Why only one abiogenesis?
April 8, 2010 1:03 PM   Subscribe

Why is it assumed that all forms of life evolved from a single common ancestor, instead of any number of different instances of abiogenesis?
posted by shakespeherian to Science & Nature (25 answers total) 9 users marked this as a favorite
IANABiologist, but I imagine that the DNA trail, that can be traced through present-day organisms back to their earlier ancestors would be a good reason.
posted by storybored at 1:06 PM on April 8, 2010

It's possible that there were multiple instances of abiogenesis, but the evidence that all living things are related is very strong, as there are DNA sequences (such as the ribosomal RNA templates) that exist in all living things. The chance that these would have evolved the same way from multiple starting points is infinitesimal.
posted by grouse at 1:13 PM on April 8, 2010

Others with more knowledge will likely come in behind me, but things like commonality of amino acid structure, DNA and RNA bases, all contribute to the assumption/theory. It's very unlikely that we wouldn't see completely different building blocks if there were separate developmental instances.
posted by dnesan at 1:15 PM on April 8, 2010

Richard Dawkins dedicates a few pages of The God Delusion to this. I believe his main point is that it is statistically most likely for life to evolve once, but I'm sure storybored's reasoning is another part of it.
posted by i_am_a_fiesta at 1:15 PM on April 8, 2010 [1 favorite]

What you are looking for is probably phylogenetic analysis. This analyzes genes. Before DNA, there was RNA. Before RNA, probably autocatalyzing sets or something. For evidence, we all seem to be working off of the same arbitrary code, which is just statistically unlikely.

As to the assumption part, here's a good one: the first mover advantage. The first organism on scene lived in a world full of available food. It doesn't take too long to cover the oceans when you're the only one around, sucking up all of the food to make more copies of you. At this point, the copies were rather sloppy, but, yeah, it would be hard for life to arise another time after someone else had come along and eaten up everything at the picnic.

Life might have arisen more than once, only to die off completely by accident. However, after it "took off" the first time, the field would change quite a bit. You'd be competing against relatives, rather than whole new branches of life.

There was some debate about the Archae domain for a while, because, hey, those guys are freaks. But it looks like they are just distant cousins who grew up in some strange places.
posted by adipocere at 1:22 PM on April 8, 2010 [2 favorites]

I'd say the strongest case here is that basically every organism on the face of the earth uses the same 20 amino acids and very similar genetic codes. The selection of these 20 amino acids, from what is otherwise a limitless pool of possible amino acids, strongly suggests that all protein-based life (essentially all life) originated from a common ancestor.

A good analogy here is written language. Written language evolved from several different starting points, hence there are many different alphabets -- Roman, Greek, Cyrillic, Hebrew, Arabic, Chinese, etc. Imagine if every written language used the Roman Alphabet. You could still have a large number of different languages (French, English, German, etc) but it would be pretty obvious that the common alphabet was a common starting point.

It's like that with amino acids. We see just one, simple alphabet; Not the multiple alphabets that would imply multiple independent abiogenesis events.
posted by u2604ab at 1:23 PM on April 8, 2010

Also there's really no reason why instead of all L-amino acids life could be made of R-amino acids. But life as we know it uses pretty much only L-amino acids. One might think that if life had begun independently a few times, maybe one of the results would have R?
posted by battlebison at 1:24 PM on April 8, 2010

One of the other arguments I've heard is that proto-life would probably consist of concentrations of lipids, amino acids, carbohydrates, etc. If there's already life around when this proto-life forms, the extant life would find the proto-life delicious. So later instances of abiogenesis might simply have been eaten by the first instance.

Damned prokaryotes, they've got no respect for biodiversity.
posted by Johnny Assay at 1:26 PM on April 8, 2010 [1 favorite]

The phrase "genetic code" refers to a particular mapping of sequences of three "letters" (base pairs) of DNA to any of 20 particular amino acids. As far as we know, this assignment is completely arbitrary, and it's exactly the same in every single known organism. So the chances that it arose independently multiple times are pretty tiny.
posted by teraflop at 1:26 PM on April 8, 2010

The Last universal ancestor article on Wikipedia lists several biochemical features which all currently living things have in common, most of which would have no particular reason to be identical if they were products of multiple independent abiogenesis events. (Some of which are already mentioned above.)
posted by DevilsAdvocate at 1:32 PM on April 8, 2010

it's exactly the same in every single known organism

There are actually a number of genetic codes but they are very similar, so the logic of your comment holds even if the underlying facts are slightly different.
posted by grouse at 1:32 PM on April 8, 2010 [3 favorites]

It is entirley possible that life did arise more than once, and all of us DNA/RNA/protein based organisms simply won out.

Everything uses the same genetic code (with a couple of modifications) - how DNA gets read into proteins. While there is some evidence that the code is somwhat optimised, it is pretty unlikely that other life would use the exact same code. The mechanisms behind reading DNA and making the RNA/proteins are highly similar across most of life - This picture shows a comparison of protein sequences of a DNA polymerase across E. coli (bacteria), yeast, a nematode, mice and humans. You can do similar comparisons for heaps of genes and see that things are all related to varying degrees (this is how we make phylogenetic trees).

Everything uses the citric acid cycle to generate energy. Some organisms have a modified version, but it looks like the common ancestor of everything (non-viral) alive today had a version of this. There is also a whole core of shared metabolism in varying functions, which is why using E. coli and S. cerevisiae is a useful way of studying human disease and why most of our knowledge of cell division comes from yeast, not human cells.
posted by scodger at 1:41 PM on April 8, 2010 [3 favorites]

This isn't evidence, but you may want to read about the principle of Occam's Razor.
posted by needs more cowbell at 2:39 PM on April 8, 2010

The genetic code is not quite universal: it is very highly conserved, but there are a few variations.

Also, I don't think that anaerobes use the citric acid cycle to generate energy.

posted by James Scott-Brown at 3:00 PM on April 8, 2010

Even if there were multiple abiogenesis events (why not?) and the resulting life forms were all perfectly equally matched (less likely, but let's run with the idea for a bit), it's not unreasonable that only one lineage would have survived to the present day. In any scenario where you have a population of things replicating and being randomly removed/killed, over time, it becomes more and more likely that one lineage will have pushed out the rest. It's a random walk through population space, except that extinctions are irreversible.
posted by hattifattener at 3:24 PM on April 8, 2010

I just read a section on this last night in Richard Dawkins' The Greatest Show On Earth, on page 408 ff ("Into a Few Forms Or Into One"). Basically, if there had been more than one origin, there would be found some organism that either did not use DNA, or used DNA with different amino acids, or the same amino acids in different combinations and "syntax", but this has not been found. It is possible that such a thing exists undiscovered in some deep ocean rift or some other inaccessible-to-us environment, but it is safe to assume that all organisms that we know of come from the same origin.
posted by matildaben at 3:26 PM on April 8, 2010

IIRC...multiple abiogenesis is seen to be rather unlikely due to numerical analysis...i.e. the number of random chemical interactions that is required for one of them to give rise to self-replicating protiens is VERY HIGH...even with the large number of molecules in the world's oceans, this is unlikely to have occured more than once in the time between when the earth cooled enough to begin supporting life, and when life actually began to appear. in fact, (depending upon who you ask) this is pretty unlikely to even have occured ONCE. so where did life come from then? quite likely (i give it about 50/50 odds, but then again i wasn't there ;) from OUTER SPACE!!! (a very fascinating read, well worth the time...)
posted by sexyrobot at 4:50 PM on April 8, 2010

Why do you assume that it is assumed that life evolved from only one common ancestor?
posted by DU at 6:03 PM on April 8, 2010

Seconding the L vs. R chirality of the amino acids. While one could make the argument that certain amino acids (of all possible kinds) are more likely to occur naturally, or are inherently more efficient at building working proteins, there's absolutely no known reason why life should only use L-amino acids instead of R.

One thing it's worth keep in mind is that, according to most current theories on the origin of life, there was a whole lot of swapping and combining of molecular components In The Beginning. While the terms "alive" and "organism" are perfectly well-defined now, it's likely that things started out a lot more vague. There were a lot of random parts floating around (by which I mean RNA-like structures, occurring by chance), and some of these parts worked better than others, and some of them happened to combine constructively with other random parts, and eventually some combination occurred that kinda-sorta managed some degree of reproduction. But was it alive? Was it distinct from its environment? My guess is that biogenesis was less of an event and more like a prolonged process.

"As far as we know, this assignment is completely arbitrary"

I made that very same statement once right here on the blue, and someone rightfully schooled me on the degeneracy of the code, which makes it more fault-tolerant. In that sense, it's not entirely arbitrary. But your point still stands—it's extremely unlikely that a competing form of life would use the exact same code.
posted by dephlogisticated at 6:50 PM on April 8, 2010

This question has been answered very soundly, but I thought you might be interested in some historical context as well. It is commonly assumed that Darwin postulated a single common ancestor for all life, but in fact he was quite cagey about it. In the Origin of Species, he suggests that "probably all the organic beings which have ever lived on this earth have descended from some one primordial form," but, knowing that this is guesswork at best, nevertheless keeps his options open:
I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number.
And in those beautiful final lines, Darwin writes,
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
(Emphasis mine.)
posted by cirripede at 9:05 PM on April 8, 2010

There's a possibility that there are other forms of life on earth that we simply don't recognise, because they live in places we can't easily access (e.g., way underground in small crevices) and they don't look like the sort of living things we're familiar with. Or they look too much like them, and nobody has thought of trying to see if bacterium #1000xyz is actually a bacterium or is really just a small self-reproducing sac of chemicals that looks like a bacterium.
posted by Joe in Australia at 10:42 PM on April 8, 2010

there's absolutely no known reason why life should only use L-amino acids instead of R.

Somewhere I read an article, I believe by Issac Asimov, where he argued that L may have won out due to some situation involving polarized light where, if you were made of R-amino acids you would take a slightly larger hit from any UV that came your way. Small enough for it not to mater if you had managed to develop things like skin, but back in the day where everybody had a single cell it might have mattered.

I read this decades ago, so don't quote me on the technical details. And, of course, a whole lot of science has gone down the pike since Issac Asimov was doing any biochemistry (or breathing) so something he based the essay on may now be considered way off base.
posted by Kid Charlemagne at 11:00 PM on April 8, 2010

It is entirley possible that life did arise more than once, and all of us DNA/RNA/protein based organisms simply won out.

...or incorporated the other forms somewhere into our own structures.

The standard story is that life is all about DNA->RNA->protein, but that story is a gross oversimplification of what goes on inside every living thing. I can think of no inherent reason why some of what goes on inside living things might not reflect organizational forms that initially developed in disparate locations.

Consider mitochondria, for example. Every animal cell contains them, but their DNA is wholly contained within them, and is separate from the DNA inside the cell's own nucleus. That's fairly strong evidence that, at some past time, life forms ancestral to today's animals simply incorporated the ancestors of today's mitochondria in some kind of symbiotic arrangement. Was that the animal cells "winning out" over the mitochondrial cells, or vice versa? I don't think so.

It might also be the case that bilipid cell membranes initially arose independently of DNA. If that's true, which of those structures has now "won out" over the other?

The more we look at the "tree" of life, the more it looks like the result of pleaching than simple branching. I can think of no good reason to assume it has a singular root.
posted by flabdablet at 11:05 PM on April 8, 2010

Others have explained it above, but I'll add my attempt at an explanation. When we look at living things, we see that they are similar in a bunch of different ways.

Some of the similarities can be explained because there is a Best Way to do something, and different groups of organisms have independently evolved to do it that way. For example, sharks and dolphins both have similar streamlined shapes because that's the best way to move efficiently through the water. This is due to the laws of physics, so it's not surprising that two different groups of animals have ended up at the same solution independently (so have humans, if you look at the shape of boats and submarines).

Some of the similarities, however, can't be explained in this way, because they don't represent a Best Way to do things. For example, there's nothing special about the genetic code that is (overwhelmingly) shared by all extant life - it's not 'better' or 'worse' than other potential codes in the way that a streamlined body shape is definitively 'better' for swimming than an angular one. Since these types of similarities can't be explained in terms of being 'better', our best explanation for them is that all extant life shares a common ancestor.

As others have pointed out, the above argument only applies to things alive today; it doesn't tell us anything about how life originally arose, or how many times it may have arisen independently.
posted by primer_dimer at 3:00 AM on April 9, 2010 [1 favorite]

Why do you assume that it is assumed that life evolved from only one common ancestor?

This question originated after I read Jerry Coyne's Why Evolution is True. Despite being very careful to explain the reasoning behind pretty much every part of evolutionary theory (speciation, sexual selection, etc.), I noticed that he sort of glossed over the bit about common descent, despite earlier pointing out that there's no fossil record for the earliest stages of life on earth. I maybe shouldn't have used the word 'assumed,' but Occam's Razor seemed like a not-very compelling argument, and it was all that I could come up with on my own.

Thanks to everyone who answered.
posted by shakespeherian at 5:01 AM on April 9, 2010

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