Tools and Brain Expansion II

Sat, 4 Mar 1995 19:35:23 EST

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This second constraint was removed analogously in the Cetacea because the
ambient temperature of the ocean, although variable, is much lower on average
than that of the average ambient air temperature experienced by tropical and
semitropical terrestrial mammals. Cetacea, in fact, experience thermal
stress in their environment in the opposite manner, where the necessity for
retention of body heat has facilitated the selection for mechanisms which
provide increased <insulation.>
The third constraint to have been removed involved the actual
enlargement of the neocortex, the mechanism(s) for which is currently
unknown. Two guesses here are (1) point mutations in DNA, or perhaps
chromosomal rearrangement(s) affecting the position, expression, and/or
timing of included genes; and (2) developmental accommodation which, after
removal of the former constraints, permitted the brain size of the affected
hominids to increase toward its <potential> (an effect similar to nutrition
on stature). In either case, neither the Australopithecine lineage leading
to *Homo* nor to the more robust lineages became encephalized as the result
of the removal of the first constraint alone (bipedality/birth canal),
because at least a million years of gracile, and two million years of robust
evolution, proceeded after bipedality without significant encephalization.
Analogously, this third constraint was removed during Cetacean evolution
(in certain species), but its modality is likewise unknown.

In summary, the minimum number of constraints which must be removed
prior to encephalization (and which can be addressed as separate hypotheses)
are, for both hominids and certain Cetacea:
(1) modification of the birth canal/rate of neonatal development;
(2) enhancement of thermoregulatory modalities/systems;
(3) mechanism(s) for physical enlargement of the brain.
In the case of the encephalized hominids, it remains unclear whether none,
part, or all of these constraints were removed ca 2 Mya, coeval with the
appearance of the habilines; there are those who entertain the possibility
that some changes occurred at that time and lead to the rudolfensis or
erectine pattern of <partial> encephalization (but with much of that species'
encephalization explained by increases in body size), with the most dramatic
encephalization occurring only in *Homo sapiens*, and that being primarily by
the reduction of body size after larger brain sizes had already been achieved
by other, <partially allometric> mechanisms. (One interesting test of this
latter possibility would be to find the point in the fossil record where
female hominid body sizes (e.g., biacromial breadth) seems reduced relative
to, say, bi-iliac width, *nes pas*?)

None of these ideas are original with me, of course, and there exists a
formidable literature on each aspect of this scenario. It thus seems to me
that we, as responsible social scientists, have first and foremost a
responsibility to become intimate with all of the relevant literature(s)
*before* we start wholesale speculation, as is characteristic of some of the
knots in this cyberspace thread.