with planets in the habitable zone produce simple organic life), which would then leave us only with the question of how likely it is that such bacterial life evolves into something more complex. That's an incredibly hard question to answer as well of course, but it's also something that we might be able to tackle within the next 50 years given a deeper understanding of how living cells work and how they came to be (and it seems we're making good progress there). So personally I'm quite confident that we will be able to get to a much better estimate of Drake's equation in the next 50 years.

    I think what will really help us to get at a better estimate
    is to answer the question whether there exists life in our
    solar system that evolved independently of that on Earth.

That's definitely the big question. But you left out the elephant in the room: is there life right here on Earth that's evolved independently? Abiogenesis doesn't simply halt after it's achieved its "goal". Perhaps we take the common-origin hypothesis for granted (that the "tree of life" has one root, and is in fact a tree), but it's actually nontrivial, strong evidence about the rate of abiogenesis, right under our noses. If it's true.

There's no hard reason why another, orthogonal family of life couldn't coexist with DNA/RNA life. We have 10^7 extant species of DNA/RNA life; that's an existence proof that hostile, competing "types" of life can coexist. Suppose we found terrestrial "XNA life" with a biochemistry totally unrelated to anything else, with no possible common origin or lateral gene transfer. Then we would know life on Earth developed independently N >= 2 times, instead of N >= 1. That would change a lot; we could update P(life | Earth-like planet) from the subjective probability range [10^-Graham's number, 1) to something more like [10^-4, 1).

What's really surprising is that this is completely untested! We genuinely don't know if there's "alien" life on Earth, because we don't know how to look for it. There's no black-box tool to approach unknown biochemistry:

    Some have postulated the existence of a 'shadow biosphere'
    on Earth, teeming with life that has gone undiscovered
    because scientists simply don't know where to look. It could
    contain life that relies on a fundamentally different
    biochemistry, using different forms of amino acids or even
    entirely novel ways of storing, replicating and executing
    inherited information that do not rely on DNA or
    proteins. [...] The trick is deciding what to look for and
    how to detect it. The usual way that researchers search for
    new organisms — by sequencing DNA or RNA — will not pick up
    life that does not depend on them.

http://www.nature.com/news/life-changing-experiments-the-bio...

    ...We could even be oblivious to unfamiliar forms of life
    right under our noses...

http://www.sciencemag.org/news/2015/02/shadow-biosphere-migh...

As soon as you get v1.0 life, it starts bioforming its environment.

v2.0 life would have to evolve in the changed environment - which would be considerably less hospitable, because the most useful chemical processes and energy sources will be colonised by v1.0 life.

v1.0 life will (more or less literally) get the low hanging fruit. v2.0 would have to evolve around the edges, which means the probability gradient is higher and therefore less likely.

But I think this actually happens with normal life as well: Individual species of bacteria try to conquer a biological niche by shifting conditions such that only they can survive. But by doing this they invariably create chemical by-products that can be harnessed by other life forms, thereby fueling their growth. It's an interesting question whether there are any biospheres on Earth that are colonized exclusively by a single species of life (I doubt it).

So although it's a very valid point I'm not sure if it's a sufficient argument against the hypothesis of having two independently evolved and fundamentally different types of life on a given planet.

Fascinating, thanks for pointing this out! Discovering a new kind of life on Earth that has coexisted with us without being noticed would be a huge event of course, and it would probably challenge what we regard as life in a general way.

This would be good stuff for a science fiction story as well.

It does make a good sci fi story: https://www.amazon.com/dp/B00M64A8EG/

Wow, thanks for the link :)

I think it's a winner take all task. If you are the first almost live form, you have plenty of time, and can evolved in an environment that has plenty of food. Something like the brown goo of Titan, with lots of hydrocarbons and other small molecules that you can pick up for free. And nobody is trying to eat you. So the reproduction process can be slow and unreliable, perhaps a few reproductions per day/week?/month??

The only way to die if a UV ray split you, or you fall into a volcano (there are no fire, because the atmosphere has very few oxygen), or a nasty oxygen molecule reacts with you, or ...

Well, there are many ways to die, but they are not targeting you, so perhaps you can hide from the sun under a rock. Or to be more clear, since you doesn't have any kind of senses or mobility, perhaps you are lucky and get beached under a rock. Perhaps the rock also makes a nice place to concentrate other molecules during the low tide.

Perhaps it's not a good idea to be near the surface and it's better to be lucky and get near a underwater sea vent with tasty hydrogen. I think the details of the first steps are not clear for anyone (in particular for me).

Once a stupid life for appears, the first thing it would do is eat all the free food. In particular, before the first life form the half of the amino acid were L and half D. We have used all the L amino acids (and created many more than the original supply), and we have also eaten all the original D amino acids. So there are very few of them floating around freely, it would be very hard to start an alternative D-tree.

A new life form will be targeted by the existing life form, like the free D amino acids. The competition will be extremely hard. So I think it's a winner take all task.

Moreover, then some moron decides to release a lot of oxygen to the atmosphere. Oxygen is extremely toxic for most life forms, and more toxic for isolated molecules that are barely alive ...

About the shadow biosphere: Perhaps we (humans) can't detect or identify them, but the bacteria and archaea can probably live there and eat them barely-alive. Our cousins can keep all the out-of-tree competition at bay.

    If you are the first almost live form, you have plenty of
    time, and can evolved in an environment that has plenty of
    food. Something like the brown goo of Titan, with lots of
    hydrocarbons and other small molecules that you can pick up
    for free.

But there's no food on Titan anywhere! There's no oxidizers for hydrocarbons to react with; hence no accessible chemical energy.

That's likely generic for pre-biotic environments. Planets don't naturally form redox gradients; if there's a large redox potential, that's reactive chemistry, which will react away to equilibrium over geologic time. It takes work to maintain such a gradient; for example, Earth's oxidizing atmosphere is actively maintained by life. Oxygen is geologically removed from atmospheres, reacting with exposed rock surfaces (oxygen weathering). It takes active expenditure of work (solar energy) to keep it existing.

There is some chemical energy in abiotic planets, but only small amounts, at very low power levels compared to Earth's aerobic biosphere (which eats something like 10^14 watts!). There's photochemistry caused by solar UV light, and there's atmospheric lightning (which is a major player in the Earth's N2 cycle). Most important is geochemical energy; i.e. primordial chemical gradients that aren't yet at equilibrium, because the two reactants are physically separated by rock layers. On Europa for example, a very slow process brings rocks into contact with the ocean, where slow serpentinization reactions can occur: oxidation of Fe++ to Fe+++ by water, with the production of H2. This would be the food source of Europa life, if it's there.

A very late reply.

I'm not sure if this is for or against your comment, but I hope you find this interesting:

"A cave in Romania that was sealed for 5.5M years" http://www.bbc.com/earth/story/20150904-the-bizarre-beasts-l... HN discussion: https://news.ycombinator.com/item?id=11682816 (889 points, 38 days ago, 163 comments)

what if life didn't evolve here but arrive on other space debris? Has that theory been ruled out?

The term you're looking for is panspermia. Nothing can necessarily be ruled out but it seems both 1) highly unlikely from a mechanistic perspective and 2) unnecessary to the explanation of life origin.

That's called panspermia, and no, it hasn't been ruled out. Main problem with it though is that it doesn't explain how life as we know it came into existence, it simply shifts its origin from Earth to somewhere else

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