Re: tree-climbing hominids
Alex Duncan (aduncan@mail.utexas.edu)
8 Oct 1995 20:28:41 GMT
In article <813146042snz@crowleyp.demon.co.uk> Paul Crowley,
Paul@crowleyp.demon.co.uk writes:
PC> I will look up the references. I am not hopeful as all those that I
>have studied are full of false assumptions. Since the time of Galileo,
>those proposing a revolutionary change in a field of knowledge have
>been advised to read the (often sacred) texts. (In his case, they
>included the Bible and works of Ptolemy.) This advice, while well
>meant, is invariably misconceived. The (sacred) texts mean a lot to
>the orthodox; they say nothing to the heretic.
>
AD> 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.
>
PC> I should have dealt with this last time. We are concerned with two
forms
>of locomotion: quadrupedal and bipedal. There is effectively no half-way
>stage. (Obviously there must have been a transition, but it must have
>taken place under very special circumstances - which is why the AAT
>exists).
>
PC> I cannot really "prove" this. It is self-evident that any viable
animal
>must have an effective mode of locomotion. Being "partially bipedal" or
>"partially quadrupedal" is NOT viable (except under those very special
>circumstances). You simply have to think of your own anatomy. How
>could it be modified so that you were less than "fully bipedal" and yet
>still remain a creature that could thrive on the ground in Africa?
>Life as a "fully bipedal" hominid would have problems enough. Anything
>less is out of the question.
Chimpanzees are "partially quadrupedal". They have a compromise anatomy
that enables them to move reasonably well on the ground while still
retaining arboreal capabilities. They do not move quadrupedally as
efficiently as other quadrupeds (and yes, the efficiency of chimp
quadrupedalism has been tested, and found wanting).
The same kind of logic could be used to claim that a creature cannot be
"partially terrestrial" and "partially aquatic", and by this logic we can
demonstrate that seals, otters, frogs, salamanders and penguins don't
exist.
PC>Your "not bipedal in the same manner as modern humans" is just not
>facing up to the issue, as is the retention of "significant tree-
>climbing abilities". If hominids ~4.5 to 2.0 Myr were terrestrial
>animals, they were either bipedal or they weren't. My own belief is
>that they were not fully terrestial - they were partially aquatic -
>but also that they were fully bipedal.
Again, we only have to look at the organisms around us to see the
problems with this statement. Using quadrupedalism as an analogy, your
argument could be used to claim that if one is a quadruped, one must only
locomote in a certain manner, one must have interlimb proportions of a
certain value, one must have specific anatomy on the distal limb
segments, etc. etc. etc. When we actually look at the various
quadrupeds, we see all kinds of different anatomies and styles of
movement. When we look at the hominoids, we see that in general,
efficient quadrupedal movement has been compromised by the demands of
tree climbing. And yet, chimps spend about 90% of their time on the
ground. Your assertion that only one kind of bipedalism would work is
also belied by a look at the animals around us. Humans are not the only
bipeds. As Chris Brochu pointed out, macropodids are functionally
bipedal, as are all theropods, some ornithischians, many early
archosaurs, etc. For the most part, all these styles of bipedalism
differ.
AD> Well, aside from the obvious question of whether or not
>> australopithecines were effective tree-climbers, the most important
>> question would be whether or not they may have built nests in the trees
>> as modern chimps do. Such nests would make sleeping in the trees a much
>> simpler business.
>
PC>This is a non-starter. Chimps usually build their nests where two
>trees meet. They use branches of about 2 inches in diameter from
>both trees. They sleep grasping branches from both trees and infants
>cling to mother. So no one falls off. If a leopard tries to approach
>the mother detects it and shakes it off. Such an arrangement is
>impossible without four grasping limbs in both parent and child.
First, I question your assumption that this impossible w/out four
grasping limbs; and, second, again you are mistaken in your understanding
of early hominid anatomy.
PC>Chimps have mastered the "safety at night" problem with a highly
>specific and well-engineered solution. Early hominids also mastered
>this problem in another highly specific manner. You know the AAT
>proposal. Savanna/mosaic proposals are conspicuous by their absence.
>
>> PC: At night? Lions, leopards and hyenas are nocturnal. Hominids are
>> diurnal and effectively blind at night. They would only injure each
>> other, and their females and young, if they were to use clubs and
>> stones whenever they heard a rustle and thought a big cat was near.
>> This must be one of the craziest ideas of all time. How much sleep
>> would any of them get each night? What's the life expectation?
>> Can you imagine the stress levels with the screams of the hyenas and
>> the lions' growls?
>>
>> AD: Here I assume PC is refering to later hominids such as H. erectus,
>> who presumably lived in fairly large groups, had a reasonably advanced
>> tool kit that included the Acheulean hand ax, and might well have had
>> fire. I agree that spending a night on the savanna under such
>> circumstances would not necessarily have been easy, but not far from
>> impossible.
>
PC>I was referring to all bipedal hominids - those which have lost the
>grasping capacity in their feet. This happened long before the use of
>fire (I put it at about 5mya.) I don't know when you date it. But
>from then until the use of fire, you have an impossible dilemma.
>
>> AD: . . . prior to 1.8 Myr, we
>> know that hominids had an anatomy appropriate to part-time life in an
>> arboreal substrate.
>
>I just don't buy this. Either they slept in the trees (like chimps) or
>they didn't. "Part-time life in the arboreal substrate" is empty BS.
For the most part, all catarrhine primates practice "part-time life in
the arboreal substrate". It is not empty BS. It is what primates do.
AD> Essentially fully modern postcranial anatomy seems
>> to arrive with the appearance of H. erectus. If we assume, for the
>> moment, that H. erectus could not climb trees and wasn't safe on the
>> ground, then I suppose we can entertain PC's suggestion that they must
>> have utilized the abundant savanna islands to sleep on.
>
PC>What are these "savanna islands"? They're no suggestion of mine. My
>islands are on the rocky marine littoral, nowhere near the savanna.
Well, another problem rears its ugly head then. There is no question
that H. erectus was capable of utilizing far drier and more open habitats
than earlier hominids. They have been frequently found in depositional
contexts indicating savanna, and even drier habitats. Additionally, they
have been found in regions (e.g. Choukoutien, Swartkraans upper cave,
Olorgesalie) where there were no bodies of water of any significant size
whatsoever.
AD> At this point of
>> course, AAT has become a non-theory for the explanation of the origin of
>> bipedalism, because bipedalism would have arisen prior to any aquatic
>> stage. Additionally, the suggestion that 2 short distance swims daily
>> (to the island and back) would have been a potent selective force is
>> problematic.
>
>I'm lost here. You seem to be putting an AAT existence into the middle
>of the savanna - or something?
I'm not putting AAT into the middle of the savanna. That's where most H.
erectus fossils are found. You suggested that hominids that couldn't
climb trees must have slept on islands. The first hominids whose
tree-climbing capabilities were as poor as ours are H. erectus.
Therefore, H. erectus must have been the first hominids who were required
to make use of aquatic resources in order to survive.
AD> But a real issue is whether or not an H. erectus band would have
>> been able to survive at night on the ground. Modern humans with tools
>> not much advanced over H. erectus (e.g., !Kung) manage it without
>> recourse to those abundant savanna islands (the islands are even more
>> abundant in the near-desert the !Kung inhabit).
>
PC>Modern humans (e.g. !Kung) have a unique and powerful resource: *fire*.
>Without that resource, hominid life on the ground is impossible.
H. erectus also had fire.
Your entire body of reasoning rests on a single premise: "any primate
biped must have a form of bipedalism that is functionally equivalent to
that of modern H. sapiens." I challenge you to question this premise.
The world is full of animals who aren't particularly good at what they do
(compared to other animals) but they do it anyway. Chimps are a prime
example. They are not "good quadrupeds". Nonetheless, their form of
modified quadrupedalism is their primary form of locomotion. I should
also point out that the same argument is a potent one against the AAT.
Humans are not particularly good swimmers. It has been pointed out 100's
of times in this newsgroup that they would have made prime prey for
African crocodylians. Once in the water, humans have virtually no
defense against crocodyles. Suggestions that humans could just run up
out of the water are a little pointless (especially if this is supposed
to be the selective force that led to bipedalism) as humans move so
slowly in the water, and quadrupeds are actually more effective waders.
The best understanding of australopithecine anatomy is that it is a
compromise between the needs to move on the ground and the needs to
maintain arboreal capabilities. They did not have to the "best" at
either one of these capabilities in order to survive. Their adaptation
was a mosaic of abilities that enabled them to utilize resources in
varied environments. Again, I point out chimps as an analogy. They are
no where near as agile in the trees as gibbons or monkeys, and no where
near as efficient on the ground as bovids or equids. Nonetheless,
somehow they survive.
Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086
512-471-4206
aduncan@mail.utexas.edu
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