I just got around to reading this 10 year old article about the X chromosome by
Eugene Harris and Jody Hey:
“X Chromosome evidence for ancient human histories,”http://www.pnas.org/content/96/6/3320.full.pdf
Even back then, the authors were remarking about the splitting of X haplotypes between The African and Non-African populations. In this case, they were comparing the PDHA1 gene which for us would correspond to the 19280872 to 19285703 positions at 23andMe.
They estimate that the age of the split would be at 1.86 million years ago. I would imagine other parts of the X would have a similar scenario, especially those areas in which we have been able to identify specific “African” blocks. But can you believe that the time to most recent X ancestor for some of our founding lines would go back before Homo erectus
even diverged from the common ancestral tree? And I thought mtDNA gave us a clue about deep ancestry. These blocks are really deep! For those of us with African type blocks and others with the Asian type blocks, that means some of our ancestors diverged nearly two million years ago and they probably would have been a different species back then and not even able to reproduce with modern humans. Yet these haplotypes are showing up in our X chromosomes.
The authors state that “the possibility of population separation before the appearance of modern H. Sapiens
does not require independent evolution of the same modern characteristics in separate populations. So long as some gene flow occurred, genes for traits that were favored by natural selection – including those associated with the transformation to modern humans – would have spread preferentially between populations. “
I haven't read the paper you cited yet, but it looks very interesting. I suppose though that depending on how small a haploblock one defines, there are going to be many blocks on the X chromosome that we share (by descent, not by convergence) not just with Homo erectus, but with other apes, lower mammal species, and even non-mammalian vertebrates. There's really not much that separates us from completely unrelated species, in terms of the amount of identical DNA. The project's results chart don't really show the whole picture in that respect, because it only focuses on those nucleotides where there is observed polymorphism.
I just checked out that block that you said was noted in the paper, in Ben's spreadsheet, and I see that it was a part of a larger highly-conserved block that I was considering posting on the project's X-SNP haploblock candidates results chart a while back (running from positions 19,133,129 to 19,351,848), but I ended up not doing so because by the time I truncated it to remove the gene, I felt it was too small a region to be particularly interesting for genealogical purposes. Perhaps it is interesting after all, from a far deeper-ancestry perspective.