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?

  • $\begingroup$ It is logically possible, however there is no hard evidence for it, and parsimony suggests against it. $\endgroup$ – Galen Jan 13 '17 at 15:27
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    $\begingroup$ @Galen I disagree that parsimony suggests against it. "Parsimony" isn't about having fewer things, it's about having fewer independent assumptions about how the world works. And life through evolution shows constant branching and extinction. Assuming this happened after but not before the LUCA isn't parsimonious. (though it would depend on where the LUCA is in the history of life and evolution to be fair) $\endgroup$ – Oosaka Apr 26 '17 at 8:30
  • $\begingroup$ @RozennKeribin I think you may be using a different usage of "parsimony" than I was. In evaluating trees via parsimony (in the context of evo-biol), we count the number of evolutionary steps required for each tree, and select the set of trees (ideally there is only one) requiring the minimum count of such steps. Adding branches at the root of the tree (before LUCA) will mean more evolutionary steps, ergo its smaller counterpart will be selected preferentially in this process. I like your usage of 'parsimony', but it may require demarcation when discussing the topic of evolution. $\endgroup$ – Galen Apr 26 '17 at 22:33
  • $\begingroup$ @RozennKeribin ... and it would have been helpful for me to demarcate the usage I was using. It is a recommendable practice in general. Although not the exact same type of example as what we're considering above, here is a pedagogical example of parsimony (in evol-biol) being used. evolution.berkeley.edu/evolibrary/article/phylogenetics_08 $\endgroup$ – Galen Apr 26 '17 at 22:43
  • $\begingroup$ @Galen that's fair, I know phylogenetics has its own use of the concept of parsimony. That said I think it's used in cases where you're building a phylogenetic tree based on actual data, traits or entities we're trying to order. I don't think this is really what the [original] question was about; the question seemed to be about whether pre-LUCA side branches could/would have existed. I think one can give an answer without satisfying the much stricter standard of building a phylogenetic tree with such branches (which requires knowing what the side branches were like) $\endgroup$ – Oosaka Apr 26 '17 at 23:42

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.

  • $\begingroup$ 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! $\endgroup$ – LanceLafontaine Oct 20 '12 at 3:04
  • $\begingroup$ I'd like a reference to rna world 3.5 years ago. what is the evidence like? $\endgroup$ – shigeta Oct 22 '12 at 3:39
  • $\begingroup$ @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. $\endgroup$ – MCM Oct 22 '12 at 3:48
  • $\begingroup$ I'm aware of the RNAWorld hypothesis and so forth - but what is the "Fossil Evidence" you refer to in your answer? $\endgroup$ – shigeta Oct 22 '12 at 4:15
  • $\begingroup$ @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. $\endgroup$ – MCM Oct 22 '12 at 4:30

LUCA was the LAST universal ancestor not the first. Naturally it competed with other extinct species. My reseach suggests that LUCA was resulted from the fusion of 2 genomes. The evidence for this is that on reconstructing the amino acid sequences for LUCA enzymes we find some contain no cystine but do contain tryptophan or the opposite.

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    $\begingroup$ Some references to support your response would be great! $\endgroup$ – Behzad Rowshanravan Oct 15 '14 at 10:48
  • $\begingroup$ Can you please add some references? Actually it is more a comment. $\endgroup$ – Chris Oct 15 '14 at 12:11

No. LUCA is a construct based on comparative analysis. Because LUCA is defined as the last universal common ancestor, no genetic evidence can be use to determine what's LUCA's ancestor looked like. The only way such a question can be answered is if by some luck, you found a fossil..or a time machine.

Given that horizontal gene transfer is all but certain in the time frames we are looking at, LUCA is unlikely to be a single organism. But more like a community of organisms, sharing a pool of genes. And a subset of these shared genes became the core of all descendants of that community. These shared genes is LUCA


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