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.
6793.
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
offspring.
"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
ago."
"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?
glenn
see http://www.glenn.morton.btinternet.co.uk/dmd.htm
for lots of creation/evolution information
anthropology/geology/paleontology/theology\
personal stories of struggle
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