Re: Becoming altricial/bipedal

Alex Duncan (
5 Oct 1995 23:29:44 GMT

In article <> Paul Crowley, writes:
>In article <44ttrj$>
> "Alex Duncan" writes:
>> Please explain to me how YOU know that the earliest hominids didn't climb
>> trees,
>I thought I had. I rely on the following propositions. Please tell
>me at what point you start to disagree.
>a) A species of ape became fully bipedal sometime between 7mya and 2mya.

Your entire argument rests on this incorrect premise. If, by "fully
bipedal" you mean "bipedal in the same manner as modern humans", then you
are incorrect. Your statement is not supported by the fossil record.
The fossil record suggests that the hominid species that were extant
between ~4.5 and 2.0 Myr WERE NOT bipedal in the same manner as modern
humans, and that they retained significant tree-climbing abilities.

You still haven't told me how YOU know earliest hominids didn't climb
trees. You have merely restated your original assertion.

>b) The adults and infants of this species became bipedal at the same
> time.


>c) A fully bipedal adult foot is not capable of grasping branches.

I will certaintly agree that the foot of modern human is not capable of
grasping branches. This isn't relevant, because the feet of the earliest
hominids were dramatically different from those of modern humans.

>d) A fully bipedal infant foot is not capable of clinging to either
> (i) branches or (ii) the mother.


>e) It is not a viable lifestyle for a bipedal maternal ape to spend
> every night in the trees (which she cannot grasp with her feet)
> while looking after a bipedal infant which cannot grasp her
> with its feet.

I would really have to say that this depends on a lot of other factors
that you haven't addressed. But, for the sake of this particular
argument I'll agree with it.

f) Such creatures would suffer devasting predation if they were to
> regularly spend nights on the ground in the forest, or in the
> savannah, or in the mosaic/savannah.

This is incorrect. "Fully bipedal" hominids are large, tool-using,
group-living primates. For a carnivore to a attack a GROUP of healthy
humans is foolhardy.

>g) They did not suffer devastating predation, so they must have had
> some other form of safe refuge during the night.

What are you suggesting -- they lived behind locked doors?

>> >You're a primate. Here's an experiment: Take your extended family with
>> >wife, grandparents, cousins, several small children and newborn infants
>> >and spend a night in a tree. You must all be naked and have nothing
>> >artificial. Imagine it every night, for twelve cold, dark hours, while
>> >keeping alert for leopards. Imagine trying to spend your days finding
>> >food for all of them while enduring such nights.
>> Whether or not I can do it is irrelevant to the discussion.
>No, it's not. We're discussing your ancestry. Your anatomy is similar
>to theirs in all significant respects. If you couldn't do it, then it's
>a fair conclusion that they couldn't. If you disagree, you should say
>what advantages they would have that you haven't. And making an
>informed judgement - and using your extensive paleoanthropological
>knowledge - you should say when these advantages disappeared.

My anatomy (and yours) is not similar to that of the earliest hominids in
all significant aspects. In fact, it is very different in many
significant aspects. You still have not provided any support for your
basic assertion.

>> Other than your unsupported claim that the earliest hominids couldn't
>> climb trees, you have provided no data to support your assertions.
>I have never suggested that the earliest hominids *couldn't* climb
>trees. I said that they could not have lived in close association
>with them, because they established a lifestyle that did not depend
>on their use.

You have repeated that argument here, and you have offered no support for

I also said that it would be very difficult for a
>transitional mother to climb a tree while carrying an infant that
>could not cling to her with all its limbs. I further said that it
>is very hard to see how she could spend twelve hours every night,
>holding such an infant, while trying to sleep in a tree.

"Transitional"??? Earlier you said that the anatomy of modern humans and
the anatomy of the earliest hominids didn't differ in any significant
aspects. Which is it?

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