It makes me wonder if our relatively little diversity is related to the uncanny valley and our ancestors eliminated any variation that wasn't enough like themselves.
But I wonder if this could be related back to the bottleneck. During a bottleneck, inbreeding-related genetic disorders become an issue. Maybe evolution selects for genetic "purging" behaviors during such bottlenecks as a means of ensuring fitness... basically during the bottleneck you'd become Nazis to escape the effects of inbreeding, and these impulses might stay afterwords.
[1] Ellyagg mentions a study that showed that humans have some male neanderthal DNA, but not female neanderthal DNA. This, to me, suggests rape. http://news.ycombinator.com/item?id=2688389