ancient genetic input to modern humans

From: Glenn Morton (
Date: Sun Feb 03 2002 - 17:16:13 EST

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    I ran across an article in my files which I don't think I had mentioned as
    evidence of the input of ancient DNA into modern Humans. Contrary to claims
    about mitochondrial DNA, the nuclear genomic data continues to accumulate
    showing a small but existent input of ancient genes into modern populasion.
    This is an old study, but as of 1999, it was still being cited favorably and
    I have found no refutation of it. Here it is:

            "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. 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). 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. The
    relevant results 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. 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 larger African population. Thus, the opsin
    polymorphism argues (mildly) against a complete replacement of
    the Caucasian gene pool by African populations.
            "The second example concerns an autosomal recessive disorder
    in lipid metabolism due to the absence of apolipoprotein C-II,
    the physiological activator of lipoprotein lipase, a key enzyme
    in very low density lipoprotein metabolism. Two deleterious
    alleles, one from a Venezuelian Caucasian family and one from a
    Japanese family, share a frameshift mutation suggesting common
    ancestry. These two mutants diverged from the normal allele at
    least 2 Myr B. P. The persistence of two defective alleles over
    such a long time is a puzzle, perhaps a consequence of small
    heterozygote advantage. But this persistence (i) argues against
    extremely small population bottlenecks throughout the Pleistocene
    human history, and (ii) favors the conclusion that European and
    Asian H. erectus have contributed to the gene pool of modern H.
    sapiens." ~ Francisco J. Ayala, Ananias Escalante, Colm O'hUigin
    and Jan Klein, "Molecular Genetics of Speciation and Human
    Origins," Proc. Natl. Acad. Sci, USA, 91:6787-6794, July 1994, p.

    One other interesting item which concerns whether or not the archic hominids
    were biologically so distinct as to be incapable of hybridization. The
    Chimp-Bonobo lines split about 3 million years ago, yet they produce fertile

    "We can get a rough idea of the date of this common ancestor
    by comparing two numbers. The first is the amount of DNA
    divergence that has accumulated between the chimpanzees and
    bonobos, and the second is the amount that has accumulated
    between chimpanzees (or bonobos) and humans. After some
    statistical correction the ratio of these numbers shows that
    the chimp-bonobo split occurred about sixty
    percent of the way back to the human-chimpanzee split. This
    means that they probably diverged about three million years
         "During those three million years many different evolutionary
    events must have taken place in these diverging chimpanzee
    and bonobo lineages. Yet none of them were so dramatic as
    to efface the essential chimpanzeeness of either lineage.
    Indeed, as I mentioned earlier, before and even after the
    differences between chimpanzees and bonobos became known,
    they had been mixed together in zoo cages. Some of them
    readily mated with each other and produced hybrid offspring.
    The handful of these hybrids that we know about seem to be
    healthy, and there is no reason to suppose that they would
    be unable to have babies of their own. As with the hybrid
    orangs, however, it is unclear how well they might do if
    they were reintroduced into the wild, or whether subsequent
    generations will continue to be normal."
    Christopher Wills, The Children of Prometheus, (Reading,
    Mass: Perseus Books, 1998), p. 189

    If interbreeding is possible after 3 million years of divergence, why do we
    think Neanderthals couldn't interbreed with us after only 500,000 years of
    divergence--the max suggested time of divergence?


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