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Brain Size and Tools I
Larry Mai (IAMLMAI@TEMPLEVM.BITNET)
Sat, 4 Mar 1995 19:33:02 EST
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I have recently followed the *brain size and tools* thread, and can
appreciate both the clarity and the (sometimes) confusion that we all feel
when attempting to understand this problem and its <solution.>
This is a very complex problem, and its appreciation requires an
extensive working knowledge of the hominid fossil record, allometric models,
anatomy and physiology, and some subtle principles of thermoregulation and
adaptation. The generality of the problem, and the synthesis required to
appreciate it, again places anthropologists in a unique position to formulate
the problem(s) as hypotheses which in this case aggregate into a scenario,
the elements of which can probably be tested only circumstantially.
For some of us this is less a case of potential teleology and the
conundra that are inevitably presented when definitions and logic are pushed
beyond their appropriate bounds, than it is a case of attempting to falsify a
scenario that seems *not* to be teleological.
In my case, I am currently having difficulty falsifying a scenario that
is constituted by only three or four central hypotheses. The actual problem
is much more complex, and the simple version is presented here.
Dramatic encephalization has occurred in at least two mammalian lineages
(the Hominidae and the marine mammal taxon Cetacea). A general scenario of
encephalization (an its consequences such as elaborate communication and
conceptualization) should <explain> encephalization in both cases. In the
case of encephalization it would seem that several *constraints* to
encephalization must be removed prior to its manifestation.
The first constraint to have been removed is well known: the problem of
the size of the birth canal in relation to neonatal brain (or body) size. So
much has been written on this that I will not elaborate here, except to
remind us that the <solution> in hominids was due partially to bipedality and
the presentation of the fetus, and due partially to a slowing of the rate of
growth in general (sometimes inappropriately called <neoteny>) which resulted
in infants presented at a <premature> stage of development (and thus smaller
absolute neonatal brain size than at the proper <mature> stage). The
removal of this constraint in hominids (at least the bipedality part)
occurred at least a million years before dramatic encephalization, according
to the current fossil record.
The removal of this constraint occurred analogously in certain Cetacea
as the result of their adaptation to an existence in a buoyant aquatic
environment in which the effects of gravity were ameliorated, and the
weight-bearing functions of the pelvis were removed, which resulted in a
pelvic girdle that eventually became increasingly fibrous, until in many
species the pelvis is a circle of bony plates attached by a cartilaginous
scaffolding. This flexible birth canal permits the birth of live young that
are larger than most terrestrial mammals.
The second constraint to have been removed is perhaps less well known:
the problem of the thermoregulation of a brain exceeding a size threshold
proportional to body size--the well known brain/body allometry relationship
characteristic of all mammals. A simple understanding of this problem can be
suggested by asking why King Kong is a physiological impossibility, and why
Gigantopithecus was probably at the limit of the absolute body size for
pongids. Imagine if you will increasing the size of a gorilla geometrically,
that is by increasing its absolute size without changing the proportion of
parts. Thus a 300 pound gorilla with a 500cc brain would if brain size were
increased to 1500cc (a little larger than modern humans) have a body mass of
900 pounds. The problem with such an animal given current climatic
conditions is that the mass has increased 3x while the ability of this
impossible animal to conduct, convect, and radiate its increased body heat,
has increased by only about 2x. This latter is a composite of both Fourier's
law of heat transfer and the mass-surface law (see Kleiber below). In other
words, such an animal if it evolved would have to sleep during the heat of
the day, subsist on plants that have a low specific heat-generating potential
during digestion, or minimize heat production in other ways, such has having
a disproportionately small brain, thus minimizing oxidation at that site. To
simple make a larger gorilla is to make an increasingly inefficient
thermoregulatory organism, to the point of diminishing returns. The current
take on the <solution> to this problem--which I prefer to think of as the
removal of constraints, is the possibility that the cerebral venous
circulation changed in one or more species, and/or the more likely
possibility that one of the species of hominids experienced mutations
(probably in the pathways of the adrenals) which resulted in an increase in
the number and/or efficiency of eccrine glands. In Newman's old observation,
hominids became <sweating and stinking> animals. For those not familiar with
the differences among primates in the capacity for <sensible> sweating (as
opposed to <insensible> sweating, primarily from the lungs), only humans
among primates are capable of evaporating 1-12 liters of water per day. The
result of this constraint removal was that the body surface area of these
hominids, whoever they were, became extremely efficient radiators of body
hear (see Falk; Wheeler), resulting in the possibility of *smaller* bodies
per unit of metabolic heat, and removing the thermoregulatory constraints
imposed--and still imposed upon other mammals--on brain size per unit of body
surface area.
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