Re: AAT and the bones
alex duncan (aduncan@mail.utexas.edu)
8 Aug 1995 23:27:39 GMT
In article <81789941wnr@desco.demon.co.uk> Elaine Morgan,
Elaine@desco.demon.co.uk writes:
>
>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?
A caution about your date here -- the molecular clock is set based upon
what we know about the fossil record. For instance, the divergence of
Pongo is often used to "set" the clock to determine the divergence dates
of the other large apes. Sivapithecus is the oldest known representative
of the pongine clade, and the oldest known Siva is currently dated to
about 12.7 Myr. This will indeed render a branching date between human
and chimp of around 5 - 6 Myr, depending on which molecular data set we
use.
The point here is that we're basing our dates on the oldest CURRENTLY
KNOWN member of the pongine clade. Other potential members of this clade
include Kenyapithecus and Afropithecus, both of which are substantially
older. It is also possible that older Sivapithecines will be discovered.
In either case the divergence date for human/chimp will move to an older
point in time. We should recognize the 6 Myr you provide as the YOUNGEST
possible given our current state of knowledge.
>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.
And then again, maybe more like 3 Myr, or 4, or 5.
snip
>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.
There are plenty of humans that either climb trees, or perform activities
that result in the same kinds of stresses on the manual phalanges. These
individuals do not show the long curved phalanges that are characteristic
of apes or A. afarensis. The most reasonable explanation for why Lucy
had arboreal adaptations is that she was a partially arboreal creature.
As far as the lack of selection pressure on phalanges goes -- every
aquatic tetrapod that I've ever seen has phalanges that are rod straight
in dorsoventral aspect.
>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:
These are all nice suggestions, but they've all been adequately explained
by less far-out hypotheses that don't require us entering the water.
>(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.
Just what exactly is it about the alternative hypotheses that you find
wobbly? In fact, maybe you could clarify in detail exactly what you
think the alternative hypotheses are?
>(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.
This being so, I'm sure you can point out all kinds of exciting changes
in the respiratory canals of polar bears, who spend a lot of time in the
water.
>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.
What evidence do you have for this supposition?
snip
>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?
Must have been a pretty substantial inland sea.
>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.
Frankly, I see this as a cop-out. I can think of all sorts of potential
skeletal adaptations to living in an aquatic environment (and no, I don't
mean growing flippers and having your nostrils move to the top of your
head). If you can't come up with something better, I will make your
predictions for you. I'm becoming suspicious that you don't want to make
any predictions because you know they will be proven incorrect. I am not
keeping my head down either. I will happily make detailed predictions
based upon my own ideas about the evolution of hominids.
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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