February 28, 2010

Stephan C. Schuster
Pennsylvania State University
Center for Comparative Genomics and Bioinformatics
310 Wartik Lab
University Park, Pennsylvania 16802

Dear Stephan Schuster:
I enjoyed your paper (Complete Khoisan and Bantu Genomes from Southern Africa, Stephan Schuster et al. NATURE vol. 463 no. 7283 February 18, 2010 page 943).  I tend to refer to the Khoisan or San by the term Bushmen because it is the name I first learned to know them and like them by and because I am pretty sure I can pronounce it.  Besides, I think it dehumanizes people to change their names.

There are so many wonderful things about Bushmen.  For one, despite claims of the Botswana government, they really do have a sustainable economy.  And although I hear repeatedly the pious remark that violence is endemic in the Third World, even worse than in the United States, I have never heard that accusation leveled against Bushmen.  And a society that has survived for tens of thousands of years makes the survival of civilizations seem brutish, nasty and short.  Few indeed survive their three hundredth birthday with society intact. 

One thing I hope will interest you is the fact that human fertility, like that of most other animals, is determined by kinship.  Except for very high degrees of inbreeding, the more closely related a couple and the more closely related their ancestors, the more children there will be.  After years of knowing this, accumulating supporting data and creating a cyber model, I can at last point my finger and say, “There.  The genetic mechanism is going on right there.”  At least I’m pretty sure.  I will be presenting this at the genetics conference in Albuquerque later this month.  If you would like a copy of my poster before then, I shall happily send it.  And I plan to post it on my website nobabies.net along with my other correspondence including letters such as this at about the time I present my poster. 

The quick and easy way to see the evidence is to take a few minutes and look at the enclosed DVD. 

This fact accounts for two remarkable things.  For one thing, it is obvious that long term survival depends on having a strict limit to gene pool size.  The fact that, as you point out, two bands of Bushmen who are sometimes within walking distance of each other seem to be more distantly related than a European and an Asian, indicates that their social horizon is limited to the band and that this limit is essentially air tight.  So while the rest of the world is plunging into an infertility crisis, the Bushmen have had the ability to go on and survive indefinitely.  As they are forced off the land into high rises, their social pool will enlarge and their outlook is no better than that of the rest of us, unless something changes, which means that people do take an interest in the welfare of their children.

The second thing accounted for is the question of whether farming has spread by cultural diffusion or by population replacement.  Consider a frontier where there are farmers on one side and hunter gatherers on the other.  Assume for a moment that the farmers have a higher fertility rate.  The two populations merge, intermarry and have children and descendants with a substantially reduced fertility.  Essentially the land falls empty.  The farmers, more fecund and with a greater population density expand into the vacuum.  As this cycle recurs, the reproductive rate of the farmers remains high because of multiple founder effects; each new community is genetically a subset of the community that gave rise to it.  Meanwhile, if any of the hunter gatherers retreat, they are likely to find sympathetic souls like them among whom they will intermarry and suffer infertility.  Thus the farming survival strategy sweeps across the landscape like a bulldozer.  It is population replacement with a little back diffusion of genes. 

In the developed world fertility has been below replacement for about thirty years.  I understand that in parts of Spain and Italy the birth rate is less than one per woman.  I suspect that is because those are the countries were the renaissance started. 

If we and our civilization are to survive, it will be necessary to produce a little more than two babies per woman.  That will not happen so long as we maintain our present strategy of random mating in large populations.

But turning it around is problematic.  It will be necessary for almost everybody to marry a fifth cousin or closer.  But where do you find one?  I can find second cousins, but that is probably a bit too close.  Only Iceland seems to have the genealogy to make rational mating decisions.  The Bushmen could have done it pretty easily and may still be salvageable.  For the rest of us I fear our last frantic hope is genomics.  We probably do not need to wait for the thousand dollar personal genome and a nationwide database before starting to work on it.  A chip optimized for a particular population might be able to work out kinship to fourth cousin with as few as a million sites examined.  I don’t know for sure.  You probably do.  I hope you will consider working on it.


M. Linton Herbert MD

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