Re: puzzle of the negrito: isolated archaic populations
Gerold Firl (geroldf@sdd.hp.com)
13 Dec 1996 20:44:22 GMT
In article <E2BCAq.8FM@liverpool.ac.uk>, dbarnes@liv.ac.uk (Dan Barnes) writes:
|> In article <584qot$p0j@news.sdd.hp.com>, geroldf@sdd.hp.com says...
|> >Remnant populations of small, frizzy-haired, forest-dwelling peoples
|> >still exist (or did within the last century) in isolated pockets
|> >throughout asia, from the phillipines, malaysia, indonesia, the
|> >andaman islands, and possibly india as well. Average height for men
|> >ranged from around 4 1/2 feet to just under 5, leading to the name
|> >"negrito", and begging the question of relations to the african
|> >pygmies. How did the negritos come to be? The answer to this question
|> >could have important implications for the history of human evolution.
|> If Ruff (1994) is correct then a reduction in stature is a response to adaptations
|> to rain forest conditions (and I'm sure sexual selection comes into it as well).
On the first count, yes - not only humans, but a variety of other
species have also adapted to rainforest conditions with smaller body
size. I'm wondering about your second point: why would you expect sexual
selection to play a role?
He
|> examined the width of fossil Apiths and H.e. (admittedly a very small sample as
|> pelvises, which he used, are rare) and showed that they fit into a general
|> climatic distribution of moderns. Since, in the generalisation of the human body
|> (a cylinder), there is no change in surface area to volume ratios with increasing
|> height he came to the conclusion that the difference in height correlated to the
|> greater volume need for water storage that a savannah dwelling hominid would
|> need.
Risky conclusion; human morphology ranges over a wide spectrum of
area/volume ratios. It's one of the primary evolutionary responses to
climatic conditions. The yahgan of tierra del fuego, who probably
represented the most advanced physiological adaptation to cold among any
modern humans, had very truncated extremities. Some of the nilo-sudanese
peoples, such as the watutsi, represent the other extreme; they are
extremely tall and slender, with proportionately longer arms and legs. I
haven't seen any data of how the area/volume (A/V) ratio of the pygmies
compares with their taller neighbors, but they may be similar. Pygmies
are not nearly as gracile as the watutsi, but because they are so much
smaller, their A/V ratio is boosted. (Note: if the human body type is
approximated as a cylindar, A/V is inversely proportional to size.)
|> From these results it is not suprising that a people who have spent a long
|> time living exclusively in rain forests would develop a decrease in height.
Right, though the relative importance of temperature regulation and
mobility in tangled overgrowth is unclear to me; both seem to be
significant.
Thermoregulatory sidebar: the human sweat system is uniquely adapted to
high-temperature activity, and constitutes one of our core adaptations.
In high-humidity environments, however, the efficacy of evaporative
cooling goes way down. Equatorial rainforests can be hot and humid, and
in such conditions *convective* cooling must be relied upon exclusively;
sweat simply does not evaporate. A high A/V ratio will increase cooling
efficiency, *regardless* of whether evaporative or convective cooling is
used. Thus, from a thermoregulatory prespective, small size is not
necessarily adaptive in the rainforest; A/V ratio is a more significant
parameter.
It
|> would seem that if Ruff's analysis is valid that it is an example of parallel
|> evolution - with two groups of people adapting to similar environmental
|> conditions.
Entirely possible. The question then would be, from which ancestral
population did the negrito evolve?
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