Re: Bidpedalism

Peter Mitchison (
29 May 1995 12:43:34 +0100

In article: <> ("David Rowe") writes:
> > it should be noted that the environmental change to savannah from
> forested >area favoured those who could range farther for food. Animals
> which could
> >travel for long distances in search of food sources such as scavenging
> >others' kills or roots, berries and seeds did better. This ability to
> travel also >developed into the ability to "run down" wounded or
> deliberately injured >animals. Now we can run marathons.
> How would you explain that when we look at other scavengers they have 4
> legs.
> Is not Quadrapedalism a more efficient form of locomotion than Bipedalism.
> David

Yes it is, but you can't get there from here. The starting point was an arboreal ape
that could get about on the ground, probably on all fours most of the time. If such
an ape were to venture the same terrestrial adaptations to all four limbs as hominids
made to their hind limbs, it would lose all other uses of its hands. It would lose all
ability to climb trees, feed and groom itself. Surely too high a price.

It is notable how little chimpanzees have adapted to their partially terrestrial
existence. They retain the ability to grip branches with all four limbs even though
this costs them energy on the ground. If they don't have far to go to find the next
meal which may well be in the top of a tree, the trade-off obviously works to the
chimps' advantage.

As a biped the terrestrially adapted hominid gained the energy efficiencies of flat
feet at the cost of some agility in the trees. Not a bad deal in the right
circumstances. By this analysis bipedalism is the only way for an arboreal ape to
become a terrestrial ape.

The interesting question is in exactly what environment an arboreal ape would find
terrestrial adaptations an advantage. Dense forest with most of the food in the tree
tops is an unlikely candidate. A treeless plain with 20 miles between meals might
fit the bill if the apes could survive there at all. Somewhere in between these
extremes is a point where the gains are just greater than the costs. Hominids must
have evolved somewhere to the right of that point.

The popular choice seems to be forest/savannah fringe but for the trade-off to work
it may be sufficient that what trees there are contain no fruit or are too big to
climb. What an arboreal ape needs is a supply of suitable trees. Even seasonality
would be a problem to an ape that needs to eat all year round. If your trees don't
contain anything edible for nine months of the year you might quickly find flat feet
an advantage even though you live in a forest.

Peter Mitchison EMail

We have all of the ingredients, we just don't know what we are cooking. Douglas Adams.