Re: AAT Theory

Peter Card (
1 Nov 1995 12:38:41 -0000

Troy Kelley <> writes:

>In article <> Gerrit Hanenburg,
> writes:
>>"At maximum running speed human bipedalism is twice as expensive
>>energetically as estimated for a quadrupedal mammal of the same body size."
>>"However,at average walking speed (4.5 km h^-1) human bipedalism is
>>slightly more efficient than is quadrupedalism in the average mammal.."
>>From:Aiello and Dean._An Introduction to Human Evolutionary Anatomy_
>> Academic Press 1990,p.272.

>It seems to me that running speed would be the most improtant variable if
>you were evolving in a savanna type environment.

Not at all. Different predators have different the modes of
attack. The sprinter par excellence is the cheetah, which can overtake
anything on legs IF it can get close enough before it starts to
sprint. Start too far away and it just runs out of juice as the zebra
or whatever canters away. It also has problems with large prey, not
the least of which is keeping scavangers away from its kill.

Human beings, and wild dogs, are cursorial hunters, capable of chasing
their prey for hours if necessary, till it can run no further.

> The fact that humans have such a slow running speed coupled with the
>fact that it is very expensive in terms of energy coupled with the fact
>that the savanna is typically a very hot environment makes one wonder why
>bipedalism would have evolved at all. I am sorry, but I don't accept the
>traditional, pat answer of cooling the body through reduced exposure to
>the sun. There are plenty ways to get cool besides "going bipedial", and
>no other animals show this tendanancy.

The point is, IMHO, that a prolonged stern chase in a hot enviroment
will cause intense heat stress, which humans can handle better than
most animals. The bipedal stance gives a slight advantage. The human
perspiration system (sweat again) is inefficient from the point of
view of water loss, but will dump heat effectively over long periods
of time.

Wild dogs have a simpler way of losing heat. They are small, and the
square/cube law works to their advantage.

"The avalanche has already begun, it is too late for the pebbles to vote"

The above article is the personal view of the poster and should not be
considered as an official comment from the JET Joint Undertaking