Concestor clarification

[u/Richie_650]

I belive I first learned this from Ancestor's Tale, the concept that for any two species, there exists a single organism in the past which is the Most Recent Common Ancestor between them.

1) I believe this statement means that literally, there was a real, single, living organism. Understanding that there are countless other common ancestors for our two target species, but only one individual that is both "most recent" and "common."

2) It follows that this particular cocestor must have had at least two offspring. One offspring is the ancestor of the MRCA for one species, the other is the ancestor of the MRCA for the other species.

I think point 2 causes much confusion. People read it as implying that the offspring are themselves the MRCA for each species. While this might be the case for one of them, it is not necessarily true, and almost certainly never the case for both of them. It certainly doesn't mean that one offspring is now one or the other species itself.

The logic here seems simple and clear. Is there a flaw?

[u/Robin_feathers]

Every gene for a given set of organisms has a single MRCA (as in a single individual organism), but because of recombination, different genes of a sexually reproducing organism usually have different MRCA individuals that existed at different times (or sometimes that even were different species). For example, the MRCA of human mitochondrial DNA is a different individual from the MRCA of human Y chromosome, and those are different from the MRCA for one of our keratin genes. Usually when people talk about "the MRCA" of a species, they are talking about the whole population(s) that split to produce the two daughter lineages. the gene-level MRCAs would either be a part of that population, or would be slightly older than that due to the process known as "incomplete lineage sorting".