Re: Last universal but not universal common ancestor of life

On 6/6/2025 5:09 PM, JTEM wrote:

 Maybe it's just badly written, but it's as near to
gibberish as anything I've ever seen from this
source:
 https://www.popularmechanics.com/science/a64969200/amino-acids-origin- of-life-order/?utm_source=social&utm_medium=copy&utm_campaign=action_bar
 First off, by "Universal common ancestor" they just mean
the "Common ancestor for life on earth."
 The popular belief is "Common Descent," as you should be
aware, and this implies (or requires) all of life to trace
back to a single point, a single ancestor.
 "Universal Ancestor" would be the ancestor of all life in
the universe, which if this piece is talking about then it
is entirely rubbish...
 First off, I do NOT believe there ever was a single source
for life on earth. This would imply that there's only a
single way for abiogenesis to occur, AND that it only could
happen once.
 Fact is, it could have happened dozens of times. I mean,
one defining characteristic of science is repeatability. So
if conditions are right, it should happen. Period. And if
conditions are not right, it should not happen. Period.
 There is a way around this, yes. You can say that Panspermia
is the right answer, that life emerged however soon after the
Big Bang it could come about, and from there rained down on
worlds... which brings us to ANOTHER complaint of mine.
 And here I do quote:
 Our understanding of these extremely ancient times will always
be incomplete, but it’s important for us to keep researching
early Earth.
 My issue here is that it's not really about time. It's not
about time at all.
 If anything rained down on the earth from space, undoubtedly
more is this magic pixie dust still exists. We can find it.
We can study it.
 The conditions of the early earth can be determined to a
greater and greater extant. We'll probably find models in
other worlds.
 I guess what I'm saying is that it's not about time it's
about conditions. Right now we have ZERO clue under which
conditions life will form, nor even if it can spontaneously
form. If we can answer these questions we'll have everything
we need to reconstruct the origins of life on earth.

https://www.pnas.org/doi/epub/10.1073/pnas.2410311121
The last universal common ancestor discussed in the PNAS article that is discussed in the Popular Mechanics article is just the last common ancestor (LUCA) of all extant life on earth.  This would be the common ancestor of archaea and eubacteria.  The PNAS article thinks that they can acquire information about life that existed before LUCA by looking at the amino acids used in protein domains that would have existed in pre LUCA lifeforms.  They tried to figure out the amino acid composition of the protein domains that have likely existed since the first proteins were evolving to be functional.  The initial genetic code would have had to make these ancient protein domains.  They claim to have estimated the frequency of use of each of the 20 amino acids before and after LUCA.
They think that the order that the amino acids were added to the genetic code needs to be reevaluated, and should reflect pre LUCA rather than post LUCA amino acid use.  Probably the main issue with their assumptions is that these protein domains have been conserved because they are the best sequences for the job that they do.  They make very stable alpha helices and beta pleated sheets that fold up into the same 3D structure with a high degree of reproducibility.  The then existing genetic code would have likely been designating for more than the amino acids that turned out to be the best for particular jobs.  What they might be observing would have had to have been selection post development of a genetic code to encode these particular amino acids more accurately.
As a side note, the article was edited by Doolittle.  So Doolittle is still alive and kicking, and still a member of the National Academy.  TO ancients may recall Doolittle as one of the early Science side combatants in the scientific creationist fiasco that resulted in the creation of talk.origins.
Ron Okimoto