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This question got me thinking about something. LUCA is the last universal common ancestor of all current living organisms, which is a very different definition from the first-ever living organism. Is there any evidence that LUCA had evolved and diverged from a now-extinct, more primitive lifeform?

To rephrase, is there any evidence to suggest that at some point, the first-ever living organism(s) had diverged into two phylogenetic groups, one being now-extinct and the other describing all current life?

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1 Answer 1

Well, what you seem to be suggesting is "Did life evolve twice on Earth?"

Your original question has an answer: Probably yes. It's not unlikely to think that the original cell evolved into two different paths and then one went extinct. However, that doesn't address LUCA. If we found fossil evidence of what we thought was LUCA, and then fossil evidence that LUCA had a genetic cousin - all that would do is push the application of the term "LUCA" on more evolutionary step backwards until both shared a common ancestor which would then be called LUCA. You can do this indefinitely until all life originates from a single cell, with countless offshoots which have gone extinct.

If you mean was there a whole other type of life - one that did not originate from LUCA and existed - then the best answer we have right now is "No." All life we know of, no matter how different and old, is still based on RNA/DNA and proteins. Fossil evidence supports this premise until about 3.5 BYA.

If there was an 'alternate' construction of life, we have not found the fossil evidence for it, and might not know what it was if we did. If the alternate form didn't utilize cells, we might not be aware we had it. If it did, but utilized different metabolisms or structures for proteins and storage - that evidence would be long gone by now.

Our best bet for answering if life can evolve differently than what we have today is out among the stars.

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I actually wasn't referring to "a whole other type of life" as you describe, or if abiogenesis had happened twice on earth, but the first paragraph (not counting the first sentence) is exactly what I was looking for, and now seems all-too-obvious in retrospect. As you suggest, if we knew of evidence which described this early now-extinct lifeform, we would have to revise our definition of LUCA. Thanks for the epiphany! –  LanceLafontaine Oct 20 '12 at 3:04
I'd like a reference to rna world 3.5 years ago. what is the evidence like? –  shigeta Oct 22 '12 at 3:39
@shigeta - RNA acts as both storage of genetic information, and can fold into proteins/enzymes (Ribosomes utilize rRNA, for instance). DNA has never been observed to naturally act as an enzyme, indicating a specialized role (and it is orders of magnitude more stable than RNA). So RNA is thought to occupy the area between Proteins and DNA, and since proteins have not been observed storing genetic information, RNA is hypothesized to be the first form of genetic storage, enabling Evolution via Natural Selection. –  MCM Oct 22 '12 at 3:48
I'm aware of the RNAWorld hypothesis and so forth - but what is the "Fossil Evidence" you refer to in your answer? –  shigeta Oct 22 '12 at 4:15
@shigeta - www.paleosoc.org/Oldest_Fossil.pdf Cyanobacteria, which are an extant species, have been traced to about 3.5 BYA. Recently (sciencedaily.com/releases/2011/08/110821205241.htm) some fossils were found of sulfur using bacteria with a very confident date of 3.4 BYA. The latest one is probably the most sophisticated, as it utilizes chemical analysis to look for organic components that wouldn't be found in natural mineral formations. –  MCM Oct 22 '12 at 4:30

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