“This suggests that when ancient humans met and mixed with Neanderthals, the two species were at the edge of biological incompatibility,” said Reich, who is also a senior associate member of the Broad Institute and an investigator at the Howard Hughes Medical Institute. Present-day human populations, which can be separated from one another by as much as 100,000 years (such as West Africans and Europeans), are fully compatible with no evidence of increased male infertility. In contrast, ancient human and Neanderthal populations apparently faced interbreeding challenges after 500,000 years of evolutionary separation.
“It is fascinating that these types of problems could arise over that short a time scale,” Reich said.
An interesting article exploring the effects of Neanderthal DNA in the populations that have them (anyone whose heritage is outside of Africa).
I found the snippet above interesting. I’ve seen a lot of discussion recently in paleoanthropology about whether or not the various prehistoric populations of humanity should be considered separate species, or just variants of the same species. If us and Neanderthals were at the limit of compatibility, it seems to show that some of those other variants were probably not compatible.
My understanding of the definition of a species is that the individual members are genetically compatible enough for breeding. If Neanderthals, separated by a few hundred thousand years, were only borderline compatible, it probably indicates that Homo Erectus was probably not, and probably should be considered a separate species, at least in my completely inexpert opinion.