AAT and the bones
Elaine Morgan (Elaine@desco.demon.co.uk)
Tue, 08 Aug 1995 16:17:33 GMT
This is where the paleontologists are fighting on their own ground and
saying "Come into my parlour"
As in all sectors of this debate, we seem to be at cross purposes about
the time scale. One line of argument is that after four million years
in the sea, the skeleton of afarensis ought to look less like an ape
and more like a dolphin.
It could not be anything like four million years. To date the
beginning, we have to ask the microbiologists for the date of
speciation. In Ralph's scenario that is not important: sympatric
speciation in the trees could have happened at any time during the
fossil gap. It matters to me because I suspect the sea flooding was
what triggered the speciation. The date given varies with whether they
are using DNA or proteins or whatever - but would you agree a mean date
of around 6mybp?
As for the end of the aquatic phase, we have fossil evidence going back
to around 4m of creatures which were now (back) on the mmainland of
Africa. Their bones suggest that fully weight-bearing (i.e. terrestrial)
bipedalism had been going on for quite some time, perhaps since say
4.5mybp. So we are talking about certainly less than 2my for the
formative aquatic period, maybe nearer one million.
Hense we're not exactly looking for a torpedo-shaped ape. With whales
and dolphins that took 70 million years, with sirenians 50 million.
No-one
that I know has speculated about the kind of pressures that induced the
ancestors of these and other aquatics, time and time again, to return
to the water. We will never know, but it's a good mental exercise
because it's hard to avoid thinking in terms of recurrent drastic
ecological events like floods
It also starts you thinking about what might have been different in the
case of an ape: answer, clearly, its arboreal habitat. In the
flooded-forest scenario, populations could have been cut off (i.e.
begun speciating) while still remaining fully arboreal as long as the
food supply lasted, and the treetops were above water. Later, when
water levels fell, and they began to discover and exploit the aquatic
food resources, they would find them more rewarding than leaves and
spend an increasing percentage of the daytime hours in the water.
But there is no reason to assume they would not sleep in the trees
as they had always done, as long as the trees were there. On the
southern end of Danakil (ex-Island) Alps the forest has never gone
away.
I do not find it at all surprising that Lucy has features like curved
phalanges and partly prehensile toes. You dont evolve flippers in a
million years, and you don't lose your ability to climb as long as you
practice it once a day. Even the gorilla, once it is grown up, only
goes upstairs to sleep, but it still recognisably ranks among the
quadrumana. There may have been a time at the height of the aquatic
interlude when some of them were on shore lines without trees. I still
don't envisage them asleep on the deep. Maybe they still climbed rocks.
In any case case there'd be no selection pressure (we are not talking
about tool-makers) to uncurve the phalanges. As for the toes, for any
biped walking on mud or sand, some degree of prehensility would be an
asset.
You might object here that I am postulating minimal skeletal changes in
the aquatic interlude, and yet I have suggested that a lot of
non-skeletal changes took place at this time - naked skin, fat layer,
disturbance of sodium homeostasis, descent of larynx, loss of estrus,
etc. - many of which S/m ers would like to post to 2.5mybp and the
savannah. But:
(a) I think bipedalism was an aquatic adaptation to wading behaviour.
and that was far from minimal. I know you don't accept that but your
alternative hypotheses really are pretty wobbly and tentative.
(b) I would think it very probable that in general soft-tissue changes
take place more rapidly. If you duck under water to dig out a
mussel, the first thing affected is the respiratory canal, not the
skeleton.
>From 4mybp onwards I see nothing much to disagree with in the analysis
of Susman et al, except that I think the hominids were physiologically
tied more closely to the waterways than any of you indicate because
they had already acquired Newman's "unique trio of conditions -
hypotrichosis corpus, hyperhydrosis, and polydipsia" i.e. nakedness,
sweatiness, and the necessity of drinking little and often.
About the taxonomy, I would like to raise one query about the "bush"
metaphor used for all those graciles and robustuses. It can be seen as
implying that bipedalsim arose once, in one place, in one group, and
the descendants of that group radiated out and acquired widely
different characteristics. Is that how you read it?
In my scenario there may have been a number of groups isolated from one
another by water in different places but all suject to the same kind of
evolutionary pressure. Perhaps some could have as Ralph suggests
speciated sympatrically earlier on. So the "bush" of bipedal hominids
could have had more than one root. What do you think?
As for the challenge to predict Ramidus's postcranial skeleton, it is
(may I say as usual?) a little one-sided. AAT-ers are to stick their
necks out while the establishment keeps its head down and waits for us
to come a cropper. Still, if that is the name of the game, what the
hell.
I predict that you will find nothing more surprising in the skeleton of
ramidus than you found on the dark side of the moon. His fingers will
be like Lucy's. Pelvis and feet will be much more hominid-like than
ape-like. They may be slightly less adapted for bipedalism but not
very much. Whatever was happening there had been happening for a long
time; it did not begin with the opening out of the forests.
Okay? Place your bets.
Elaine
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