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domestication. Domestic horses have one fewer chromosome in their set of
chromosomes than Przewalski's horses (just as we have one fewer chromo-
some than our closest living relatives the chimpanzees). Either a pair of chro-
mosomes fused together in the domestic horse lineage, or a chromosome
split into two in the Przewalski lineage. The resulting chromosomal mis-
match does not seem to give problems in crosses between these two groups
of horses—the hybrids are healthy and fertile.
There are two possible explanations for how this dif erence in chro-
mosome number arose. The fi rst is that the real ancestors of domesticated
horses were not Przewalski's horses at all, though they were undoubtedly
closely related to them. Instead they were a dif erent species, with one fewer
chromosome than the Przewalski's horses. This wild horse species, like the
Przewalski's horses, also ran free across the steppes of Central Asia.
In this scenario, groups of Asian nomads domesticated some members
of this wild species several thousand years ago. Then, as the domestic horses
and their nomad owners proliferated, their wild ancestors were driven to
extinction, as had happened to many horse species before them. The only
Asian wild horses left were Przewalski's horses. They too were being driven
to extinction, but unlike the true ancestors of the domesticated horse a few
of them luckily survived in the Munich and Prague zoos.
The second and much less likely possibility is that domestic horses are
descendants of a mutant strain of Przewalski's horse, one in which two
chromosomes had become fused. This scenario requires the close juxtaposi-
tion of several unlikely events: a chromosome fusion that spread through
a local population of Przewalski's horses, followed by the domestication
of some of the animals that carried the fusion, and fi nally the extinction of
all the wild members of the Przewalski's horse group that carried the fused
chromosome.
Both of these scenarios are possible, but I am inclined to wield Occam's
razor and slice away the second less likely one. If the fi rst scenario is the right
one, it demonstrates once again the enormous impact that humans have had
on the wild ancestors of our domesticated species. We humans may have
driven not one wild Eurasian horse species to extinction, but two!
 
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