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Re: A. anamensis - Nat. Geographic
KAMI925 (kami925@aol.com)
20 Aug 1995 13:02:39 -0400
On the subject of A. anamensis, I think it is too early to predict exactly
where in the family tree both anamensis and ramidus fall... Ramidus has
yet to be determined as a biped-- the one diagnostic feature recovered,
the pelvic bone, is still in the process of being analyzed at UC Berkeley.
It has been determined via stable carbon isotope analysis that ramidus
did indeed live in a wooded grassland. This factor tends to place ramidus
more along the lines of the australopithecines as opposed to being a
direct offshoot of the pygmy chimp. Chimps have a very high quality diet
and rely heavily on their arboreal skills to meet their dietary
requirements. It has been found that animals that live in a wooded
grassland environment eat a more varied diet, including both low quality
and high quality foods. How does all of this tie in with anamensis and
ramidus? It is called the three-way split theory, and it was developed by
one of my former professors, Dr. Stan Ambrose at the University of
Illinois- Urbana-Champaign. Prior to the Messinian Salinity Crisis
(@5mya) most of east Africa was dense woodland. The salinity crisis
caused a general cooling of the earth, causing the dense woodland of east
Africa to become separated. The common ancestor of the homonoids were
thus separated into three different regions. Genetic drift and other
possible factors caused a physical speciation... yet all three still
occupied the same ecological niche. After the salinity crisis passed, the
separate forest regions merged into one-- and thus you now had three
distinct species fighting for the same ecological niche. The species that
eventually became chimps took a high quality diet, the species that became
gorillas took a low quality diet... what happened to the third species?
Well, two's company, three's a crowd... the third species went to the
savanna and lived in not an arboreal environment, but a mosaic
environment. Since ramidus also lived in a mosaic environment, that would
have the tendency to make people think that it was somehow linked to this
third species that eventually became hominids. Chimps have a very well
established niche, and if you assume that competing for niches led one
species to go to savannah living, then one would have to ask, why would
the chimp line feel any niche pressure that would cause a speciation? If
there was no niche pressure, then no speciation, and ramidus could not be
an offshoot of a chimp. However, I go back to my original claim that
making any judgments is premature. If ramidus proves not to bipedal, then
we must ask ourselves what other types of selective pressure existed in
the woodlands that would compel the chimps to speciate and move to the
savannah. Also, if ramidus does prove to be a biped, then we must first
exclude other bipedal selective pressures to determine whether or not
niche is the only reason bipedalism developed. As for anamensis, from
what I have read, it does appear that it is in the direct line with
afarensis, however, more work is needed to determine its
paleoenvironment. If anamensis, so far a definite biped, is found to
have lived in a closed environment, then anything goes. If the selective
pressure for bipedalism is the result of something other then ecological
niche competition, then any of the hominoids had the equal potention
towards bipedalism. This then in turn could implicate ramidus was a
direct offshoot of the pygmy. And as for ramidus, right now I am leaning
towards ramidus as being a speciation of an ancestor common to both it and
anamensis.
Karen A. Mini
University of Chicago
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