"All living things share a common ancestor"

That all living things share a common ancestor is something I see repeated over and over as I read and watch various pieces about evolution. The implicit assumption that appears to be present here, but one that I have never seen explicitly addressed, is that if all living things share a common ancestor, then we must be sure that abiogenesis has happened only once.

Why do we believe this to be the case?

Couldn’t it be possible that, for example, plants and animals don’t share a common ancestor?

There are so many similarities at the molecular level, things that could easily have been different, that it is inconceivable that they didn’t have a common ancestor. This includes basic biochemistry, but also DNA and protein sequences. We can even reconstruct how things are related–with some uncertainties–on the basis of sequences.

a god(s) could could create anything he/she/it wants including a single origin of matter and let the laws of nature roll or to creating individual life forms that look related when they aren’t.

a strong suggestion of common ancestry is a whole lot of biochemistry that is common to living plants and animals. it is possible that some of that could have independent origins from non-divine causes though it would probably be a vanishing small chance.

Sure, but that’s completely outside the bounds of science and is not useful in addressing the OP.

No, the universality of the genetic code merely tells us that the living product of one abiogenesis event became totally dominant.
Life could have arisen umpteen jillion times, but so far as anyone can tell, only one such event produced organisms whose offspring are alive today.

Is the argument that all (or all to the point of being statistically insignificant) life forms that have ever existed took on all their current form and function based solely on some mutation that gave them a survival advantage? AND that mutation survived into several generations of mutation without being lessened AND spread throughout the entire population of that organism… and that’s why we have sharks and giraffes and crabgrass and birds?

What sort of odds are we looking at here?

it context for my full posting.

Though there are those that claim that this apparent universality might be an artefact of just not looking far enough, or not looking for the right thing – there’s quite a lot of microbes out there, the overwhelming majority of which probably have never been examined with respect to their exact biochemical makeup, mainly because most of them aren’t even known at this point. So it’s really just the case that, while we haven’t got any conclusive evidence for a second, independent abiogenesis, there’s still some investigation yet to be performed before we could even approach calling our search ‘exhaustive’.

I’ve also heard the claim that nanobes, being too small to contain the usual DNA-replication mechanisms, might be an example of life on a different basis, or perhaps a remnant from an earlier RNA-world – if they are appropriately called ‘life’ at all, that is.


Could you clarify your point then. I don’t understand what you’re getting at.

There are a couple of lineages I’ve read about (quite a while ago) and can’t remember if they were given as examples of completely off-the-line branches or if they were just branches that aren’t related to anything that’s CURRENTLY living. Were the Burgess Shale creatures in the line of descent that all animals are descendants of? What about the Ediacarian fauna?

Yes, you have to turn over all of the rocks to be sure that there isn’t a cabbage under one of them, but we’ve turned over a lot of rocks without finding anything remotely cabbagey.

The Burgess Shale finds were oringally thought to include many lines that were dead ends and totally unlike anything we knew. Further investigation has shown that this was wildly overstated and that most if not all were related to modern lines.

Exactly. In fact, the advantages of a head start make it likely that once life has arisen and proliferated, any completely new genesis of life would be easily outcompeted or simply eaten in its first feeble stages by the already-established biome. So odds are good that we’d see the same life we see now whether or not it arose only once or a jillion times. Early bird gets the earth.

I don’t think you quite grasp how odds work. You can’t take something that exists and calculate odds against its existence.

You can make odds for or against something that might happen in the future, but the thing about the past is that it is the past, and since it did happen, obviously the odds against it happening were worthless.

What are the odds against something that has already happened happening? Put it this way; any bookmaker would be delighted to take that bet. What are the odds that last year’s football season didn’t end the way it did? None at all. Absolutely zero.

If you really want an answer as to how life on this planet could have just happened, without any divine influence, I’ll put it another way for you: in an infinite universe, quite literally everything has either already happened or will happen in the future. Therefore the odds against anything being possible are so small as to be completely negligible. The odds against what has already happened (life on planet Earth) are zero.

I sincerely hope that helps, but I also sincerely doubt it was the answer you were looking for.

No, the argument is that all life forms that have ever existed took on their current form and function primarily as a result of numerous mutations, down through their respective lineages, that conferred selective advantages. AND that the resulting traits were passed down from generation to generation in each of those lineages without being lessened (because, you know, that’s how adaptation works; those selectively-advantageous mutations become more frequent throughout a population until they become “fixed” - i.e., the allele frequency in the population is 100%). And that’s why we have sharks and giraffes and crabgrass and birds.

Abiogenesis could have happened multiple times; we are simply the descendants of the population which “won”. Further, each abiogenesis event would likely not have produced only a single individual, but rather a population of similar proto-organisms.

Considering the full range of diversity on Earth, multicellular plants and animals (and fungi) are actually quite closely related. The greatest biochemical divergences are found among the bacteria and between them and other single-celled organisms called Archaea. But all organisms we know of share the same basic genetic code.

It’s also possible that earth life is the product of an amalgamation of different types of early life. So there might have been RNA protolife and membrane protolife, and either the RNA life learned to parasitise the membrane life, or the membrane life captured the RNA life. And there might have been other sorts of self-organizing systems that were incorporated into the first bacterial cells.

And we have to remember that for the first few billion years on earth, nothing existed except bacteria. Or maybe it would be better to say bacteria-like life. Because it is possible that there were several sorts of life that arose independently on Earth. But this seems unlikely, because one sort of life would probably have an advantage over the other sorts, and would spread over the entire planet and eat and destroy the other sorts. And as has been pointed out, while it’s possible some sort of life that seems unrelated to all other Earth life might be discovered tomorrow, we have never discovered any organisms that don’t share the same Earth-type biochemistry.

But if we did discover an organism that was radically different from all Earth life, it might be a relic of an independent biogenesis on Earth, or it might have been introduced from some other biosphere. And for all we know, Earth life might not have originated on Earth. If we find organisms on Mars or Europa, we’ll either find that they share Earth biochemistry which means that life can travel between planets, or they’ll have an independent biochemistry. Or we’ll find nothing.

But the funny thing is that life on Earth arose very very quickly after the Earth cooled enough to have liquid water. So either abiogenesis is a very likely outcome anywhere there’s liquid water, or colonization from extraterrestrial sources is a very likely outcome anywhere there’s liquid water, or Earth is somehow atypical. But we have no reason to believe Earth is atypical, other than possessing lots of liquid water, and so the first two possibilities are the most likely.

Have we actually turned over a significant number of rocks yet, though? I honestly don’t know jack about this (i.e. what percentage of microbial organisms thought to exist are actually known, and of how many of those we know the biochemical mechanisms – I recall reading an article making the case that, since laboratory conditions typically are geared towards life with a ‘conventional’ biochemistry (nutrient solutions etc.), it might be relatively easy to miss what we’re not really looking for, especially since there might not be much of a difference in outward appearance).

Edit: Here’s the article I was thinking about.

You are not going back far enough. Many Burgess fauna members do not belong to extant phyla – but all (except sponges) are metazoan animals. We don’t know a lot about the Ediacara fauna, but it seems likely that they share common ancestry with metazoans, parazoans (sponges and extinct relatives), and animal-form protists.

But, and this is most important, all living things except bacteria, archaeans, and cyanophytes (“blue-green algae”) share the eukaryotic cell, which is a highly complex and standardized organic structure, almost certainly something that evolved only once.

I think the same thing is true of prokaryotic cells, the building blocks of bacteria, archaeans, and cyanophytes. But my knowledge of paleocytology is incomplete.

The best example is the genetic code. Our DNA is written in four “letters”, A, C, T, and G. These letters are organized into three-letter “words” called codons, so there are therefore 64 different codons. Each of these codons corresponds to a single one of the 20 amino acids (some aminos have more than one codon coding for them) or to the “stop codon” (which, in this language analogy, can be considered a punctuation mark), and a “sentence” of codons corresponds to a particular chain of amino acids, or protein. The blueprint for which codon corresponds to which amino acid is called the genetic code.

Now, I lack the combinatoric chops to calculate exactly how many different possible genetic codes there could be, but it’s somewhere in the vicinity of 21^64, a fantastically huge number (too large to even be called astronomically large). And yet, out of all of those possible genetic codes, every single life form we’ve ever examined has used the exact same one. That’s far too long a longshot to be coincidence, so there has to be some reason why they’re all the same, and the only explanation anyone’s come up with that makes sense is that all life is descended from some single species that used that code.