Talk:Last universal common ancestor/Archives/2013/July

"However, the formal test was ambiguous with respect to the community of organisms hypothesis"

I respectfully think that the Wikipedia Community has misunderstood what the Biological Community has published. It's a minor misunderstanding, perhaps, but still a misunderstanding.

First of all, let's refer to every separate incident where organic molecules initially came together to form a cell-like structure as an "abiogenesis event." However many abiogenesis events there may have been, it's fairly safe to say that most of them quickly became extinct and contributed nothing whatsoever to the modern biosphere. However, let's say the lineages (progeny) of up to a dozen separate abiogenesis events survived beyond the Late Eoarchean into the Early Paleoarchean.

The thing to keep in mind is that there are myriad other chemically possible nucleotides besides the A, T (DNA only), G, C, and U (RNA only) monomenrs found in modern DNA and RNA, and for that matter there are myriad possible amino acids besides the standard 20 found in modern proteins. So, a microbe descendent from some other now-extinct abiogenesis event would most likely not even use the AT/GC rule or the same 20 amino acids that modern organisms do. As a result, microbes descendent from separate abiogenesis events would garble each other's genes and proteins and make them useless. (This would convert transferred genes into noncoding DNA, essentially erasing them as usable, expressable genes.) Thus, if 2 ancient bacterial cells were able to partake in horizontal gene transfer and actually use each other's genes to express proteins, it would imply that they had already shared an even older common ancestor at some point.

That being explained, the formal test favors a single LUCA over, for example, separate abiogenesis events being compatible by chance (very unlikely). Contrary to what the Article implies when it says the test was ambiguous regarding an early microbial community, I don't think anyone in relevant fields doubts that an early microbial community existed. At some point, though, there was an individual cell within that community, and while that cell was never alone, its descendants and only its descendents would survive beyond the Paleoarchean Era. That cell was a member of an early microbial community within which it struggled for survival, but it alone was LUCA in that its descendants survived. (Afterwards, there would be no shortage of horizontal transfer among LUCA's descendants.)

Quick side note: LUCA would have been an early DNA cell, just shortly post-RNA world. As far as the earlier explanation on horizontal transfer, though, it applies to the A, U, G, and C in RNA just as easily as the A, T, G, and C in DNA.

My suggestion for the Article: Let's make it clear that LUCA was a member of the early microbial community, and that while it was never alone it was the one from its time whose descendants survived. Suggesting that there's any doubt about the existence of an early microbial community, or that of LUCA, or that the 2 are mutually exclusive competing hypotheses, is rather misleading. The Mysterious El Willstro (talk) 19:32, 16 May 2013 (UTC)

Quite.Petter Bøckman (talk) 22:27, 16 May 2013 (UTC)

All right, folks

With Petter Bøckman's clear approval, and based on everything I've learned earning a B.S. in Biology, I have changed Passage A to Passage B in the Article.

Passage A:

"However, the formal test was ambiguous with respect to the community of organisms hypothesis, since it did not require that the last universal common ancestor be a single organism, but allowed it to be a population of organisms with different genotypes that lived in different places and times. The formal test was also consistent with multiple populations with independent origins gaining the ability to exchange essential genetic material effectively to become one species."

Passage B:

"While the formal test overwhelmingly favored the existence of a single LUA, this does not imply that LUA was ever alone. Instead, it was a member of the early microbial community. Given that many other nucleotides are possible besides adenine (A), thymine (T, DNA only), guanine (G), cytosine (C), and uracil (U, RNA only), it is extremely unlikely that organisms descendent from separate abiogenesis events (that is to say separate incidents where organic molecules initially came together to form cell-like structures) would be able to complete a horizontal gene transfer without garbling each other's genes, converting them into noncoding segments. Similarly, many more amino acids are chemically possible than the twenty found in modern protein molecules. These lines of chemical evidence, taken into account for the formal statistical test by Theobald (2010), point to a single cell having been LUCA in that although it was a member of the early microbial community only its descendents survived beyond the Paleoarchean Era. With a common framework in the AT/GC rule and the standard twenty amino acids, horizontal gene transfer would have been feasible and may have been very common later on among the progeny of that single cell."

I hope you all appreciate this clarification! This is what the formal test favored. As Saey (2010) noted, a single LUCA with subsequent horizontal transfer among its descendents is anywhere from 10²⁸⁶⁰ to 10³⁴⁸⁹ times more likely than separate abiogenesis events coincidentally having the same nucleotide and amino acid framework so as to be able to partake in horizontal transfer without turning each other's genes into jibberish. The statistical test by Theobald (2010) reaffirms that statistic. The Mysterious El Willstro (talk) 18:42, 1 July 2013 (UTC)

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