Why didn't LUCA go extinct?

Otangelo Grasso, formerly of this parish, is asking at Uncommon Descent:

Let’s suppose there was a first Last Universal Common Ancestor (LUCA) or a small population of it. How did it overcome deleterious harmful mutations, in order not to go extinct?

The choice of venue is noteworthy - why ask such a question in a forum where most of the frequenters do not believe there even was a universal common ancestor? Mr Grasso appears to be cheerleading, rather than actually seeking an answer.

The obvious answer to Mt Grasso’s question may be: the same way organisms today overcome deleterious harmful mutations and don’t go extinct.

There’s also some confusion in the question. What exactly is a “first last” UCA? Logically there can only be one “Last” UCA, and if there is more than one UCA (which if there are any UCAs there undoubtedly are), the first one won’t be the last one. Mr Grasso may as well be asking for the tallest shortest horse. His question makes no sense. Perhaps that’s why he asked it at UD - such confusion would be unremarkable there, where many are culled and few are cogent.


P.S. The highest lowest point is in Lesotho, and the lowest highest point is in Tuvalu.

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IIRC, the time between the origin of life to LUCA was about 500 million years. IOW, LUCA was already a battle-hardened veteran of the natural selection wars. He wasn’t going to be scared off by a few deleterious mutations, not him.

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I’m fairly sure LUCA dates from 2021…

In addition to all the other problems, this seems to be a Texas Sharpshooter fallacy. There were likely many, many different early life forms, but only one of them was the last universal common ancestor. What’s the chance of LUCA surviving? We don’t know. What’s the chance of one out of all the early life forms surviving to become LUCA? We also don’t know, but presumably much higher.

Putting this in terms of human genealogy, there was a point before which every human either left no extant descendants or was the genealogical ancestor of all modern humans. What is the chance that the lineage of the human MRCA survived to the modern day? Out of every other person on earth at the time, the probability of that one person’s lineage surviving would be miniscule, approaching zero. But what is the chance that at least one person’s lineage survived to the modern day? One, since humans exist today.


At the microbial level especially, it seems rather meaningless to point out a single cell and say that it is that cell that is the Last Common Ancestor of any vast, descendant lineage of cells. Given that descendant cells from that putative LCA would be picking up DNA from sources outside of the expanding lineage that one is following. Put that way, LUCA still can still serve as an interesting point of conversation, but I’m rather doubting there ever was such a thing.

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Hi Andrew
This is an argument about RMNS being the mechanism. If God drove the process then the chance problems go away as now you have a deterministic mechanism. Can life survive without DNA repair? Did God engineer that mechanism? The strength is O’s argument is how did we get DNA repair by RMNS?

Is it? What about drift? It’s not RMNS.

What chance problems?

Depends on whether the life in question is competing with other life that has it, prolly.

Do you think carefully before writing these questions?

Why would an omnipotent God need to engineer anything?

How is a question a strength of an argument?


You give too much importance to the randomness of mutations.

In principle, you could have a deterministic system where each possible mutation is tested in a systematic coordinated way. This would be hard to implement, because it would require central coordination of all mutations. But if we could do that, the results we would see would not be much different from what we are currently seeing. Determinism vs. randomness is really not important, except that random mutations makes for a simpler implementation.


It’s not an argument about any mechanism. It’s showing how fallacious Otangelo’s argument is.


That question is not relevant to LUCA, since LUCA had DNA repair systems. This is a confusion you appear to share with Otangelo.


Hi Neil

Without a random initial change as the mechanism probability arguments are not very interesting. If I am not mistaken O is arguing against mechanisms which initiate with random change.

I disagree with that. The environment is still subject to apparently random change (earthquakes, volcanoes, meteor strikes, tsunamis, weather extremes. Even if mutations were deterministic, the use of trial and error methods to adapt to changes in the environment would lead to apparently random drift.


Look, regardless of what the mechanism was, Otangelo’s argument is severely flawed. It’s a Texas Sharpshooter fallacy. As I wrote above, it’s like saying that the chance of humans existing today is miniscule, because the chance of the human MRCA’s lineage (out of every other lineage) surviving to the modern day is almost zero.


Hi Andrew
I don’t think your human analogy works for an ancient single celled organism. Different mutation rates due to fast generation times with immature repair mechanisms if any, small populations (his assumption) and more time.

Again, it’s not about any mechanism. It’s simply a fallacious argument.


Hi Andrew
If you read his article carefully you will see it is about a mechanism. The mechanism is the random change of a sequence (DNA) and then the effect that of mutation either beneficial, neutral or deleterious. The clue is in the papers he cited at the bottom. The argument is that the majority of the mutations will be deleterious thus breaking down the functionality of the sequence. This is part of the sequence problem.

Otangelo’s argument might be about a mechanism. But all I’m saying is that the type of reasoning he presents is fallacious, no matter what the mechanism is.

I read it carefully, and the supposed point of the article is completely independent of the mechanism. It would apply equally badly to any mechanism, whether DNA, RNA, some other genetic material or some other form of mutation and inheritance.

The issues with Otangelo’s article are nothing to do with mechanism, but to do with faulty reasoning and contradictory terminology.

Please take your irrelevancies elsewhere.

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The claim is that the majority of the mutations will be deleterious thus breaking down the functionality of the sequence.


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