"Bottleneck" for non-Africans — one of the reasons for the lower "diversity" among non-Africans is that approximately 64±6 thousand years ago, their ancestors passed through a population "bottleneck." In other words, as a result of some catastrophe, almost all non-Africans died or degenerated, and only a small group survived. Figuratively speaking, in the end, the descendants of just one couple survived, and all genealogical lines of practically all men on the planet converge to them. What kind of catastrophe or other calamity, such as a plague, occurred is unknown, and two hypotheses carry the most weight — the eruption of the Toba volcano, the largest known in human history, approximately 70 thousand years ago, and cooling in the northern hemisphere. Climatologists claim that the rank of the cooling's catastrophic nature is higher than the rank of the Toba eruption. However that may be, here is what happened (see the diagram above).
Diagram of the evolution of haplogroups of modern humanity. On the horizontal axis — the main Y-chromosome haplogroups of humanity, on the vertical axis — the absolute time scale. The common ancestor of the alpha haplogroup lived approximately 160 thousand years ago, the common ancestor of the beta haplogroup (or haplogroups from B to T) — 64±6 thousand years ago (from the article A.A. Klyosov and I.L. Rozhanskii, Advances in Anthropology, 2012b). The diagram does not show the archaic African lines A00 and A0 (the latter has now replaced line A1b on the left side of the diagram in the nomenclature); an updated haplogroup tree will be shown below.
Materials:
Our ancestors did not come out of Africa. Klyosov 2013
Re-Examining the "Out of Africa" Theory and the Origin of Europeoids (Caucasoids) in Light of DNA Genealogy. Klyosov 2012