[ExI] The Yamnaya question

[Rafal Smigrodzki]

About 5000 years ago the male ancestors of most of our list members burst
forth from their Yamnaya Russian steppe domain and started merrily
slaughtering their way across the world. Nowadays known as
Proto-Indo-Europeans, over a period of a few thousand years they spread
their language and their genes from Norway to Sri Lanka and the Gibraltar,
ruthlessly exterminating local men and taking their women. With the
invention of guns and ships they continued their expansion to the Americas,
the tip of Africa and to Australia, before finally running out of steam
sometime in the 1960's.

This was the most successful single demographic expansion in the history of
our species, at least as measured by the acreage settled or conquered, and
there are many hypotheses as to why the Yamnaya succeeded to this extent.
Nazis, who called the Yamnaya Aryans, believed they were simply superior to
other people. Some researchers say that the Yamnaya won because they had
carts and horses. Nobody knows for sure.

It occurred to me that the answer might not lie in the intrinsic genetic
endowments of the Yamnaya and is not explained merely by technological
superiority but rather it is related to a quirk of genetics, the optimum
mating distance.

It is well established that mating with close relatives dramatically
reduces evolutionary fitness - this is why the children of incest are so
often sickly, deformed and mentally slow. On the other end of the spectrum,
mating among organisms that are so far from each other genetically that
they are just barely able to have offspring also leads to diminished or
zero fitness, as in mules and tigons. In between these extremes there is an
optimum mating distance, where the parents are not so close as to share
large numbers of deleterious rare mutations but not so far as to have
incompatible adaptations. Their babies are stronger, faster, resistant to
infections and smarter, an effect that in some situations is called
heterosis, or hybrid vigor.

One of the typical features of prehistoric life in established communities
was a very restricted ability to travel due to presence of hostile
neighbors. Thus, after an initial settlement of an area, for example when
the Anatolians swamped the local hunter-gatherers throughout Europe, each
small location would become reproductively isolated from even relatively
close neighbors (we are talking 100 miles distance or less) and thus
increasingly inbred. This splintering into tiny tribes and inbreeding has
been observed in many locations, from Europe, to Africa to Papua. The
effect persists even as neighboring tribes, such as modern Yanomamo, raid
each other to steal women and slaughter competing men - since they are in
effect stealing from their extended families, they still remain inbred.

The Yamnaya developed their culture around horses, carts and a nomadic
lifestyle. By themselves these technologies may not have dramatic genetic
implications except for one thing - they allow long distance kidnapping of
women. Long-distance raiders can not only increase the number of mates
available to them but also can increase the genetic distance between
themselves and their mates. Geographic distance translates into genetic
distance, and a technology for fast movement on the Earth's surface
translates into an exploration of new genetic possibilities.

The spread of Yamnaya thus resulted in the generation of less inbred
offspring, which in turn used their mobility technology to extend their
reach with each generation, consuming the inbred, sessile populations in
the process. The Yamnaya genes were not intrinsically "better" than the
genes of e.g. the Anatolians or Dravidians but the offspring of far-away
invaders and their local slaves was more genetically diverse, and thus
fitter.

This mechanism may be responsible for the demographic transformation of the
ancient world and it does have implications for prospective parents today -
a bit of genetic adventurousness and out-of-the-shtetl thinking may go a
long way towards having beautiful and smart babies.
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