Re: An alternative to ST and AAT

Paul Crowley (
Sat, 16 Nov 96 20:13:49 GMT

In article <> "Susan S. Chin" writes:

> Paul Crowley ( wrote:
> : I'm sure that the proto-hominids would have achieved a high degree
> : of social co-operation. This implies terrestriality and not a
> : rain-forest habitat. So its use would have been new.
> How are you so sure of this social cooperation in early hominids?
> Afarensis' sexual dimorphism suggests a high degree of competition among
> the males for access to females.

I've partly answered this in a reply to Gerrit. I suggest that
reasons for dimorphism should be looked for in the wider ecology.
Chimp males demonstrate a high degree of competition, but show
little dimorphism; this is probably because if they got much
heavier they would not be able to sleep, or generally function,
in the trees. What would happen if a group of chimps forged an
existence away from trees? The major restraint on the dimorphism
would be lifted and I'd suggest that we would see something like
A.afarensis. Large dimorphism certainly does not necessarily
imply a single-male harem type of social structure, and the
assumption that it does is far too facile. For example, highly
dimorphic baboons do not have one.

The real question is "Why is Lucy so small?". I'd suggest a
niche where a small female could forage just as well as a big
one; where large distances did not need to be covered; where
heat elimination was a problem during the day; where it was
warm at night; and where predation was minimal.
(Does anyone want me to suggest a location?)

> Punctuated equilibrium notwithstanding, evolution of functional
> morphological anatomy such as hominid arm length would *not* occur
> instantaneously. If Lucy and other afarensis are able to walk
> effectively, not necessarily "optimally" but obviously not in a waddling
> manner, there is no evolutionary reason to quickly reduce the arm
> length. Selection over time, possibly over the course of the afarensis
> lineage might favor a reduction of arm length. But if it's not detrimental
> to effective bipedalism, there's no evolutionary pressure to reduce the arm
> length quickly as you alluded to. They are called "primitive retentions."
> If it ain't broke, don't mess with it...

Arm length (or leg length) is just the sort of thing that would
rapidly attain optimum size. Natural populations of mammals have
wide variations in limb length (glance around at some H.s.s) and
over a few thousand generations, there is no question that the
"right" one is always reached. If Lucy had a long forearm, it was
because she needed one. The question is: "What did she need it
for?". If she had short legs and big feet, it was because she
needed them. But what for?

Primitive retentions are of a quite different nature; they are
essentially non-functional; selective pressures have little
effect on them.

> You're projecting modern Homo sapiens' ideas of what is healthy. I
> seriously doubt our earliest ancestors choice of diet necessarily takes
> this into account. If anything, fatty substances such as bone marrow from
> carcasses is thought to comprise the early hominid diet. Even today, most
> human diets are much higher in fat than is thought "healthy." So if our
> ancestors didn't eat much fat, where did this dietary pattern come from.
> Without a consistent, reliable source of food, early hominids likely
> weren't worrying about their waistlines or cholesterol level, IMO.

No scenario of hominid evolution claims that our ancestors had
a high proportion of animal fat in their diet. That, I hope, is
the main reason I feel it's unhealthy. (I'm sure I'm also affected
by the large number of people I known who've died suddenly of heart
attacks or who have had bad hearts.)

The "fat appetite" may be like the "salt appetite" in that it might
indicate a deprivation of essential substances in our evolutionary
past. (BTW, I believe that our "salt appetite" might only have
arisen in the last 10 Kyr or so.)

> Hypotheses about the origin of bipedalism would be useful scientifically,
> but they should proceed at a very fundamental level. There may be many
> differing pressures which caused that first ape to stand up and decide 2
> legs are better than 4. Will one hypothesis ever adequately explain these
> various selective pressures? I have my doubts there...

I'm afraid that IMHO you have scarcely begun to think about the
problems of bipedalism. It was not brought about by a variety of
differing pressures that caused an ape to "decide". Like most
in the discipline you have forgotten about the females. The ape
in your mind is male. Think of the reproductive unit - the
mother/infant dyad. Ask how a _mother_ with a large infant on
her belly (or on her back) could decide to walk or run bipedally.