The need for ancient humans to keep cool during the day might explain
their lack of body hair but not why they walked on two feet.
Bipedalism didn’t evolve as a way for ancient humans to keep cool during
the heat of the day, according to a new model published today (December
12) in Proceedings of the National Academy of Sciences. But once
hominins did start walking on two feet, it ignited another change that
allowed them to stay cool—the loss of body hair. The new model explains
why similarly sized mammals that walk on all fours and that may tend to
overheat have not given up their coats.
“If you are already walking upright for other reasons it actually makes
the advantage you get from losing hair bigger than if you were on four
legs,” said David Wilkinson of John Moores University in Liverpool, who
authored the study along with Graeme Ruxton of the University of
Glasgow. “You are moving more of your body up above the ground and sweat
evaporates more easily” than it can if you were on all fours, because
more air will circulate around you, Wilkinson explained.
Wilkinson and Ruxton came to this conclusion after analyzing a
mathematical model of body temperatures during activity at different
times of the day for quadrupeds and bipeds with and without fur. The
model is an update to a previous theory by Peter Wheeler also of John
Moores University, who proposed that both hair loss and bipedalism were
driven by our need to cool down. His theory was that switching from four
to two feet would reduce the amount of an animal’s body in direct sun
and thus increase its ability to stay cool.
But Wheeler left out a critical factor, Wilkinson said—animal movement.
Stationary animals could just hang out in the shade during the peak of
the day to avoid overheating, he noted, while activities such as
foraging likely forced early humans into direct sunlight more often.
Taking movement into account, Wilkinson and Ruxton’s model predicted
that modern human ancestors would generate much more body heat
metabolically as they traveled and hunted than the sun could cause,
suggesting that standing upright to avoid the sun, as Wheeler’s model
proposed, would have done little to fight overheating.
“In Peter’s models, he had a nice thermal advantage to standing
upright,” said Wilkinson, “but now that vanishes in our version of the
model.”
The new model further showed that four-legged creatures do not shed body
heat as quickly when they lose their fur, suggesting that the loss of
body hair would only have been a significant advantage to ancient humans
if they were already walking on two feet. Thus, Wilkinson and Ruxton
argue that bipedalism arose first—for some reason other than heat loss,
such as improved observation of dangers, appearing larger to predators,
or freeing the hands for tool use and carrying—then hair loss began, as
a way to combat overheating.
The addition of animal movement to the model was key, said Sarah Elton
of the Hull York Medical School, UK, who was not involved in the study.
“In any environment you move between parts that are shaded and parts
that are in open sun…. sometimes you are sheltered from the wind or not.”
But while Elton is generally in praise of the model, she pointed out
that, “at the end of the day, it is just a model and models stand and
fall on the type of evidence and also on the sensitivity of the model
itself,” or the degree to which it is affected by variations in the
assumed parameters, such as the climate, early humans’ movements,
availability of shade, and so on. “There are other ideas” about why
humans may have dropped their body fur, such as selective pressures
imposed by the opposite sex, like a preference for hair-free mates.
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