Universal common ancestry

[jimbob]

http://www.planet-science.com/about_sy/events/pdfs/Steve_Jones_Transcript.pdf

It's basically due to the difference in reproductive systems. Men can spread their sperm around all over the place, which tends to result in a few powerful men having lots of children, while the less powerful have significantly fewer. That's not possible for women because of the whole pregnancy thing, so the distribution of children tends to be more spread out across many women.

If you look at this diagram (from here), it shows how the idea of the most recent male or female ancestor can be constructed. That picture shows for women, but if you do the same for men, there will tend to be more than just two or three children at each point, and so it will converge back to the common ancestor much more quickly.

The important point to remember is that this does not say that there were no other people alive at the time, or that this is the ultimate ancestor, both of which are obviously inconsistent with the male and female one occurring at different times. Instead, it is simply the most recent ancestor that all men or women share. Lots of people will also share many ancestors from both earlier and later, and some of the earlier ones will also be common to everyone.

The bit being missed in the question is that it it is last demonstrated common ancestor.

This is shown by looking at y-chromosomes for men, which are only carried by men, and by mitochondrial DNA which comes only from the mother.

None of my daughters will carry my Y-chromosome. My son will carry his mother's mitochondrial DNA, but he won't pass it on.

 

[Roboramma]

A female has every ancestor that her father has regardless of what genes she inherits.

If the sentence I originally quoted was trying to make the claim that every man alive today possess a gene or genes that he inherited from a common ancestor, it failed to articulate that. It simply says they have a common male ancestor. I'm just pointing out that if every man alive today has one male ancestor in common, then every female alive today must also have that same ancestor in common, even if the women didn't inherit the genes used to demonstrate this common ancestor in men.

I second this, and I think it's obviously necessarily true. It's also worth pointing out that "mitochondrial eve" and "y chromosome adam" are not the most recent common ancestors of all humans alive today, they are, as jimbob points out, just the most recent ones that we can nail down specifically. Chances are that the most recent ancestor common to all living people lived considerably more recently than either of them.

As to the OP, I agree with wowbagger when he said:

I think that, for most practial purposes, "UCA" and "earliest nexus" essentially mean the same thing.

So I don't understand John Hewitt's objections.
Specifically:

To me, this paper merely suggests some previous nexus leading to some earlier divergence but that is simply what one would expect. It is certainly not the imagery conjured up by the term UCA. I am also left wondering what all the fuss is about with this paper.

What is different from that and the "imagery conjured up by the term UCA"?

 

[John Hewitt]

So I don't understand John Hewitt's objections.
Specifically:

Well, in the first place, Wowbagger's use of quotes was inappropriate - I did not use the term "earliest nexus" at all, I used the term "some previous nexus," which could mean any nexus during or prior to the emergence of genes. It certainly does not mean earliest nexus.

Secondly, I find the terms nexus and UCA to be incommensurable. The term UCA implies some particular originating type of organism or individual whose unique capacity for reproduction led to a subsequent widespread dispersal and divergence. A nexus can arise within an established evolutionary tree as a narrowing in the width of intergenerational data flows by virtue of there having been, at some time, a small population of reproducing individuals.

Focus on the UCA concept implies focus on the properties of those very first organisms. Even if it is taken to mean individuals within the resrtricted population that created a nexus, the fact is that the nature of such restricted populations of reproducers are well known to be highly contingent and often more or less arbitrary. As a result, the application of the UCA concept always has a high dergee of arbitrariness about it. On the other hand, and at the risk of banging my own bioepistemic drum, one can understand a nexus in terms of data processing, as a narrowing down of an intergenerational data flow without creating the same contingency. Thus, a discussion focussed on data flow could hope to be less arbitrary than one focussed on the properties of individuals.

Thirdly, the UCA concept is both chemically and biologically unspecified and its relationship to the experiments described in the paper referenced by the OP seems to me obscure.