(Aiello, 1993). There is evidence of regional continuity, and also of the incorporation of traits from one into another population (Aiello, 1993; Spuhler, 1993).
Does the molecular evidence favor any particular model of the origin of modern humans? The HLA evidence contradicts the Noah's Ark model and evinces that the ancestral population of modern humans was at no time smaller than several thousand individuals, a result also consistent with the mitochondrial DNA data.
Two recent molecular studies favor some degree of regional continuity over complete African replacement. The first study concerns polymorphisms in the genes for green and red visual pigments. Color vision in animals is mediated by light-sensitive pigments consisting of a chromophore covalently linked to a protein moiety (opsin). The genes coding for opsins in the red and green pigments are located on the long arm of chromosome X, whereas the one for the blue pigment is on chromosome 7 (Deeb et al., 1994). In humans, the red and green opsin genes are highly homologous and consist of six exons. The duplication of these two genes has been dated to 30–40 Myr B.P., shortly after the divergence of the Old- and New-World primates.
The green and red opsin genes have now been sequenced in a sample of 16 chimpanzees, 7 gorillas, and 4 orangutans, yielding a total of 14 biallelic polymorphic sites (all in either exon 2 or 4) (Deeb et al., 1994). Six of these polymorphisms are also found in humans, which indicates that they are of ancient origin predating the divergence of humans and apes.
One of these trans-specific polymorphisms involves the amino acid residue 65, which in the green opsin gene of orangutans and humans can be either threonine or isoleucine (Deeb et al., 1994). The relevant result is that this polymorphism has been found in Caucasians (the Ile-65 allele in 4 out of 120 individuals) but not in a sample of 56 individuals of African ancestry and 49 of Asian ancestry (Deeb et al., 1994). It is possible that the Ile-65 allele may eventually be found in African populations. It is also possible that it may have been lost from African and Asian populations in recent times, that is, after the emergence of modern humans. But since this polymorphism is millions of years old, loss of the allele over the long period since the migration of H. erectus out of Africa is more likely than a recent loss. In the replacement model, migrants from Africa colonize other parts of the world and replace any preexisting populations within the last 200,000 years. It would seem unlikely that the polymorphism would have been passed on to Caucasian populations and become thereafter lost in the