Some
notes on problems I can see with the thermoregulation theory of the origin of
bipedalism
Reliance on the
The
benefit of vertical posturing to get thermoregulatory relief only applies in direct
sunlight at or around
The
convection/sweat cooling part of his theory relies heavily on there being just the right height of vegetation on
the ground. If there is no vegetation at all his calculations are that there
would be very little to gain from bipedal stance. In fact only when the grass
covering is in the range 70-120 cm is there any benefit at all - and even then
not much.
Even
a modest density of small bushes in the local habitat would spoil this model
and negate the theory.
It
is tempting to argue that the same principles would occur in a mixed woodland
environment but if it did so at all, the principles would be vastly diluted.
Wooden Environment for A.
afarensis
Ever
since Johanson (1976) we have known that Australopithecus
afarensis did not live on the savannah and that therefore the origin of
bipedalism did not happen there at all.
This
alone is enough grounds for dismissing Wheeler’s theory.
Common sense tells us that hominids would
not have gone and stood in the mid-day sun. The idea that this was a factor in
early bipedalism is just ludicrous. Have extant chimps/Bonobos or gorillas ever
been observed doing anything like this? Even if they did, what are they going
to do there? Walk around upright admiring the view? Isn’t the whole idea that
they’d be doing this for food? And what kind of grazing would they be able to
do standing upright? Picking off grass-seeds is the only one I can think of.
Scavenging, digging up roots, or whatever else would need them to bend down -
negating the thermoregulatory benefit again.
It
has been argued that this is at least a idea which would work at the level of
individual selection - so that a single hominid able to go out into the mid-day
sun would incur benefit from extra grazing - but this would only be true if the
benefit exceeded the cost.
The
benefit? A couple of hours of extra grazing time.
The
cost? Extra effort to get upright (in early bipeds), extra heat gain through
radiation, extra water loss through sweating, greater risk of predation.
I
can’t see how this would be a profitable venture. If an individual tried it I’d
predict that the behaviour would be maladaptive and soon be selected out.
Biological
If
humans had evolved on the savannah we would not have such dilute urine (1/3 of
typical savannah living animals) and we would probably not sweat as a cooling
mechanism at all.
Sweating
only makes sense as a cooling mechanism if the animal happens to live next to a
reliable source of fresh water, like a river.
Experiments only on a
theoretical Model
All
his experiments were theoretical, on a 1:5 scaled (61 cm) model.
He
could have easily performed some experiments with human subjects to determine
the amount of heat-loss and water cost of bipedal movement in savannah
environments.
The Paper
¼ of
his paper is spent outlining the basic idea that air travels faster and is
cooler further from the ground. His use of complicated-looking mathematical
formulae is impressive at first sight until you realise that they are just
stating the obvious.
The
paper would have looked far less impressive if it would have had a photograph
of him with his 5:1 scale model of a hominid!
Sweating Assumptions
In
the second paper (on water budgeting) it was not clear how he made his
calculations on water loss. He states at one point that he assumed the sweating
of his hypothetical creature would be that of a Patas savannah-based monkey but
he doesn’t make it clear whether this is the assumption he’s then using in the
water loss calculations. If he is then clearly the water budget calculations
would be distorted in his favour.
Wouldn’t
it have been better to state the assumption of the sweating capability and then
vary it in his model? We could then have determined at what point of sweat
cooling efficiency his model would fail and compare it with humans.
Also
it seemed that he made the assumption that the hominid still had all its hair.
This is understandable in the context of proposing an origin for bipedalism but
again it would have been interesting to see his calculations using different
levels of hair cover to see how that factor effects the sweating argument.
It
seems to be assumed that convective heat loss is a separate thing entirely from
sweat cooling so that paradoxically the need for sweating would be less if you
stand bipedally than if you get down on all fours. This would have been easily
to test with human subjects.
It
seems very counter-intuitive to me that convective heat loss (convection = “transmission of heat through
gases/liquid by means of currents”) and sweat cooling can be considered
separately. Surely convection would cause more sweat to evaporate, not less. I
would have thought that humans would sweat copiously in a hot savannah
environment and require a great deal of water to replace the water lost.
Early Bipedal Hominid Effort
He
didn’t seem to factor in any calculation for the extra effort and hence energy
an early biped would have to expend in order to get itself upright - a most
unnatural posture. Therefore his assumption of 1.7 x BMR probably was not
enough.
His arguments are
self-defeating.
If we became bipedal to keep cool we would have to live in the open savannah,
but on the open savannah we would not have had enough water to survive (as he
admits himself we would need at least 1.5 kg/day - and this is using all if his
assumptions). If the hominids went closer to a water source, there would
undoubtedly have been more bushes and trees which would have removed the
thermoregulatory advantage he is suggesting.
This
also places another assumption onto these hominids – that they could walk
fairly long distances - surely unlikely at the beginning.