Check your buddy's sloping brow, he might be a Neandertal!

Possible ancestral structure in human populations:

Using sequence data from the Environmental Genome Project, we find strong evidence for ancient admixture in both a European and a West African population (p ~ 10^{-7}), with contributions to the modern gene pool of at least 5%. While Neanderthals form an obvious archaic source population candidate in Europe, there is not yet a clear source population candidate in West Africa.

5% isn't jack, but, that's enough to introduce lots of novel alleles which might reshape the larger population into which admixture results in introgression. Remember, probability of fixation of a novel allele in a population is 2 X the selection coefficient. Admixture can be thought of as operationally a hypermutational event as a spike in strange mutants enters the gene pool.

Via Dienekes.