Re: Bipedalism and other factors
Pat Dooley (patdooley@aol.com)
9 Jul 1995 19:45:42 -0400
Re: Bipedalism and other factors
>From: herwin@gmu.edu (Harry Erwin)
I wrote:
>> 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.
The human diving reflex is more than a simple nervous system adaptation.
For example, in simulated deep dives under laboratory conditions it was
found that the circulation to the limbs was completely shut down and
the heart rate slowed to about 12 beats per minute.
Other unique adaptations that facilitate human diving include:
1 .Hairlessness ( to reduce drag on descent and ascent)
2. Downward pointing nostrils (that stops water from being forced into the
nasal cavaties)
3. Descended larynx
4. Bipedalism (keeps legs, spine and head in one plane)
It's a sign of progress when you are forced to admit the existence of
biological
innovations that seem to support diving. However, your suspicion that the
"emergence of modern man with the appearence of sea-going cultures might
be
related" has some major difficulties. It implies that identical
adaptations arose
amongst unrelated cultures, (extremely unlikely), or that we are all
descended
from a single, modern sea-going culture. That, too, seems unlikely.
Your 1000 generations figure is also suspect. Aborigines have lived in
Australia for upwards of 50,000 years (3000-3500 generations) isolated
from the rest of humanity yet they show no signs of any special nervous
adaptations, despite a small population base, and a unique environment;
i.e. a situation where you might expect some evolutionary changes to
occur fairly rapidly.
>
>>
>> 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.
>
Not unfortunately, at all. It provides a better starting point than a
quadrupedal gait.
>>
>> 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.
>
What the web does now does not necessarily explain why it evolved.
You also skipped the issue of vestigal webbing.
>--
>Harry Erwin
Pat Dooley
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