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Mononoke wrote:To the best of my knowledge there is considerable evidence to suggest that all life on this planet can be traced to a common ancestor. Are there any credible alternative hypothesis.

None that explain the high degree of congruence of living genetic codes, no. There is perhaps some latitude in arguing about whether there was a single cell from which we all descend ... I tend towards the view that there was. In essence, every DNA base in every organism is there as a result of copying (or miscopying) from a prior DNA (or RNA) template (the only exception, AFAIK, being telomeric repeats). The normal route of base-ancestry is parentage - a chromosome is replicated in its entirety and the two copies passed into two new cells. (In the case of sexual species, two chromosomes are merged into one, and half thrown away). If you follow such parental copy-chains backwards - regardless of whether you are following asexual or sexual lineages - you will get coalescence: the further back you go, the fewer individuals from ancestral populations will be represented in the tree-of-ancestors. This is simply the result of survival at every level being a sampling process, with much the same dynamic as fixation of alleles by selection/drift. Absent HGT, the inevitable result of this is coalescence upon a single individual, just as allele fixation is coalescence upon a single ancestral sequence. If we had two distinct genetic systems, we would get two UCAs. But we don't.

The role of HGT is to crown a different individual, ancestral to the 'mainline' LUCA, as the ultimate ancestor of every DNA base in the modern population. Our 'mainline' LUCA was a member of a broader population, and if HGT occurred between a descendant of her contemporaries and one of her descendants, then we would look to the LCA of those two organisms as the common ancestor of all DNA. If there were other HGT events, we would need to gather these threads together also. The "DNA LUCA" is the ancestor of 'mainline LUCA' and all contributors of subsequent HGT. The same applies to endosymbiosis - the archaeon and bacterium and blue-green alga that gave rise to eukaryotic cells had a common ancestor.

This does not preclude separate origins for Life, but only one variety has left descendants. The commonality of the genetic system (DNA-RNA-protein) suggests it is unlikely that we have received some bases by a copy-chain pointing back to a source independent of 'DNA LUCA''s own coalescent ancestors, unless it too hit upon DNA independently, including one or more of 'our' 4 bases. 'mainline LUCA' and 'DNA LUCA' are both, I think, singularities.

Um... where is the fossil or other real evidence of the UCA?

I say real, because of the GIGO rule.

Is it merely a theoretical construct, or an actual organism?

asyncritus wrote:Um... where is the fossil or other real evidence of the UCA?

I say real, because of the GIGO rule.

Is it merely a theoretical construct, or an actual organism?

What's "real evidence" and how does it relate to the GIGO rule?
What's the difference between a "theoretical construct" and an "actual organism" ?

How does skeletal fossil differ from a molecular fossil? When does a skeletal fossil stop being a skeletal fossil and become a theoretical construct? In what way does a skeletal fossil imply an "actual organism" where a molecular fossil does not?

Please define your terms.

asyncritus wrote:Um... where is the fossil or other real evidence of the UCA?

I say real, because of the GIGO rule.

Is it merely a theoretical construct, or an actual organism?

The theoretical construct is that it was an actual organism. There is another theoretical construct that would have it as a set of organisms exchanging genes. Like the Big Bang, it is inferred from observation and theory and extrapolation - for any current population, fewer and fewer individuals in ancestral populations are represented as ancestors as we go back in time. Extending this to the 'population of populations' - all life on earth - the logical conclusion is coalescence upon a single individual. Her contemporaries left no descendants to the present; she left loads.

A fossil? Why yes, I have it right here. As you would understand, it is a bit of a prize due to its rarity value. You can have a peek for a thousand quid!

In some sense, and in a more than metaphorical one, we are all "her" fossils: that is, the genes of living creatures are many-times-removed copies of the LCA's genetic material.

Though, of course, imperfect copying, possible substitution (or "takeover") of one genetic material for another, and lateral genetic borrowing have so overwritten the record that any sort of precise "reconstruction" of that far-removed LCA may present insuperable difficulties.

Though I wouldn't like to bet on that...

But that doesn't mean "approximate" reconstructions -- ones at least bounded and constrained by the evidence and physical principles -- can't be usefully approached.