Is all life on earth descended from a single cell?
Is all life on earth descended from a single cell?
Cells divide and make new cells, is all life on Earth rooted in one super ancestor cell? Or are there parallel paths to cell creation?
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We haven't even fully grasped the system of self-propagating chemical reactions that increasingly met more criteria of "life" before a single cell was formed. Everything suggests that the process is super rare so it's safe to assume it only happened successfully once. Tectonic plate movement has erased geologic evidence of conditions around the origin of life but given that all known forms of life share the same basic elements (H, C, N, O) and reactions, it is harder to argue against a single origin than for it.
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so it's safe to assume it only happened successfully once
It's safe to assume that it has happened successfully at least once.
The universe is vast and the cosmos even moreso, and while we may indeed be the only form of life in this universe that can wonder about and traverse space, we know with 100% certainty that this universe has the characteristics to produce life.
We cannot be certain or safely assume that it's only happened once. The amount of space we've explored even with our telescopes is about the same amount as a cup of ocean water versus the ocean.
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I think all we can assume is that life as we know it became dominant, either by sheer luck or by outcompeting other variants. We can’t reasonably discard that life happened more than once here.
It might seem like an astronomically rare event because that’s the only kind of life we know.
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It's not only the same elements, but the fact that all forms of life incorporate self-replicating DNA molecules.
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Probably maybe, maybe not?
It depends where you out the boundaries of life. The unfortunate answer is that this may not be a scientific question, or at least not a question the scientific method can answer because we just won't be able to get evidence from the period if time where we could answer this question.
But a few things to think about along the way. There are two big considerations when it comes to the evolution of cells, and what purpose they serve. The first is an issue around concentration and the second is on gradients. Firstly, one of the biggest issues you have around creating the conditions for the evolution of life is concentrating enough material, in an area to have meaningful concentration density for life processes to occur. This is one of the main arguments against life starting in the oceans. You would need massive concentrations of basic chemical ingredients to get any kind of meaningful interactions because of this. The second point is around gradients. Almost all life processes are modulated through some kind of a barrier, usually a cell membrane, where a bunch of stuff is on one side of the barrier, and by either moving through or interacting across the barrier, "work" happens, that work being the stuff of life. You need a barrier to set up some kind of a concentration gradient separating compound A from compound B.
Cells as phospholipid bilayers, accomplish both of these things. It creates an opportunity to keep things concentrated and it also gives an opportunity to create gradients.
Life is a steady process held out of thermodynamic equilibrium with its environment. There are many such systems that are not living systems that have this same property. It's in these systems that complexity can arise.
It could be that in the proto biological phase of earth, there might have been a system where cell like structures could have emerged through a simple chemical, thermodynamic process. In such a system, so long as there is material and energetic flow, it might be that these kinds of concentration gradients naturally arise.
Now, that's more on the speculative evolutionary biology side.
On the "what evidence we do have" side, among bacteria and phages, we see a far more significant diversity in cell membrane or cell wall type materials than we see among eukaryotic life. If you put your chalk line down at eukaryotes, almost assuredly there was one common ancestor. This is just accepted biological theory at this point.
However, there is far more diversity when you get into bacteria, archea, and phages when it comes to the molecular construction of how they keep the inside bits from the outside bits separated. But it's not clear if these point to a LUCA or not.
To summarize. It's pretty hard to imagine life without cells because it's almost impossible to create the kinds of concentration of materials necessary to do so, and we basically define life as the stuff which happens across one a gradient, which we don't see happening outside of the concept of cells. If might we'll be that cells evolved first, through some arbitrary natural process, and life processes emerged across a simple system where some chemistry could become concentrated and barriers existed to create gradients.We have some evidence that there are a wide range of materials used for cell membrane, however, this doesn't them out a LUCA.
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That’s an open question, so nobody knows the right answer. We weren’t there to witness it happen, and any evidence we do have is very indirect at best.
This leads to many possibilities, and it’s difficult to figure out how likely they are.
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We weren’t there to witness it happen, and any evidence we do have is very indirect at best.
Homicide detectives don't need to have been there to figure out whodunit. There's tons of "indirect evidence" to help biologists put the pieces together, and while we may not yet have a single conclusive or concise answer, the fact is that some hypotheses have been elevated to theories, and some theories have risen above and proved to be stronger than others.
Not that long ago, people thought humans were put here, fully formed, by gods. We gathered evidence which led to the conclusion that that scenario is factually untrue. Eventually, we may do the same for figuring out the exact origins of life on Earth (and there's some good theories).
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Homicide detectives have lots of evidence to work with, and they’re all reasonably fresh.
The case of Ötzi (the iceman) is really tricky to solve, since there’s very little evidence to work with. We know what happened to him, but everything else about the case is a subject of much speculation. I would argue that solving the origin of life is even harder to solve.
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We were all, at some point, just some really spicy clay.
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No, at least not in the way religious people might want you to think. There were likely multiple single-celled organisms that became the early ancestors to the various life on Earth.
So in that way, we all have a common ancestor, but it probably wasn't one cell in particular, just like all apes (including humans) and all modern monkeys didn't come from one single old-world monkey.
The reason this is plausible is because convergent evolution has been observed countless times throughout nature. Bats have wings, insects have wings, birds have wings; fish have scales, snakes have scales, pangolins have scales. And so, a single cell from way back when only needs to have been able to survive, not necessarily have all the exact same adaptations as other cells. As they mixed and shared DNA and split, some of them would have coalesced into our early ancestors, some would have become ancestors of plants, some would have become ancestors for fungi, etc.
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Yes, most likely, all organisms on earth originate from the Last Universal Common Ancestor (LUCA).
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☝️🤓I mean, it isn't "most likely".
Unless there are multiple independent sources of life then, by definition, the last universal common ancestor is the thing that we all originated from
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