Natasha, a macaque in an Israeli zoo, began walking exclusively on two legs after almost dying from an illness.

Why Humans Walk on Two Legs: Other Theories

Scientists claim that walking on two legs was one of the keys to humans' development from ancient ape-like ancestors. Walking on two legs saved energy and allowed the arms to be used for activities like hunting, crafting simple tools and interacting with objects. Charles Darwin proposed long ago that having two limbs free to use tools constituted a key element of advanced intelligence [source: National Geographic].

Prior to the chimp-treadmill study, which was the first study examining upright walking in adult chimpanzees, scientists debated a variety of explanations for why humans walk upright. Many of these explanations invoked the idea of energy conservation in one way or another.

One theory proposed that walking on two legs freed the arms, which could then be used to collect food to bring to the family unit. If a primate walked on all fours, he couldn't carry much food back to the family without embarking on multiple hunting or foraging expeditions. It required less energy to provide for a family if the male could walk upright and return to his mate and young with enough food for all of them. The female could then stay with the young and take care of them, ensuring their health and protection from predators.

A second theory proposed that hominids started walking upright when traveling through water. Chimps do this today, rising up on their hind legs to wade through a pool or creek.

Still another theory proposed that our ancient ancestors rose up on their hind legs in order to cool themselves. By standing upright, they exposed less of their bodies to the sun.

Changing habitats and ecological conditions can have a dramatic effect on animal behavior, sometimes forcing species to adapt, flee or die. Some researchers believe that several million years ago, a warming climate and declining forest habitats meant that our forebears had to undergo longer journeys to find food. Walking on two legs made these journeys less taxing. A related possibility is that the changing climate forced primates to become primarily ground-dwellers, rather than living in trees and forest canopies. Food sources became more plentiful on the ground, where primates would have had an advantage by walking on two legs.

The final theory asserts that our ancestors never had to leave the trees to learn to walk on two legs. Instead, they learned while still living above ground. Orangutans provide a modern-day analog, as they often stand on two legs on tree branches and grab onto other branches with their front limbs in order to stay balanced.

The treadmill study may provide the best case for explaining why humans evolved to walk upright. But a fascinating story from 2004 shows that nature continually surprises and confounds even the most experienced researchers. That year, a five-year old black macaque named Natasha, living in a zoo in Israel, almost died from a bad case of the stomach flu. After regaining her health, Natasha inexplicably began walking upright all the time -- and with remarkably good posture. While monkeys often walk upright for short periods, they never do so consistently and straight-backed like a human. Three other monkeys living with Natasha had the stomach flu, but none of them displayed her post-illness behavior. The veterinarian treating her said that brain damage might be the cause but that he had never heard of a monkey walking only upright before.

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