Re: trichotomy revisited

Pat Dooley (
6 Dec 1994 02:00:09 -0500

In article <>, (Phillip Bigelow) writes:

Scenario 3
3) The disease infected baboons, and spread to other African apes all
_after_ the split between the common ancestor of chimps/gorillas and the
hominid lineage. In this scenario, the reason that hominids didn't get
infected is because they were genetically different enough from other
that the virus wouldn't infect them in the first place. This explaination
would mean that the hominid population on the African mainland was fully
exposed to the baboon virus, but was extremely resistant to it. As a
result, the hominid population didn't _need_ to evolve the genetic marker
for viral suppression that is found in the DNA of other African apes. The
other African apes, being further down the "family tree" than were the
hominids, _had_ to evolve a genetic marker for viral suppression, because
they are more closely related to the baboon. An analogy would be that
don't get the feline leukemia viral infection because they are so
far-removed genetically from felids. In fact, humans don't have any known
genetic marker for suppression of that virus. We probably wouldn't need
evolve a genetic marker for it, anyway.

Please note that all of the scenarios outlined above are strongly
on the timing of splits in the primate clades, and on the timing of when
virus first appeared. It is also dependent on _if_ the virus first
in baboons in the first place. Given that:

I think scenario 3 could just as easily explain the pattern of the genetic
marker in apes, without having to resort to an exotic island off the
coast as a refuge for hominids. I would be curious if anyone can point
any weaknesses in scenario 3.

There are just a couple of slight weaknesses. The Baboon marker gene is
present in a wide range of primate species - right down to bush babies.
The idea
of one branch of apes missing it while far more distantly related primates
were getting it can't be sustained. The range of species covers just
every habitat, so that leaves only one option. Our ancestors weren't in
Africa when that bug was going around.