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First Family and AAT
Gerrell Drawhorn (szpltdwn@dale.ucdavis.edu)
25 Sep 1995 02:13:28 GMT
I have three points to make regarding the AAT.
1) The AL-333 Locality suggests that A. afarensis was poorly adapted
to dealing with even moderate levels of water. The lithostratigraphy and
taphonomy of this "First Family" site indicates the group was incapable of
escaping from a slowly rising water level while they were trapped on a
spit of land along an embayment or slow moving river. Clearly these hominids
were utilizing littoral environments- but why would they have so quickly
lost the "Aquatic" adaptations that would have profoundly benefitted them?
I reason that A. afarensis probably did not have Aquatic ancestors
("wading" ancestors perhaps, but many primates do this, including lowland
gorillas and bonobos...it's a pervasive catarrhine character).
2) Bipedal adaptations in early hominids appear to be mechanically
designed to deal with increasing gravitational forces applied to the hip,
hindlimbs and foot. Swimming would seem to completely release the
hindlimbs from these verticle forces - selection would take the climbing
foot, ankle and leg of an ancestral hominoid on an opposite trajectory
than ultimately attained in hominids. Even wading by a quadruped would
reduce these forces to some degree.
I find it difficult to envisage a situation where the lateral
system in the pelvic musculature, the increased surface areas and
angulation of the knees, the development of the longitudinal and
transverse arches of the foot, etc. could arise if animals were in an
environment where they were buoyant. Either a lightly-built knuckle-walking
or (more generalized) brachiator (not as specialized as hylobatines)
would seem to provide a better progenitor than an aquatic form.
3) Many of the soft-anatomical features cited by the AAT proponents
of an aquatic past are found in related hominoids and cercopithecoids.
Digital webbing is found in siamangs (hence the name Symphalangus
syndactylus), gorillas and bonobos.These are probably adaptations to the
brachiatory hand of hominoids. I suspect the diving reflex, and many
other AAT traits would also be found in our near relatives. The fact that
Japanese macaques, Nasalis, and swamp monkeys (C. mitis) are capable of
swimming suggests a broader capability not restricted to humans. If so,
there may be many traits attributed as derived from a human AA phase
that are clearly "pre-adaptations" from an earlier set of features. Until
we know what the distribution of these characters are in hominoids (ie.
can it clearly be stated that no other hominoid except humans has a
"diving reflex"), these characters should be given very low weight in any
argument.
Jerry Drawhorn
Dept. of Anthropology
University of California
Davis, CA 95616
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