disadvantageous intermediates?

Alex Duncan (aduncan@mail.utexas.edu)
1 Aug 1995 00:50:06 GMT

We haven€t heard anything from Pat Dooley for a while. Did we chase him

Pat several times referred to something that I found disturbing. He
suggested that the earliest bipedal hominids would have been
€disadvantageous intermediates€, and wouldn€t have been able to survive
in the mosaic/savanna environment. Then, curiously, he cited Dawkins to
support his view.

A quick reading of Dawkins will make it obvious that either Pat didn€t
read it (he may have heard about it), or he did read it and totally
misunderstood it.

In fact, Pat was using what Dawkins refers to as €the argument from
personal incredulity.€ This argument takes the form €I find it nearly
impossible to believe that (name your favorite structure) could have
EVOLVED, therefore it must have been created.€ I want to make it clear
that I am not calling Pat a creationist here. But his form of argument
is similar to the €personal incredulity€ argument.

Would an early bipedal hominid have been able to survive in the African
mosaic if it was not as well adapted to bipedalism as modern humans are?
I provide a Dawkins quote on a different subject to illustrate the
falsity of the problem of disadvantageous intermediates. Hopefully,
everyone can make the appropriate analogies.

begin Dawkins quote

What use is half a wing? How did wings get their start? Many animals leap
from bough to bough, and sometimes fall to the ground. Especially in a
small animal, the whole body surface catches the air and assists the
leap, or breaks the fall, by acting as a crude aerofoil. Any tendency to
increase the ratio of surface area to weight would help, for example
flaps of skin growing out in the angles of joints. From here, there is a
continuous series of gradations to gliding wings, and hence to flapping
wings. Obviously there are distances that could not have been jumped by
the earliest animals with proto-wings. Equally obviously, for any degree
of smallness or crudeness of ancestral air-catching surfaces, there must
be some distance, however short, which can be jumped with the flap and
which cannot be jumped without the flap.

Or, if prototype wingflaps worked to break the animal's fall, you cannot
say 'Below a certain size the flaps would have been of no use at all'.
Once again, it doesn't matter how small and un-winglike the first
wingflaps were. There must be some height, call it h, such that an animal
would just break its neck if it fell from that height, but would just
survive if it fell from a slightly lower height. In this critical zone,
any improvement in the body surface's ability to catch the air and break
the fall, however slight that improvement, can make the difference
between life and death. Natural selection will then favour slight,
prototype wingflaps. When these small wingflaps have become the norm, the
critical height h will become slightly greater. Now a slight further
increase in the wingflaps will make the difference between life and
death. And so on, until we have proper wings.

There are animals alive today that beautifully illustrate every stage in
the continuum. There are frogs that glide with big webs between their
toes, tree-snakes with flattened bodies that catch the air, lizards with
flaps along their bodies; and several different kinds of mammals that
glide with membranes stretched between their limbs, showing us the kind
of way bats must have got their start. Contrary to the creationist
literature, not only are animals with 'half a wing' common, so are
animals with a quarter of a wing, three quarters of a wing, and so on.
The idea of a flying continuum becomes even more persuasive when we
remember that very small animals tend to float gently in air, whatever
their shape. The reason this is persuasive is that there is an
infinitesimally graded continuum from small to large.

end Dawkins quote

>From Dawkins R (1987) €The Blind Watchmaker€ pp. 89-90. New York: WW
Norton & Co.

The point here is that the earliest hominids didn€t have to be very good
bipeds to take advantage of the open spaces between trees that developed
at the end of the Miocene. All they had to be able to do was move from
one patch of trees to the next before being gobbled by a predator. If
you can€t imagine them doing that, then the patches of trees in your head
are too far apart. Move them closer together and try it again.

Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086