Re: Why is Homo sapiens hairless?

Rohinton Collins (
17 Nov 1996 20:17:46 GMT

John Waters <> wrote in article

> 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.

I'm sorry, I missed the flow of logic in that last statement.

> (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.)

This is, quite frankly, nonsense John. A very small selection pressure (of
perhaps 1 or 2 percent) may have a significant evolutionary effect over a
long period of time. No-one needs to die on a regular basis. An early
hominid who had less hair and more sweat glands than his fellow hominid,
could stay out longer during the day hunting/gathering/scavenging, and
would therefore be reproductively more successful. No deaths are required.
All animals can detect when their body temperature has risen, even by as
little as 1 degree, and take shelter. Early hominids would have been no
exception. The selective advantage described above would have been all
that's necessary for the gradual evolution of hairlessness over many
generations. This 'history of death due to overheating' idea you have is
most worrying since it simply does not make any sense.

> 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.

But there weren't significant deaths due to overheating in order to
facilitate this evolutionary event.

> 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.

Exactly. If by 'form of behaviour' you mean 'death due to overheating' then
you are absolutely correct. This 'behaviour' is not witnessed in any extant
hominoid species, so what makes you think that a hominid species would
'behave' this way?

> If the conditions are too hot, the sensible individual
> stays in the shade. The foolish individual dies.

There is no sensible or foolish John, just advantageous characteristics
(less hair, more sweat glands) due to mutation or population variation.

> 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?

Again, no-one died of overheating, at least not in the terms you describe.
See above.

> 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?

You seem to suffer, Paul, from a condition I call 'assumptive disorder'.
You base assumption upon assumption, with logic flow alarmingly absent. I
won't even attempt to criticise the above paragraph until I can understand

> >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.


> > 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?

Not after using clothes, no.