Out of Africa
lamoran at gpu.utcs.utoronto.ca
Sat Nov 16 13:29:39 EST 1991
Keith Robison (robison at nucleus.harvard.edu) writes,
"I believe that the molecular evidence for an African origin of humans
is rather certain.
1. mtDNA sequence analysis (see ref. below) generates a tree in which
one must climb eight nodes from the root before finding a branch
which contains non-African sequences on both sides of the branch
(the tree was rooted with a chimpanzee sequence)."
This is an interesting point but I don't see why it is relevant. Let's assume
that Homo sapiens sapiens arose in Eurasia and that the original population
split in two with half migrating to Africa. At some later point in time
another group from Eurasia also moved to Africa. In fact, there may have been
several such incidents over time.
Keith, can you construct an Out-of-Africa senario that is so obvioulsy more
reasonable and more simple? Any senarios that I can think of still require
many migrations between continents. For example, an early population
would have had to migrate out of Africa but some descendents of this group
returned. Or, several distinct populations left Africa at different times.
I am not arguing that the data REFUTES the Out-of-Africa hypothesis...I am
simply trying to point out that it doesn't support it nearly as strongly as
most people believe. You have focused on one of my key objections. With all
due respect I do not believe that you have thought skeptically about what
you are saying and why it makes the Out-of-Africa hypothesis "rather certain".
However, I admit to being somewhat nervous about making statements such
as this in case I have missed something obvious. Please do not hesitate to
correct me if my logic is flawed.
Keith then says,
"2. One can climb several more nodes from the point described above
before reaching a branch with large numbers of non-African
sequences on both sides of the branch."
This is the same point that you raised before but it gives me an opportunity
to point out that Cavelli-Sforza's data shows a clean separation between
African and non-African populations while Wilson's data does not seem
reasonable to me. Even if your logic was correct (which it isn't IMHO) I
would be reluctant to rely so heavily on such data. At the risk of getting
off on a tangent how does one explain the wide separation of some populations
that we expect to cluster together on the dendrogram? How does one explain
data that shows that Australians are more closely related to African
pygmies than to Asians?
Keith concludes with,
"3. The three closest ancestors of humans, pygmy chimps, common chimps,
and gorillas, all live in Africa (pygmy and common chimps diverged
relatively soon after the chimp--human--gorilla split).
Based on these facts, if one assumes a non-African origin of humans
one must also assume wide-spread migrations of humans into Africa and
a similar influx of chimps and gorillas. An African origin of the great
apes is by far a simpler explanation."
The current residence of those species that are most closely related to the
genus Homo has nothing to do with your argument that "the molecular evidence
for an African origin of humans is rather certain.". This illustrates one
of the other points that I was trying to make in my original posting namely
that molecular biologists often get confused over the difference between
the origin of the GENUS Homo and the origin of the SPECIES Homo sapiens.
It also seems to be quite commmon to use non-genetic arguments, as Keith
does, to support the molecular data instead of the other way around.
(The data in question here is the current home of gorillas and chimps. It
doesn't seem likely to me that I can discover this by looking at my sequencing
gels! We have to rely on field biologists...the ones that actually know what
these organisms look like. The real data would be the fossil evidence that
demonstrates that the earliest fossils of gorillas and chimps are from
Africa. If the earliest fossils were from central Asia then it would not
matter what the field biologists had to say about present day distribution.)
Let's restrict discussion of the Out-of-Africa hypothesis to the RECENT
origin of Homo sapiens sapiens and the possibility that this population
replaced existing populations of Homo sapiens by spreading from an African
center. This event is postulated to have occurred about two hundred thousand
years ago or less. Alternatives to this hypothesis are that modern humans
arose somewhere else, especially that they arose gradually throughout the
inhabited world by evolving from the indigenous populations. For the purposes
of debate and discussion we will assume that Africa was the original home of
the genus Homo. There is no molecular data that either refutes or strengthens
that assumption and the validity of the assumption should not have any
affect on the Out-of-Africa discussion.
Laurence A. Moran (Larry)
Dept. of Biochemistry
University of Toronto
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