Re: Bipedalism in different substrates

From: Pauline M Ross (pmross_at_ross-software.co.uk)
Date: 07/17/04 

Date: Sat, 17 Jul 2004 17:01:16 +0100

On Sat, 17 Jul 2004 01:46:52 GMT, "Bob Keeter" <rkeeter@earthlink.net>
wrote:

>Now, Pauline. Is this an answer to the question posed in the quote you
>so generously included above? OBVIOUSLY something favored bipedalism
>or we would not be bipedal. Wading is one of the options as is the need
>to free up the forelimbs for flying, but some options can be discounted
>quite easily for one reason or another. Im asking that you apply the same
>logic to the "wading" hypothesis as you did elsewhere. Step by step,
>with your own intellectual honesty vetting the conclusion of that logical
>process! 8-)

The logic in the quote above is that if a species shows adaptations
for activity X, then it must have been doing a certain amount of X. So
if hominids show adaptations for bipedal locomotion, then they must
have been doing a certain amount of bipedal locomotion.

But (so far as I know) there are no specific anatomical adaptations
just for wading. In other words, a species which wades is not
distinguishable at the skeletal level from one which is terrestrial
(just as otter skeletons are apparently indistinguishable from weasel
skeletons).

So all you can say is that hominids must have been doing a fair amount
of bipedal walking, but whether on land or in water is impossible to
say, and as for why - well, living analogues are the best place to
start looking for reasons.

>Yep. BUT the sampling techniques used for these isotopic studies depend on
>powdering and even vaporizing a small sample of the enamel for the mass
>spectrometer. I would think that it would be difficult to the point of
>impossible
>to extract enamel from only one year's deposit much less any subset of a
>year.

OK, thanks, but that means that the 'tree-rings' are probably not
relevant in the context of isotopic analyses.
>
>When analyzed the carbon isotopes showed a carbon accumulation that could
>be explained by the diet of a savanna-living hyena. Hyenas eat meat, one
>way or another, year round.

Yes, but that doesn't mean that the hominids ate exactly the same diet
as hyenas, does it? It means only that they had the same *proportions*
of C3/C4 foods in their diets, which was intermediate between pure C4
(grazers) and pure C3 (browsers and frugivores), but nearer the C3 end
of the spectrum.

It is probably safe to presume that the hyenas were living mainly on
meat, but you can't make the same assumption about the hominids. They
*could* have been entirely vegetarian and still had the same C3/C4
proportions as the hyena. And the variation between individuals is
quite large (-5.6 to -11.3).

>Could you point me at that paper on the isotopic variation by the way? 8-)

Nikolaas J van der Merwe, J Francis Thackeray, Julia A Lee-Thorp,
Julie Luyt 'The carbon isotope ecology and diet of Australopithecus
africanus at Sterkfontein, South Africa' JHE 44 (2003) 581-597.

They get a range of C4 from 30% to 60% (with Makapansgat included that
becomes 0% to 60%). I only seem to have the printout, but I have a
couple of other papers by Sponheimer and Lee-Thorp if you want them.

[Snip termites]

>> >[Bob] ABSOLUTELY! If there was every a profound point of agreement
>> >you have found it. We were always NEAR water, just like most other
>> >totally non-aquatic mammals!
>>
>>[Pauline] Hey, we agree on something :-)
>
>Does that mean that you REALLY do agree that even totally non-aquatic
>mammals are still "tied" to water? What kind of "non-aquatic" mammal
>would you be referring to? After all that would set a "threshold" for
>example.

I agree that we (ie our ancestors) were always near water. I don't
think I would agree that most other totally non-aquatic mammals are
always near water - it kind of depends what you mean by 'near'. Is a
chimp always 'near' water? How about a giraffe?
>
>If a mammal was "less connected" to water than an "X", it is by definition
>non-aquatic! That would be a very powerful statement dont you think?
>8-) Powerful and at the same time very "sticky" from a logical standpoint
>wouldnt you say? ;-)

You're in binary mode, Bob - aquatic/non-aquatic. In the real world,
there is a continuum.

>If anything, I would offer that the "sweating" is clear and "obvious"
>proof that our ancestors (you pick the point in time) had to exist in
>a hot, and at least relatively dry climate. The simple reason is that its
>only in such a climate that sweating is at all efficient.

I agree. Hot, dry and with abundant water to replenish that sweated
off. And Paul's point about salt is a good one.

You see, again you are in binary mode. You assume that a hominid must
have cooled off *either* by dunking in water *or* by sweating, but the
two could easily have developed in concert. When you are right beside
the water, you take a dip to cool off; when you have to spend time
away from the water, you sweat to cool off. Hominids probably always
exploited a variety of local resources, so sometimes they would be in
or near the water and sometimes not.

[Snip maths]

>Pauline, have you ever spent any time in a very hot, very dry place?
>You will go through water like its going out of style. But quite frankly
>so long as you have plenty of water to push through your marvelously
>efficient evaporative cooling system you can survive some very ugly
>ambient temperatures.

Yep. Lived in Texas for 3 years, toured all round the SW states, did a
fair amount of walking around in very hot, very dry places. I found I
had to drink pretty much continuously, although I suppose anyone who
was used to being outside would need to drink less.
>
>I agree entirely with the proposition that a hominid would be in woodland
>areas near water, but I would also contend that at least the margins of
>the true grasslands and virtually all of the "open woodland" areas would
>be viable. 8-)

Yes, absolutely. I think hominids were always generalists, exploited
whatever was available.

> So long as they can be close enough to open water to get a
>good filling drink once a day, and you will be surprised at just how much
>a hominid can drink after a long day in the sun, they dont even need
>a water skin or ostrich shell.

I'm not sure that humans can drink just once a day. A lot of
open-grassland grazers can do that, but I don't think humans can.

>> [Pauline] OK. Clubs are a possibility, digging sticks a bit more of a stretch,
>> but we can agree that it is all conjecture - or, as I prefer to put
>> it, there is no evidence... ;-)
>
>Did you say that! If so, Im claiming a major coup! "There is no evidence"
>is a damn-fine phrase. Remember it.

Oh, I will :-) But remember that PA has precious little hard evidence
of any kind. A few well-scattered fossils and artefacts, some
ecological data, extant species and some genetic data. And every last
bit of it disputed and argued over. It's astonishing how much
information people manage to glean from so little base data, but it
does suggest caution; your Sponheimer/Lee-Thorp paper is based on just
4 samples of A. africanus, for example.

>> [Pauline]But I do wonder why you feel such a missionary zeal to reform all us
>> poor misguided folks...
>
>Perhaps the same reason you want to "reform" all of us poor misguided
>savanna folk!

But I don't. I actually don't care what you (or anyone) thinks.
Everyone is welcome to look at the evidence (such as it is) and make
up their own minds. I enjoy our discussions, but I have no thought of
'reforming' you; the objective (for me, anyway) is simply to clarify
my ideas and learn something in the process. 

-- 
Pauline Ross

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