Re: AAT reply from Elaine Morgan
David M woodcock (dmw@engin.umich.edu)
24 Dec 1994 12:23:18 GMT
The "molecular clock" work gives 5 -6 Ma as the time of divergence
between African apes and hominids, that is about the start of the
Pliocene. Now the A.ramidus find supports that date: it could
easily be no more than a million years removed from the common
ancestor.
The evidence -- foramen magnum placement, length of arm bones --
supports White's conclusion that A.ramidus at 4.4 Ma was bipedal,and
certainly not a knuckle walker.
Now this appears to be an embarrassment for the AAT.
Either bipedalism predates the African ape/hominid divergence
which seems most likely -- leaving the awkward question as
to why chimps and gorillas don't exhibit all the other "aquatic"
adaptations AAT supporters cite, or the aquatic interlude must have
lasted less than 1 million years. And if we choose the latter
we must also suppose that knuckle walking either evolved
directly from quadrapedalism in the African apes or by
coincidence they went through a bipedal phase --for a different
reason.
It would be even more of an embarrassment if A.ramidus
is actually Pan ramidus -- a bipedal chimp at 4.4 Ma. But this
seems likely, otherwise we must suppose that the proto-hominid
[Sivapithecus aka Ramapithecus] started with thick molar enamel
at 9 Ma, switched to thin til about 4.2 Ma, then switched
back to thick. Not impossible obviously- thickness of enamel, like
"hairiness" is apparently reversible. But likely ?
However, assume ramidus was an australopithecine :
Does a 1 million year isolation on an offshore island explain
the Benveniste and Todaro results mentioned on this group earlier ?
Not as well as Todaro's suggestion of an Asian origin for Homo.
And that seems increasingly likely given recent H.erectus finds
in Asia at 1.7 and 1.9 Ma. The Todaro case has two points.
1. there is evidence that humans were exposed to the retrovirus, but
lost their defense over a long period of nonexposure, thus
nonselection.
2. Thus when exposed again they were vulnerable, so were a
good vector for passing the virus to the domesticated cat which
with the advantage of much shorter generations has evolved a defense.
This is much better evidence for the bulk of present day humanity's
ancestors being out of Africa for most of the last 2.2-2.5 Myr,then
it is for them being absent from 5.5 to 4.5 Ma.
Could the "aquatic" adaptations have evolved in 1 Myr ?
Some could've no doubt, but bipedalism seems more problematic...
And an aquatic phase is not the only possible explanation:
To understand the problems involved it helps to understand the
primate with which the process of selection for bipedalism began.
If we accept the "molecular clock", divergence between the orang line
and the other large hominoids was 8-10 Ma. As it happens the
Sivapithecus population appears to have been split at about that
time as the continuity of the forests that stretched from E.Africa
to S.Eurasia at the start of the Miocene was eroded.
The process had begun about 15 Ma, by 8 Ma it had proceeded to
the point that most African apes were apparently extinct.
The case is very good that Sivapithecus is the orang's ancestor; but
this implies it was the ancestor of the African hominoids as well.
Fortunately we have a good example of Sivapithecus; it was a
quadraped like the related dryopithecines. However, it was also
an ape, not a monkey which means it's lower torso was more rigid
than a monkey's. This facilitates suspensory feeding and brachiation
in the trees, but makes quadrapedal locomotion on the ground
less effective. Indeed, the supreme brachiator, the gibbon --in
which this trend has gone the farthest, always moves bipedally
when forced to move on the ground. The gibbon is also small;
bipedalism for small "quadrupeds" is easier than for large ones:
female Sivapithecus probably weighed about 14 kilos.
The point: even for Sivapithecus -- quadrapedal locomotion was
probably not much superior to bipedal if at all.
Moreover, Sivapithecus had more reason to spend time on the ground
than did the ancestral gibbon. It had to improve at one mode or the
other --for in the late Miocene it had competition -- fierce
competition that would drive all other Miocene apes to extinction --
the monkeys. Different monkey species were developing different
digestive specializations tailored to different diets. Monkeys had
never specialized as suspensory feeders; they had flexible spines;
they were much more at home on the ground.
And since the forests were growing sparser and breaking up the ground
was becoming increasingly more important to Sivapithecus. The
forest floor was an opportunity as well as a challenge for this
ape -- as judging by its dentition and the diets of present
Pongo,Pan and Homo it had catholic tastes. The other more specialized
dryopithecines were wiped out; Sivapithecus found more food on
the forest floor. It already built nests,rain huts from foliage;
it was pre-adapted for tool use, much as was the ancestor of the
Galapagos finch that probes for insects with a cactus spine.
It carried food back up trees, it carried tools -- termite
probes, sticks, hammer stones for nuts, etc -- females
carried infants perhaps longer, perhaps more than one. The
deteroraton of their habitat increased the dangers for
both young and old, probably favoring more frequent births.
So the balance tipped toward proto-hominids becoming more effective
bipeds rather than attempting to revert to effective quadrapeds.
So why didn't they opt for knuckle-walking ?
It allows fast movement on the ground for an ape with a
relatively stiff spine. However, at c 9 Ma Sivapithecus had neither
long enough arms or a number of other traits that would have made
knuckle-walking superior to quadrapedalism.
By 5.5 Ma there were proto-hominids pre-adapted for knuckle
walking. They were credible bipeds ,tho with short legs, nonlocking
knees, still evolving pelvises. When they became more arboreal,
tho probably heavier, and the lengths of their arms increased they
shifted to knuckle walking. It facilitated movement thru dense
foliage. One population eventually filled the niche of
large forest herbivore previously filled by a dryopithecine.
A Trigger for the divergence of Pan,Gorilla,Homo ?
-------------------------------------------------
It appears the divergence wasn't very dramatic at the time.
The common ancestor was already an adequate biped; and there's
no evidence that any of the other "aquatic" human traits were
acquired at that time. Pliocene climatic changes destroyed a lot of
proto-hominid habitat. Some groups began to spend more time on the
savanna, others spent more time in the lusher mountain forests produced
by rifting. Eventually adaptation to different biomes led to
speciation.
The proto-hominids survived; selection on the savanna favored
improved bipedalism. Several nonobvious factors favored survival.
There were fewer types of large carnivores on the savanna than there
are today. Very probably the modern big cats were absent; certainly
the canids were. Hyenas are night hunters. Thus foraging during
the day wasn't quite as dangerous as one might think. Tho we've
no evidence of stone tool making til 2.6 Ma, we've every reason
to suppose tool use predates it by millions of years, probably
back to 7-8 Ma.
This scenario makes several predictions:
1. Hominids found at 6 Ma will be bipedal; but will look like
good candidates for ancestors of Pan.
2. The not-very-well-adapted bipedal ape will be found in Miocene
forest -- not savanna or seashore.
3. No one will ever find transitional forms between Pan and Gorilla
and the dryopithecines that once appeared to be their
ancestors ; there were none.
4. There are hominid tools in S.Africa used and made by A.africanus and
A.robustus tho probably no stone butchering tools.
Elaine Morgan asks:
>Fat. Yes, aging captive overfed apes (esp orangs) will become obese.I
>stated this in Scars of Evolution. But the fat adheres to the
>underlying tissues, is not bonded to the skin as in Homo and
>aquatics. Among primates only Homo has fat babies and infants. Why
>should that be?
In land mammals fat provides calories when food is scarce and
insulation. Unlike tropical apes temperate zone Homo had to cope
with an erratic food supply and large temperature fluctuations.
Human babies can be born at any time of the year, not just during
the high productivity season; fat is starvation insurance.
AAT was an interesting idea when Hardy first proposed it, but
like the conventional savanna bipedalism/brain/tool hypothesis
current at the time the evidence accumulated and the analysis
done over the last 3 decades hasn't supported it. It's becoming
clear that the traits that distinguish humans from other
apes did not all appear at once, but accumulated over thousands of
millenia of divergent evolution.
-- David Woodcock
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