Re: Why is Homo sapiens hairless?
John Waters (jdwaters@dircon.co.uk)
17 Nov 1996 14:09:34 GMT
Susan S. Chin <susansf@netcom.com> wrote in article
<susansfE0z5Jt.6pz@netcom.com>...
> : 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.
JW: For what it worth, baby elephants have quite long hair.
This is gradually lost as they become larger.
>
> 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").
JW: Yes, this is an important point. We have the same
number of hair roots as a Chimpanzee. By constrast we have
nine times as many sweat glands. Both the increase in sweat
glands, and the reduction in the growth rate of body hair,
imply an evolutionary history of death due to overheating.
(If a species merely becomes uncomfortable in hot
conditions, there will be no evolutionary selection
favouring a change in hairyness etc. It is only when
individuals start dying of overheating that selection will
be made for hairlessness etc.)
It is generally agreed that the ancestral elephants were
much smaller than the present ones. Furthermore, the
ancestral elephants were probably as hairy as the present
day baby elephants. And for the same reason. So it was a
case of growth in size versus hairyness. Selection favoured
size, and a reduction in hairyness.
However, there is no evidence that the hominids grew in
size. So the question arises as to what could have driven
them to live in conditions under which a fair proportion of
them would have died through overheating. After all, this
is not the form of behaviour witnessed in other primates,
or even present day HG Tribes.
If the conditions are too hot, the sensible individual
stays in the shade. The foolish individual dies. In such
cases, whose genes go forward to the future gene pool.
Those who die of overheating? Or the sensible ones who stay
in the shade?
Of course, if the individuals who died were babies, who
were not able to move into cooler conditions, this might
create the necessary degree of involuntary evolution. Do
any anthropoids have helpless babies? Is this a fair
comparison?
> 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?"
JW: I don't know. Are you suggesting a drastic change for
hominids here? This is beginning to sound like the Red Sea
Theory. Has Paul converted you to his cause? I would have
thought the transfer would have been relatively gradual.
After all, you can only force the pace of evolution just so
far. It's not just sunlight.
>
> : 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?
JW:: This is a bit like saying how long is a piece of
string. Nobody can possible know how long the Chimpanzees
have been living in Senegal. Nor whether they have been in
a similar habitat for the last 5 myrs. Nevertheless, the
implication is that if they can survive in this sort of
habitat now, their (very similar) ancestors could have
lived in that sort of habitat.
The Senegal habitat is described as very arid. Midday
temperatures can reach 42 degrees centigrade. At night the
temperatures can drop to 17 degrees.
>This hair reduction in hominids is thought to
> have happened *gradually* over the entire hominid lineage
JW: Really? By whom? I'd be grateful for some references.
> : 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.
JW: I couldn't agree more. Though I prefer your previous
statement emphasising hair reduction, rather than loss.
That said, is the evolution here a one way street? Could a
hominid species become more hairy as an adaptation to a
cooler climate?
John.
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