[ExI] Discovery Suggests All Complex Life Came From

[Ben]

Me:
 > ... not convinced that all life on earth had to originate from a 
single cell, but from a /population/ of similar cells.
... Does anyone have a convincing counter-argument?


BillK:
 > Yes. Every earth species uses the same DNA proteins.  Multiple 
origins are far more unlikely than everything coming from one origin.


John Clark:
 > The evidence strongly indicates that everything alive today is 
descended from just one organism, LUCA (the Last Universal Common 
Ancestor) that lived between 3.5 and 3.8 billion years ago. But that 
doesn't mean that LUCA was the first living thing, in fact it almost 
certainly was not; it's just that lines of descent other than LUCA's 
ended up going extinct. Perhaps LUCA had some beneficial mutation, or 
more likely LUCA just got lucky.


I'm not arguing against LUCA, not at all, and I agree with what BillK 
and John Clark say.

Instead, I'm arguing against the idea that LUCA had to be a /single 
individual cell/. I'm saying that it could well have been a single 
/type/ of cell, with more than one individual. And that this seems more 
likely.

It's not beyond the bounds of possibility that a set of 
randomly-generated protocells had the same set of molecules for storing 
information, and the differences between them were negligible. This pool 
of protocells would be LUCA.

If you have a set of circumstances that generate a variety of 
mechanisms, how likely is it that there will only be one single instance 
of any one mechanism? It's impossible to say, I know, without knowing 
how big the environment is that produces those circumstances, and what 
the circumstances actually are. How many protocells existed, 3.8 bn 
years ago? We have no idea. It may have been half-a-dozen, or it might 
have been trillions. I'd imagine that a few billion might be a 
reasonable guess, though.

As an analogy, suppose there are half a million valid english words, and 
a single one of them represents LUCA. If you randomly generate, say, 2 
million valid english words, what's the probability that the LUCA word 
will only crop up once?

Without knowing just how many protocells were being generated, and the 
exact constraints of the chemistry, we'll probably never know the 
answer, I'm just saying it doesn't /have to be/ a single, solitary cell, 
and it seems unlikely that it would have been.


Ben Zaiboc