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October 07, 2004

Common Ancestors

Some of the posts and comments that got lost during the recent server failure were about a paper in Nature on common ancestors.

Here is the abstract:

Nature 431, 562 - 566 (30 September 2004)

Modelling the recent common ancestry of all living humans

1 Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2 7609 Sebago Road, Bethesda, Maryland 20817, USA
3 Department of Statistics, Yale University, New Haven, Connecticut 06520, USA

If a common ancestor of all living humans is defined as an individual who is a genealogical ancestor of all present-day people, the most recent common ancestor (MRCA) for a randomly mating population would have lived in the very recent past. However, the random mating model ignores essential aspects of population substructure, such as the tendency of individuals to choose mates from the same social group, and the relative isolation of geographically separated groups. Here we show that recent common ancestors also emerge from two models incorporating substantial population substructure. One model, designed for simplicity and theoretical insight, yields explicit mathematical results through a probabilistic analysis. A more elaborate second model, designed to capture historical population dynamics in a more realistic way, is analysed computationally through Monte Carlo simulations. These analyses suggest that the genealogies of all living humans overlap in remarkable ways in the recent past. In particular, the MRCA of all present-day humans lived just a few thousand years ago in these models. Moreover, among all individuals living more than just a few thousand years earlier than the MRCA, each present-day human has exactly the same set of genealogical ancestors.

I have read the full paper, and with one qualification the findings seem pretty robust. The crucial issue is whether the migration rates assumed in the model are plausible, and the authors have been quite cautious about these: for example, they assume only 5 migrants per generation between Australia and New Guinea, and only 50 per generation between Arabia and the Horn of Africa and between North and South America.

The necessary qualification, which the authors themselves recognise, is that the results would be invalidated if there are any regions that have been totally isolated for millennia. In the lost discussion Razib suggested the Andaman Islanders as a possibility. I have found this paper on the genetics of the Andaman Islanders. The data imply that many of the islanders, especially on Great Andaman, have some mixture of recent ancestry from the mainland, but there are still some with no sign of recent mixture. So unless there has been earlier migration from the mainland, of which all traces have been lost, there may still be a few islanders who do not descend from the MRCA of the rest of the world.

With this exception, and maybe a few others, the conclusion seems sound that we all have common ancestors in the comparatively recent past. As I mentioned in the lost discussion, this point was anticipated by R. A. Fisher in a letter in 1929, when he said “King Solomon lived 100 generations ago, and his line may be extinct; if not, I wager he is in the ancestry of all of us, and in nearly equal proportions, however unequally his wisdom may be distributed”.

Of course, it does not follow that all of the ’common ancestors’ have contributed equally, or even at all, to the genes of all their descendants. At a distance of 100 generations, a single line of descent from a single ancestor would contribute less than 1 part in a billion billion billion to a descendant’s genome. Since there are only of the order of a billion base pairs in the human genome, this means in practice that he would be very unlikely to contribute anything at all. It is only through multiple lines of descent that a remote descendant is likely to inherit anything genetically.

Posted by David B at 04:36 AM