Hi John, I enjoy your blog very much. Ive been reading a lot recently on human origins and genetics (most recently, for example, Nicholas Wades book Before the Dawn). One issue that I find does not seem to be clear in popular science accounts, and I thought you could clarify--- around the so-called Y-Chromosome Adam (or Mitochondrial Eve). Have geneticists determined that Adam was actually an individual in the ancestral population? Or is this shorthand for what I understand to be a deme or a subpopulation within the ancestral population.
There is a unique ancestor for the Y chromosome, so it is really an individual. As in the case of the mtDNA, this would not be the only man who was alive at that time, it is just inevitable that at some point all the other Y chromosome lineages have become extinct.
The reason you haven’t heard too much about it lately is that there is a huge dispute about how long ago the Y chromosome ancestor lived. Most estimates put it within the last 70,000 years, which is too young. But we don’t really know how much too young. I expect we’ll have better estimates upon whole-genome sequencing, but not yet.