WALKING TALL - AND COOL

How come humans ended up walking on two legs? Because they found it cooler that
way, says a British researcher.

On the African savanna about 4 million years ago, our ancestors found that
walking upright exposed less skin to direct sunshine and enabled cooler air to
circulate more freely over their upper-body regions, according to Peter Wheeler,
director of biological and earth sciences at Liverpool Polytechnic University.

In the past, other scholars proposed that bipedalism evolved because it allowed
humans to look over tall grasses or kept their hands free for carrying babies or
wielding tools, But Wheeler's theory concentrates on "the single most stressful
feature of the environment at the time-the strong solar radiation and the lack
of drinking water," he says.

Using a calculation based on a model of an Australopithecine, the apelike
creature that evolved into early hominids, Wheeler found that walking upright
would have been a much cooler mode of locomotion, requiring less drinking water
than the traditional knuckle-walking. "I estimated that a knuckle-walker would
have needed about 2.5 liters of water a day, a biped about 1.5 liters," he says.

By walking upright, early Australopithecines could also wander greater distances
to collect food in the intense heat of the day. It became advantageous for them
to lose the thick fur of ancestral apes, which protected against direct solar
radiation but prevented heat loss from the skin. Wheeler believes his theory
thus explains why humans are the only naked apes. Further, the cooling effect of
walking upright allowed humans to become taller and thinner and for their
brains-sensitive to overheating-to increase in size.