Re: Out of E.Asia II

David Marcus Woodcock (dmw@engin.umich.edu)
8 Nov 1995 05:24:01 GMT

In article <47nl28$3fm@scotsman.ed.ac.uk>, <jamesb@hgu.mrc.ac.uk> wrote:
>dmw@engin.umich.edu (David Marcus Woodcock) wrote:
>>
>>And of course, there is the Todaro and Beneviste data and
>>argument.
>
>Do you have a reference for this? Does the Todaro and Beneviste argument
>still hold up today ie has a human resistence gene been found yet? Do you
>think it could fit the Danakil Alp story as well as ooea?
>
>James Borrett.
>

To answer your question I must give you Todaro's argument:

Kathleen Hunt gave a nice summary of the case T & B's made in their:

"Evolution of type-C viral genes: evidence for an Asian origin of man",
Nature 261:101-108.

H>...states that humans were exposed to the baboon virus in
H>the past, and carry the endogenous virogene. BUT, humans may not have
H>been exposed to the baboon virus as *recently* and *repeatedly* as
H>other African primates have.
H>
H>Here's a quick summary:
H>
H>1) If you look at all the *other* genes in humans, chimps, gorillas,
H>baboons, macaques, and many other primates, using DNA-DNA
H>hybridization to compare all their non-repetitive cellular DNA, you
H>find that human DNA is very similar to chimp and gorilla DNA and that
H>all three of them are rather far from the baboon. For instance, chimp
H>DNA has 54% homology to baboon DNA, gorillas have 50% homology, and
H>humans have 56% homology. The expected result.
H>
H>2) If you compare JUST THE VIROGENE, though, you get a strikingly
H>different result.
H> 2a) The chimp and gorilla virogene are both about 10% homologous
H>to the baboon virogene. But the human is MUCH farther from the baboon
H>(3%, close to the limit of detectability). In fact the human virogene is
H>exactly as far away from the baboon as the gibbon & siamang virogenes
H>are (gibbons and siamangs are Asian apes not very closely related to
H>humans).
H> 2b) If you then look at the virogenes of other groups of primates
H>that have Asian and African species, the Asian species consistently come
H>out oddly far away from their close African relatives, and always in the
H>direction of being farther away from the baboon. (ex: African colobus
H>has 24% homology to baboon virogene, while the Asian langur, a close
H>relative of the colobus, has only 5% homology. Compare the results for
H>overall DNA: colobus has 64% homology to baboon cellular DNA,langur has
H>a very similar 62%.Similar patterns for Asian vs. African cercopithecine
H>monkeys.)
H>
H>So, what on earth is going on with the virogene? Why does it show a
H>pattern of divergence that varies not only with taxonomic relatedness,
H>but also with geographical habitat? The authors' hypothesis is that
H>ever since the virogene got into the Old World primates, the gene has
H>been slowly diverging as the various species evolved. BUT, the gene
H>has diverged *more slowly than expected* in African primates. Some
H>selective force has operated to keep the African primates' virogenes
H>rather similar to the baboon virogene, and prevented them from
H>diverging the way they normally would.
H>
H>Next key point: in baboons,the virogene can spontaneously pop out of the
H>genome and re-emerge as an infectious virus. This doesn't happen in
H>other anthropoid primates. AND, "endogenous type C virogenes have
H>previously been shown to restrict the replication of highly related
H>viruses in the same cells."
H>
H>Thus, the authors speculate that baboons have *continued* to spread this
H>virus ever since it got into their genome,and that African primates have
H>retained a baboon-like virogene as a cellular defense against this
H>omnipresent baboon virus. This explains why Asian primates have such
H>different virogenes; they aren't in contact with baboons and don't need
H>protection against the baboon virus. (In support of this theory the
H>authors point out that cells from baboons, geladas, and mangabeys, which
H>are all African monkeys, restrict division of the baboon virus, while
H>cells from Rhesus monkeys [Asian] and from humans are susceptible to
H>infection by the virus.) Thus, the authors propose that humans evolved
H>in Asia. As I'm sure you've noticed, another model consistent with this
H>evidence is that humans evolved in Africa, but in isolation from other
H>African primates. In the AAH model, this would be the Danakil Alps.
H>
H>That's it in a nutshell.It certainly is evidence that some part of human
H>evolution took place in isolation from other African primates. I'm very
H>interested to know what this is thought of this paper now. DNA-DNA
H>hybridization was pretty much the only technique they used, and I note
H>that they didn't sequence the virogene, which certainly must be possible
H>now. Also, all the australopithecine fossil discoveries in the last 20
H>years show that humans have been in contact with African primates for at
H>least the last 4.5 million years -- wouldn't that be enough to reverse
H>the human virogene's drift from the baboon virus, at least enough so
H>that it wasn't as divergent as the virogenes from Asian primates?

A good point. This is why T&B's Asian origin idea and my ooea
scenario accounts for this data much better than Morgan's Danakil Alps
idea. According to my ooea scenario most ancestors of modern humans
would have been in Eurasia from as early as 2.5 mya to .1 mya
[ for Africans, 0 mya for others]. No significant re-exposure for
a long time. [ This time period also takes into account ooea I ,i.e.
H.erectus originated in Asia].

--David Woodcock