Multilple origins of life on earth

[FrankJ777]

From what I understand, all life on Earth is descendent from a common ancestor, which was probably a self replicating molecule that spontaneously formed billions of years ago. As I think I understand it this single molecule self replicated, and succesive generations of the replicated molecules mutated, etc. forming all of life today, and there for all living things are related. I've heared it hypothisised that this process might be common on plannets that are conducive to life. So, I've been wondering if there is any evidence that this process might have occurred several times on earth. Is there any evidence that there are organisms on Earth that don't trace there lineage to the same origin as us? Should we expect that the process that started life to have happened more than a single time? Also would there be a way to tell if an organism was spawned from a separate lineage?

 

[phinds]

I expect that evidence of multiple origins of life on Earth would be big news and I have never heard of such a thing. On the other hand, I'm not sure it's clear that it can be excluded. It's an interesting question.

 

[Borek]

The idea isn't new, I believe I read about it in one of Hoimar von Ditfurth's books back in seventies. Version he was describing was that the life probably started several times but it was also several times wiped out by cosmic scale cataclysms (early Solar system being full of planetoids bombarding planets). The latest approach was lucky enough to survive and eventually becoming what we know.

But I was always under impression it is a speculation without evidence.

 

[atyy]

Present evidence is consistent with universal common ancestry - which does not rule out multiple origins of life.

As an example, Theobald's "A formal test of the theory of universal common ancestry", published in Nature a few years ago (available at http://theobald.brandeis.edu/publications.html) states

"The theory of UCA allows for the possibility of multiple independent origins of life. If life began multiple times, UCA requires a ‘bottleneck’ in evolution in which descendants of only one of the independent origins have survived exclusively until the present (and the rest have become extinct), or, multiple populations with independent, separate origins convergently gained the ability to exchange essential genetic material (in effect, to become one species). ... Furthermore, UCA does not demand that the last universal common ancestor was a single organism, in accord with the traditional evolutionary view that common ancestors of species are groups, not individuals. Rather, the last universal common ancestor may have comprised a population of organisms with different genotypes that lived in different places at different times."

Carl Woese's essay "On the evolution of cells" is an interesting read (but keep in mind Theobald's later work when evaluating the details of Woese's proposal). http://www.pnas.org/content/99/13/8742.long

 

[Pythagorean]

It really depends on what you call the "origin of life". At what point at which an aggregate of interacting molecules becomes considered living? I believe the RNA world hypothesis indicates that three major cell types occurred independently and separated in time. We only have "record" of the DNA line, really, which represents a very stable form of coding for life. Molecular aggregates based on less stable coding (i.e. ribozymes) might still be considered living depending on where you draw the line.

 

[phyzguy]

The explanation that I have heard for why there weren't multiple origins of life on Earth is that, once life had successfully evolved once, any more primitive life that attempted to evolve would be quickly eaten.