Why do Humans Walk Upright?
Posted by shadmia on July 17, 2007
Bipedalism (Standing or moving on two appendages) is one of the defining characteristics of being human. But bipedal motion is not an exclusively human activity. Certain birds like the ostrich, which can attain speeds of 65 km/hr, and the penguin use bipedal motion effectively. Other bipedal animals include kangaroos, gibbons and giant pangolins.
Bipedalism seems to be an adaptive reaction to the environment. There have been many theories as to why humans evolved as bipeds.
- Postural feeding hypothesis – Humans learned upright posturing for picking fruits or keeping the head above water while searching for water plants.
- Provisioning model – Food gathering. Using bipedal motion was an efficient way of of transporting food. It was also useful for carrying the young as well as holding tools and weapons.
- Thermo-regulatory model – Dr. Peter Wheeler, a professor of evolutionary biology, proposes that bipedalism raises the amount of body surface area higher above the ground which results in a reduction in heat gain and helps heat dissipation. In other words, standing up keeps you cooler.
- Turn-over pulse hypothesis – About 2.5 million years ago climate change caused drier conditions which severely depleted the forests causing humans to cross greater distances in search of food. Bipedalism made their journey easier by freeing up their hands to carry food and weapons.
A more recent theory has been proposed – Because it’s just plain easier!! or in scientific jargon “energetically less costly.” In an experiment to prove this hypothesis researchers trained five chimpanzees to walk on a treadmill while wearing masks that allowed measurement of their oxygen consumption. The chimps were measured both while walking upright and while moving on their legs and knuckles. They performed the same tests on humans in order to compare the difference in the amount of energy necessary to move around. The results were published in the Proceedings of the National Academy of Sciences (PNAS)
It turns out that humans walking on two legs use only one-quarter of the energy that chimpanzees use while knuckle-walking on four limbs. And the chimps, on average, use as much energy using two legs as they did when they used all four limbs. One of the chimps used less energy on two legs, one used about the same and the others used more, said David Raichlen, assistant professor of anthropology at the University of Arizona.
“What we were surprised at was the variation,” he said in a telephone interview. “That was pretty exciting, because when you talk about how evolution works, variation is the bottom line, without variation there is no evolution.”
If an individual can save energy moving around and hunting and spend more of it on reproduction, “that’s how you end up getting new species,” he said.
Walking on two legs freed our arms, opening the door to manipulating the world, Raichlen said. “We think about the evolution of bipedalism as one of first events that led hominids down the path to being human.”
On the lighter side and just to have a little fun, check out these two video clips below of how adaptive dogs can be: