Re: Bipedalism and other factors
Harry Erwin (herwin@gmu.edu)
Tue, 04 Jul 1995 10:47:03 -0400
In article <3t7gvd$972@newsbf02.news.aol.com>, patdooley@aol.com (Pat
Dooley) wrote:
> herwin@osf1.gmu.edu (HARRY R. ERWIN) wrote (in part):
>
> >It's easy to envision early hominids having some skill with water (i.e.,
> >behavioral adaptation), but without a great deal of skeletal adaptation
> in
> >that direction. Look at cats--tigers take to the water easily, while most
> >species avoid it. I suspect this can be tabled as having no strong
> >evidence in either direction. In any case, hominids probably did not
> >spend most of their time in the water. Not with limbs adapted to climbing
>
> >and bridging.
>
> Jaguars take to the water even better than tigers. They collect a
> substantial
> proportion of their prey from water. However, neither they, nor any other
> primates, has the ability to dive to any depth.
>
> The upper limit on unassisted human diving performance
> is about 250 feet. Some human groups regularly dive to a depth of 80
> feet. These aren't just learned capabilities - there are physiological
> adaptations to support them, including conscious control over
> breathing, a heightened diving reflex that slows the heart rate
> down from 72 to 35 beats per minute, and an ability to hold ones
> breath for 3 minutes or more.
These are features that could have been acquired fairly late, especially
since they are nervous system adaptations, an area where biological
innovations are known to be particularly easy. Fixation would have taken
no more than 1000 generations. My suspicion is that the connection of the
emergence of modern man with the appearence of sea-going cultures might be
related.
>
> Such features would not be surprising if our closest relatives
> could achieve some significant fraction of these capabilities.
> But, there is no sign of such capabilities in them or in other
> fully terrestrial mammals.
>
> It can also be argued that human limbs are partially adapted for
> swimming and diving. In particular, a human being can swim or dive with
> their arms, legs, head and body in a plane. That improves efficiency
> and is a feature of most semi-aquatic and aquatic mammals.
Ah! I see your argument. Unfortunately, those features are also useful for
arboreal movement of small apes.
>
> There is also the issue of residual webbing in humans. That useless
> flap of skin between your thumb and forefinger is the only thing
> that restricts the movement of your thumb back another thirty degrees;
> other apes don't have such a flap. A significant percentage of humans
> still
> have further vestigal webbing between their fingers and toes.
The thumb/first-finger web is the primary path by which large external
forces are coupled to the hand structure in man. In the apes, the thumb is
reduced or (in the gibbons) adapted to swing out of the way and the forces
are transmitted via the fingers, which are vulnerable to breakage and
other injury and so must be fairly tough and indelicate. Yes, the web
restricts thumb mobility, but it allows the thumb and fingers to be
delicate manipulatory appendages with a reduced risk of damage.
--
Harry Erwin
Internet: herwin@gmu.edu
Home Page: http://osf1.gmu.edu/~herwin (try again if necessary)
PhD student in comp neurosci: "Glitches happen" & "Meaning is emotional"
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