It seems like all life on earth descended from one species, that was a cell. So all our life on earth is made out of cells. But couldn't life be completely different somewhere else?
It's a very simple object--kind of a thermodynamic primitive for resisting entropy. The organelles within cells are themselves cells. I'm not sure what life would look like without cells but it would probably rely on some type of otherwise naturally occuring gradient like a tide pool or mineral formation.
I don't doubt there is some relationship, but that isn't the definition of domain. Otherwise we'd have 1 domain.
> Archaea (/ɑːrˈkiːə/ (listen) ar-KEE-ə; SG: archaeon /ɑːrˈkiːən/ ar-KEE-ən) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes.
The three-domain system divides life in Archaea, Bacteria, and Eukarya. In this system, Archaea and Bacteria can be grouped together as prokaryotes.
In the two-domain system, the division is between Archaea and Bacteria. In this case, eukaryotes are seen as a subgroup of Archaea.
Hope to have cleared up some of the confusion.
Correct. All notions are tools constructed by and for human minds for their own purposes. It's possible to hypothesize a definition-breaking example for nearly everything that isn't a mathematical abstract. What counts is useful definitions. In the case of "life" we already have many examples of things that don't perfectly fit the definition, like viruses and prions.
The local star, especially if close enough, creates lots of gradients.
Cells (as in the bit that makes it a contained volume) are both a thing that forms spontaneously without biology, and very useful for isolating one blob of chemistry from others right next to it.
There is however life on earth with huge cells and multiple nucleuses: https://en.wikipedia.org/wiki/Plasmodium_(life_cycle)
But also, the Viking test was more general than "cells", it would have ben positive with anything that could, to simplify, eat the food it was given.
More precisely, they are just cell membranes, not cells. Cell membranes form spontaneously from many fatty substances, the root cause being that oil and water do not mix, i.e. fatty substances are hydrophobic.
A dead cell or cell membrane does not exchange chemical substances with the environment (except passively, i.e. when molecules or ions diffuse from high concentration towards low concentration).
A living cell has not only a cell membrane that separates the interior from the exterior, but it also has molecular pumps and ionic pumps embedded in the membrane, which selectively pump some substances in and other substances out. Without this controlled exchange between interior and exterior, the growth and multiplication that define life are impossible.
What is characteristic for life are the molecular pumps and ionic pumps. The cell membranes have only the passive role of preventing the substances that have been pumped in one direction to come back to the place from where they had been taken by the pump.
But it would be really interesting, if there were other possibilities too we could think of.
A hundred years ago nobody thought something like quantum entanglement could be possible. Maybe there will be a similar eye opening surprise in extra terrestrial life.
But off course it’s reasonable to start looking for all the things we can already imagine, and make observations in a way we already know of.
We simply don't have a framework other than basic physics and chemistry to speculate in this area. The possibility space is too large and we don't know the fitness function.
That doesn’t mean your idea is wrong, but if we assume regular cold chemistry (i.e. not stellar insides or other exotic states/physics), membranes are just the sane and obvious default. Which conditions could make them less effective than something else?
I'm struggling to imagine what usable energy generation in a high entropy environment would look like.
That's what I believed too, but I have doubts now after reading another comment that defines it as energy gradients. As far as I understand the concept, a crystal is low entropy as well. Or is it the same thing? Or the combination of the two (Use energy to keep entropy low)?
> membranes are just the sane and obvious default
The less sane one could be... Magnetic confinement fields I guess?
It helps to look at how biology actually works; we get these images of things like DNA copying where the next amino acid is just sitting there, all but busting in the door to be copied. It gives a very false image of how biology works. In reality it's an insane world down there with a whole bunch of the cell just jiggling around thermally all the time, and the mechanisms all work by waiting to be bashed into by the right thing. Everything is so fast and the rate of jiggling is such that it all ends up working out. But it's nothing like anything we'd design or even very easily wrap our head around. I've sometimes wondered if we were somehow otherwise the same, but not biological, how long it would take for us to ever invent biology. We are certainly nowhere near it now.
Crystals and solid-phase in general doesn't have an obvious route to chemistry occurring anything like the way it does in biology, and indeed, it doesn't have an obvious route to chemistry occurring at all at any scale.
In science fiction, anything goes. If you want to talk about physical processes we've never even heard of, hey, go nuts. I've got my own pet theories.
If we want to stick to science, we can in fact use science to put some bounds on things. We don't have to know everything. Physical reproduction in the case you are talking about would have to be incredibly, incredible slow, plural orders of magnitude slower than Earth life. Reproduction of some energy signature might be possible, but without the ability to increase the physical substrate it can run on, that's all it would be. We actually have examples of that today; basically a laser fits the bill here, a crystal (in at least some cases) that can give a particular energy pattern the ability to reproduce. Clearly not life, though.
It actually reminds me of theories about life arising on Pluto or something. While we can't necessarily prove it is impossible, I've seen theories about using superfluid Helium as a base liquid for life, we can say it would have to be by necessity excruciatingly slow by Earth standards by the obvious ways the temperature affects chemistry speed. I can't prove a solid is completely impossible to be alive, but it is so, so much slower that it has some serious problems with its sun burning out in the metabolic equivalent of just a few thousand years of Earth metabolism.
I like both modes of thought. I do not like people mixing them willy-nilly. It wrecks them both and you very much get a worst of both worlds.
It's something that I've been very interested in for a long time, especially the idea of "scale of life" - could there be a brain the size of a solar system? How would we even understand it? What about life which operates on a different time scale (in the solar-system brain example, it would operate on a different scale on two axis - space and time).
Our lifespan also seems quite arbitrary. Some sophisticated animals live quite a bit longer than us and others live a tiny fraction of a percent as long. One would imagine that between solar systems there is a much greater degree of variance.
It would be difficult to understand the time horizons of a collectively intelligent swarm of fruit flies or a serene pod of whales, but imagine if interstellar life has a geologic component to its life cycle?
I believe, In the slim chance we encounter some kind of 'life', it wouldn't be even considered as life by our standards.
Just a simple quiz to exercise the brain: - should life move at all? - what's the maximum distance between molecules we can still consider as one entity? - can a state of matter be 'life'. Not a matter itself, but just a state of it?
The reality might be very boring. Way too boring for a good story.
Life? Don't talk to me about life.
I think the only wiggle room there is "nontrivial".
Martian isn't a broad synonym for alien or extraterrestrial. It just means life on mars
I am unaware of any understanding of evolution or early life on Earth which supports an idea of some giant contiguous life-form.
That said, Mars is not Earth, and our definition of life is ever evolving (is a virus alive? is fire alive?) so, again anything is possible, but not everything is likely.
Okay? People have no idea what pre-cellular life looked like so the entire earth being a giant cell is not out of the question.
Cells are much too complex to appear spontaneously, without evolving from an acellular form of life. Cell membranes can form spontaneously from fatty substances, but no cell can live without complex molecular pumps that are embedded in the cell membrane and which bring in some substances from outside and which expel some other substances from inside. Such pumps must be the result of a long evolution that preceded the first cells.
Nevertheless, any form of life that is not made of cells cannot live free in a fluid like water, but it must be attached to the surface of some suitable rocks, where a flow of nutrients must exist.
The most plausible hypotheses about the appearance of life on Earth suppose that before the appearance of cells, life was restricted to pores inside rocks from hydrothermal vents, where a flow of dihydrogen and of alkaline ions could provide the energy, and metallic sulfide minerals could provide the catalysts for organic syntheses.
A stray breeze blowing away chemicals would effectively kill life. A "cell" having a barrier would allow that chemical process to happen.
Life may not require cells in the way cells on Earth evolved, but they'll require cells of some sort.
But then I start to wonder if evolutionary pressure would encourage ones that form colonies and eventually closed topologies and cells. And then others might specialize to hijack those colonies...
One of the indicators of life I like to use, that seems to be useful in the general sense is that life is good at decreasing (fighting against) entropy at a local level. So fire for example increases entropy. A living being takes in raw materials plus energy, and produces a lower entropy (higher organization) of those inputs. Like plants making sugar, and pulling apart CO2 to make free oxygen. Or animals taking in sugar and oxygen, the output is higher entropy (CO2) but you have higher organization of material (lower entropy) locally within that organism.
This also means that life doesn't necessarily need to reproduce (although that is how life that we know of works). You could theoretically have a living being that kept evolving its own form but never developed the ability to copy itself. It just keeps consuming energy and environmental items, and maintaining its own internal complex structures.
If you take away my food, I will still have energy to find more. If you take away my air, I will still have air (albeit quite time-limited) to find more. That gradient is how we understand something to be alive.
I think if we go further down the logic hole and start questioning that by virtue of it being something you can start yourself with a tinderbox, that fire is not life, we question the entire premise of ideas like abiogenesis which argue life is inevitable when certain chemical conditions being met.
Not really. Everything you described is a property of chemistry and physics, not biology. Coals smolder under high residual heat and a lack of oxygen, not because of gated ion channels, osmotic barriers or any other evolved structure.
> This is akin to clonal propagation to ensure survival that we see in many species, or the release of many progeny over an area banking on the reality that most will not survive, but some will.
It's really not. Fire has no genome, nor a biological urge to reproduce. Cinders are a result of physics - pieces of organic matter fragment, heat causes rising air, some fragments are light enough to rise on hot air. If they are still hot when they land, fire can start at a new location.
Homeostasis seems to be important.
Reproduction seems important, but that's "just" a survival strategy. It doesn't necessarily seem like a requirement for something to be "alive".
I add the context part so we can say a virus is alive if the context of host cells is present, and otherwise not. Similar to how we are alive in the context of being on the Earth's surface, but not on the Sun.
Like what a gas cloud with intelligence?
We live in a universe with distinct rules, saying life doesnt have to have cells is basically fantasizing about other dimensions where we're made out of hot dogs or something. Saying life doesn't require cells is just redefining what words mean, since a cell literally means the smallest functional unit of life.
That comparison is ridiculous.
We live in a universe for which many of the rules are unclear and we have only grasped some small subset of the possibility space dor emergent structures those rules enable.
We have a hard enough time coming up with a clear definition of which of the emergent structures that we do know about qualify as "life". You seem to accept it as settled that viruses are not alive and are mere chemistry but it seems to me like drawing a line in the sand between two similar processes.
When we're talking about cells, that seems like an easier thing to define, but I would posit that is another definition that could prove to be slippery if an alien biology without a common ancestor is discovered.
We have to be careful about how we define life and cells in relation to each other to avoid just creating tautologies rather than useful definitions.
For example, how sure can we be that by defining life to exclude viruses, we haven't defined life in such a way that we have excluded processes that can support consciousness?
I think it is pretty easy to imagine life which has "cells" that we fail to recognize as such because they serve a similar function but with a sufficiently different form that we would fail to see or recognize them in a microscope.
Now, many of those imaginings may be scientifically impossible but that doesn't justify comparing all of them to "a dimension where we are made of hotdogs."
But the only life we know... is cells. Unless you count viruses. Which most biologists don't.
That is we are not trying to discover whether a thing is alive or not, we are trying to decide what Alive means.
This confusion creates the illusion that terms like Life, consciousness, emotions, reflexes, etc. are well defined while they in practice are not.
I hope it wouldn’t be a huge breakthrough for the virus
... But the kind of thing people mean by "life" colloquially is really multicellular or at least eukaryotic life, which only arose on earth like 600 million years ago, which is only like 15% of the time that life in the more strict sense (bacteria/archae) has existed here.
I don't really see much of controversy in what I said here?