Re: Why is Homo sapiens hairless?

Susan S. Chin (susansf@netcom.com)
Sat, 16 Nov 1996 17:50:17 GMT

Susan S. Chin <susansf@netcom.com> wrote in article
: <susansfE0qpt3.6M7@netcom.com>...
: >
; > the hypothesis was that hominids
: > lost their hair gradually as they left the forrested
: environments that
: > the ancestors of todays great apes lived in (modern
: descendants being
: > gorillas, chimps, and orangutan). Once this bipedal ape
: was out in the
: > open, not necessarily a savannah, but not a covered
: arboreal setting
: > either, the need for heat loss was such that selection
: favored loss of
: > hair which apes living in forrested environments still
: have. Makes sense
: > to me...

: JW: Ummm. Trouble is that the only mammal species with
: limited hair which live in a comparable environment are the
: Elephant and Rhinoceros. In their case, their surface to
: mass ratio can explain the condition. Hominids were
: somewhat smaller and lighter. All other species of mammals
: in these environments have a full coat of hair/fur. Why
: should the hominids have been any different?

Comparing elephants, rhinos and other mammals living in "comparable"
environments to early hominids isn't particularly useful for several
reasons:

Phylogeny - hominids vs mammals. You would have to assume that the
hair on primates and on mammals are similiar in origin: a primitive
shared characteristic which arose before the split, therefore homologous
characters. Frankly, I don't know the answer to that one. But the degree
of relatedness of great apes to hominids is obviously much greater than
to non-Primate mammals.

The hair on pongids and the reduction of hair in hominids then
would be a more useful area of comparisons in terms of ecology and
adaptations to hairlessness (=less hair, not "no hair").

There are likely many reasons dating back to
the origins of the other mammals you mention which made hair loss or
reduction not a useful or necessary character evolutionarily speaking.
If hair or fur is the primitive condition, then for hair loss to occur
there would have to be very strong selective factors in favor of it. Did
these other mammals change their environments about 4-5 mya from an
arboreal and therefore shaded environment to one where exposure to sunlight
was as drastic as "Night and Day?"

: Similar species, with similar physiologies living in
: similar habitats, should have similar degrees of hairiness.
: Chimpanzees in Senegal live in similar habitats, but have
: not become hairless. If anything, bipedalism would have
: reduced their exposure to solar radiation, so they should
: have got more hairy to compensate.

This depends on how long those chimpanzees have lived in Senegal. How
similiar is the habitat? This hair reduction in hominids is thought to
have happened *gradually* over the entire hominid lineage (not clear at
what point we became "anatomically modern" in our hairlessness). Unless
the same can be shown for modern chimps living in Senegal, that is not a
good comparison either.

: It is not clear to me how much bipedalism would
lead to : increased solar heating of the skull ( viz a viz a
: Chimpanzee). Hot adapted people have frizzy hair. This
: reduces the impact of solar radiation on the skull, while
: allowing maximal flow of air over the head. I would expect
: this kind of adaptation in the hominids.

You are looking at the hominids as one homogenous group. Current theory
has Homo erectus out in Europe, Asia, Indonesia and whereever else that
I've missed by 1.5mya+. Very different environments, very different
adaptations. This is still consistent with modern human variations in
hair structure based on geographic origins.

: But reduced body hair? No chance. Until the development of
: the female subcutanous fat, there is no reason to think
: that the hominids would have been any less hairy than their
: hominoid cousins.

GRADUALLY. This hair loss came GRADUALLY.

Susan

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                                             susansf@netcom.com