As I've already mentioned, genetic anthropologist Stephen Oppenheimer has pointed to a very similar gap in the genetic evidence, which led him to propose a major bottleneck event stemming from the eruption of Mount Toba. While the timing of the Toba eruption makes Oppenheimer's interpretation problematic, the genetic evidence he cites does indeed suggest that something very unusual must have happened in South Asia very early on, at what might well have been a critical turning point in human history.
With all this in mind, let's return to the genetic map I presented two posts ago:
(Click on the above to enlarge the image.) The map, representing a phylogenetic tree based on mtDNA (mitochondrial DNA, representing female lineages only) is from a recently published article, Correcting for Purifying Selection: An Improved Human Mitochondrial Molecular Clock, by Pedro Soares et al., June 2009. (Among the co-authors, by the way, is Stephen Oppenheimer.) What makes this tree especially interesting is, first of all, the effort by the authors to correct for the effects of natural selection, and secondly, the introduction of timing estimates based on what the authors claim to be a new and improved molecular "clock." The number displayed next to each node represents their estimate for the origin of that node, in thousands of years. Thus, for example, the origin of M is given as 60.6, i.e., 60.6 thousand years ago.
Each node is color coded according to its most common geographical occurrence, as indicated in the key at the upper right. What's unusual here is the fact that the M haplogroups for South Asia (in violet) are presented farther to the right, and slightly lower, than the M haplogroups for East Asia (in blue), implying that the South Asian haplogroups appeared at a later date than those for East Asia. And sure enough, the date for South Asian M is 49.4, while that for East Asian M is 60.6. If the Out of Africa migration proceeded in an orderly, predictable fashion, from west to east, we would expect to find the oldest haplogroups in South, not East, Asia.
So once again we must ask: what gives?
The first point to be made regarding the new research is the fact that the dates they've come up with "render an out of-Africa dispersal prior to the Toba eruption in Sumatra at ~74 kya less likely." Given that Oppenheimer is one of the co-authors, this is a particularly significant conclusion. Nevertheless, the inconsistency between South and East Asia must be accounted for:
In the context of the southern-coastal-route model, it should be noted that although the distribution of haplogroup M has also been used to support the southern route model, the age of haplogroup M in India, at 49.4 (39.0; 60.2) kya, is significantly lower than in East Asia, at 60.6 (47.3; 74.3) kya . . . At face value, this could suggest an origin of haplogroup M in East Asia and a later migration back into South Asia, suggesting that it may have been a ‘‘pre-M’’ lineage that initially crossed South Asia. . . Southeast Asia may [therefore] be the point of origin of haplogroup M . . . Alternatively, if M dispersed with N and R through South Asia, M may have been caught up in a subsequent bottleneck and founder effect so that its age signals the time of re-expansion rather than first arrival (p. 752 -- my emphasis).
There will, of course, be a great many refinements and no doubt revisions of all the many phylogenetic trees that are now being published. Nevertheless it seems clear, not only from these results, but others already published, that the gap I've noted, based largely on cultural evidence, is reflected in the genetic evidence as well. While such a gap might at first seem to be a stumbling block in our understanding of the earliest migrations of modern humans out of Africa, it is also, as I see it, a tremendously important clue, which could explain a great many other mysteries, some of which are so obvious as to have been taken for granted by the great majority of investigators.
(to be continued . . . )