Re: Bipedalism in different substrates
From: Pauline M Ross (pmross_at_ross-software.co.uk)
Date: 07/19/04
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Date: Mon, 19 Jul 2004 09:34:04 +0100
On Sun, 18 Jul 2004 13:41:20 GMT, "Bob Keeter" <rkeeter@earthlink.net>
wrote:
>Your description provided pretty much matches the notional environment
>that Reed suggests, at least for the 3.4-3.18 mya timeframe.
[Snip]
>"[...]Thus the reconstruction predicted from the mammalian
>community is closest to that suggested by the pollen recovered from the
>deposits."
>That last is possibly the most telling I think. The pollen analyses and the
>reconstruction from the mammalian fossil record (Reed's) are closest to
>agreement. Pollen analyses show evergreen brushland close to forested
>areas. This is still not "contradictory" with your description above, just
>a slightly different spin I think.
Yes. The drawings in the encyclopedia show the whole area, but the
location of deposition is just one small part of it. Evergreen
brushland and forest is what you would expect to find around the edge
of a permanent lake. The raw data in Reed's paper (table 3, p. 298)
shows that both Hadar sites (Sidi Hakoma and Denen Dora) had high
proportions of aquatic species.
>Not that it could possibly prove a thing, but would you expect to find more
>zebra fossils around a water hole at the edge of a forest or spider monkey
>fossils in what was a grassy plain?
I would expect to find *all* fossils near water, since that's where
they are created (except for a small number in caves). That's why
Reed's paper is so interesting, in that she bypasses the taphonomic
bias of location of deposition by looking at the morphology of the
fossil species. Most people look only at the terrestrial fossils and
ignore the aquatic ones, thereby creating a different kind of
taphonomic bias.
>We also know that the mere existence of a wet environment does NOT
>greatly influence the evolution of primates, at least not in the sense of
>the influences that we would be discussing.
Most primates are highly arboreal, so water on the ground doesn't
affect them at all. They can stay in the canopy. The great apes are
different in being larger than average so they can't use the lighter
branches at the extremities to get around. Inevitably they are more
likely to have to come down to ground level and (if there's water
around) get their toes wet.
[Snip]
>So, if the ancestors of chimps, bonobos and gorillas have been consistently
>exposed to a wet environment, actually WETTER than typical hominids
>perhaps, and for a longer term than the hominid line, why are they not
>better
>adapted to that environment than your notional "aquatic hominid"?
Well, now, how do we know what sort of environment the ancestors of
chimps, bonobos and gorillas have been exposed to? We have a fair idea
where they were some 15+ Mya, and we know where they are today, but in
between? How many fossil chimps have been found? None, yet quite a
number of hominid fossils have turned up.
So either chimps and gorillas are actually descended from some of
those hominids (a bit radical, but it has been suggested), or they
were somewhere else altogether at the time. And since virtually all
fossils are deposited in water, the 'somewhere else' would have to be
somewhere without large bodies of water.
>> [Pauline] And did they in fact 'move out' at all? Maybe they stayed put but the
>> open (or more open) areas came closer to them? Maybe they stayed put
>> and increased their day range to include open (or more open) areas?
>
>I suspect that you are more right than you may be willing to accept in that
>first question and implied answer. The open areas "came to them" I suspect
>because of the overall dessication of Africa during the period in question.
>Things
>got dryer and the forest retreated leaving species in ever increasing "open
>space".
I'm quite happy to accept that the open areas 'came to them' to some
extent. I don't think, though, that any species would stay put when
their preferred habitat was retreating. As it got drier, they would
have stayed with the trees, around lakes, along rivers, in wetter
valleys, but patches of grassland would have been closer. So instead
of their territory being entirely forest, it would start to include
some open patches. And it is likely that in drier times they would
have to increase the size of their day ranges (as savanna chimps have
larger day ranges than dense forest populations).
[Big snip]
>But "evidence" is not a hypothetical "conclusion" drawn from that bone.
>The bone is irrefutable. The interpretation of the distal end wear pattern
>as being from performing the Austrailian crawl to excess, is NOT evidence!
Interpretations are not hard evidence, true. But an interpretation by
a reputable scientist in a peer-reviewed journal is certainly
acceptable *at some level* as evidence (at least until a new fossil
turns up or someone finds a flaw in the methodology). And while the
bone itself is irrefutable, virtually everything about it (age, size,
shape, markings, etc) is subject to interpretation to some degree.
[Snip]
>And the fact that our closest cousins have spent millions of years more
>than hominids, living in exactly the environments you would say our
>ancestors were molded by, have NOT adapted with ANY of the
>above "aquatic features" is a non sequiteur?
As I said above, there is no evidence of precisely what environment
they lived in. We can make assumptions, but that is all they would be.
>If you take the four great Africa apes (us included) and ranked them
>in order of their tolerance for dry climates, it would appear to be
>Gorilla, bonobo, chimp and last (and most tolerant) hominids.
>Yet the underlying theme of the AAH is that hominds got to where
>we are today only because of adapting to a wet environment!!!
And if you ranked them in order of their tolerance for *wet* climates,
you would get (perhaps) chimp, bonobo, gorilla and (last and most
tolerant) hominids. Modern humans are tolerant of almost any kind of
climate, including desert and polar. Fossil hominids are found in (and
therefore tolerant of) well-watered environments.
>>[pauline] What I really like about the aquatic explanation is its power to
>> suggest very simple solutions to otherwise puzzling features. Take
>> concealed oestrus, for example (or extended receptivity, whichever you
>> prefer). It's a real stretch to find a sensible terrestrial
>> explanation for that, and it usually ends up in a convoluted tangle of
>> pair bonding and home bases and other social factors. In water, it's
>> easy - no scent to indicate oestrus, so those individuals who were at
>> it all the time were more likely to hit the right moment and therefore
>> left more progeny.
>
>But that could only applies if the creatures are essentially in the water
>all of the time!
No, not really. A dip in water washes away the scent, and it takes a
while to reappear. Even a couple of dips a day would be enough to
select for greater receptivity.
> And even then, let me offer a reference or two and
>let me see if you would still like to hang on to this assertion connecting
>concealed oestrus to aquatic connections.
[Snip hippo reference]
A search on hippo breeding brought up this line: "When the female
hippo is ready to breed she goes out to choose a mate and he must
behave in a respectful manner as she enters his refuge." Not much need
for overt signs of oestrus if the female takes charge. Primates tend
to have some signal for oestrus.
>I would also recommend a bit of research into the whole
>"concealed oestrus" issue among aquatic mammals. 8-)
>I would recommend starting with:
>http://www.zoologi.su.se/research/tullberg/papers/99-CA-40.pdf
Well, that's an interesting paper and no mistake (although nothing
about aquatic mammals). It does make the point that receptivity
outside oestrus is a fairly common feature in primates, so humans may
not be so far from the norm. He also suggests that constant
receptivity may simply be a result of modern human living conditions,
and that the natural condition would be much the same as with the
other apes, which is certainly thought-provoking. Maybe there simply
isn't a puzzle to be solved here!
>But if the terrestrial explanations have hard and fast, existing and
>observable
>analogs elsewhere in the animal world, would you call them "convoluted"?
There is no analog anywhere in the terrestrial animal world for
year-round subcutaneous fat; for furlessness, there are only pigs (but
these things have many analogs in the aquatic world). The standard
explanation for human furlessness involves a level of heat-dumping
unseen elsewhere in nature in a medium-sized animal, combined with an
unusually prolific use of water and body salts, possibly combined with
a locomotory method which is vanishingly rare. Wouldn't you call that
convoluted?
>
>On the other hand if the aquatic solutions began to defy the existing
>examples,
>and even contradict each other in modern species, do those start to feel
>convoluted and contrived to you?
Both subcutaneous fat and furlessness are commonly found in aquatic
species, and are very rare in medium-sized terrestrial species.
Where's the contradiction?
-- Pauline Ross
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