Re: Why is Homo sapiens hairless?

Rohinton Collins (
14 Nov 1996 00:42:06 GMT

Paul Crowley <> wrote in article
> In article <>
> "Susan S. Chin" writes:
> > . . 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
> > to me...
> But hair insulates from the heat of the sun;

Hair insulates the body. It protects the body from solar radiation, but
also prevents efficient heat loss - this is why you will never see a
savannah baboon out in the midday sun.

> and then there's the
> cold at night, the thorns, the poison ivy . . .

How about shelter and perhaps, god forbid, clothing?

> And why should
> the need for heat loss be so compelling?

Perhaps this was our ancestors' reason for hair loss. In order that they
may brave the midday sun in order to hunt/forage/scavenge whilst its furry
cousins were forced to seek shelter.

> What was waddling little
> Lucy doing that needed such energy?

Can you please dispense with the incredibly subjective and misleading word
'waddling' Paul? A. afarensis was a fully adapted biped. There is no reason
to assume that her locomotion was markedly less efficient than ours just
because it was different.

> And some more questions:
> Why is it retained on the head?

Because this is where the sun is shining all day, think about it. It
protects the scalp from intense solar radiation (rays hitting the body,
would, on average, hit at a more oblique angle) Even with this protection
we still get sunstroke (overheating of the brain) if we stay out in very
hot sun for too long. In these instances, the increased temperature of the
brain due to the insulation of the hair is far outweighed by its insulation
from the sun.

> Why do males have more body hair than females and children?

This probably does have a lot to do with hormones ;) although this is no

> Why do babies have virtually none at all?

Babies, children and women have just as much body hair as men. It's just
that their hair is very fine, almost to be invisible.

> What relationship does its loss have to sweating?

This has been explained many times Paul, do we have to go over old ground?
It is very simple: Less hair means more air movement next to the skin.
Sweat will thus constantly have fresh (low humidity and thus easier to
evaporate into) air in which to evaporate. Have you ever taken off a
sweater or T-shirt when you have been perspiring heavily? Have you not
become cold quickly, assuming cool and breezy conditions (not necessary,
but they amplify the effect)? This, Paul, is increased heat-loss efficiency
from perspiration due to a hairless body at work!

> Why did the sebaceous glands (formerly for oiling the hair) not
> atrophy, but become gigantic after puberty, especially in males?

The sebaceous glands also keep the skin in good condition - they stop it
from drying out. Do you like the way your skin feels after washing it with
soap? Acne is a medical condition relating to overproduction of certain
hormones. It may also be bacteria-related.

> Why was body hair never recovered?

Because the reasons for its evolution still exert pressure. Either that, or
we have other means of reproducing its beneficial effects, whilst having
lost its non-beneficial effects.

> This was sweating, which was more efficient without the hair.

If you understand this, why your above question which I have taken time to

> (Let me concede that this whole narrative may be thought to
> contain a large element of speculation. ;)

Oh alright. The whole thing is a 'just-so' story. Nothing is based on the
evidence we have to hand, be it archaeological or morphological. It's just
as well I haven't wasted any time reading any books on the AAT. From what I
have read in this newsgroup, and from published papers, it is complete and
utter nonsense. Shellfish? Ha!

> Paul.

I do not claim to have all of the answers Paul. But those I do have are
logical and parsimonious, relying heavily on the physical evidence, and to
a lesser degree what we may extrapolate from the ethology of the primates,
especially the great apes. I try to stay as much away from nested
speculation as possible. I wish you would.