HOW HUMANS EVOLVED TO WALK UPRIGHT

What would you do if your family had only a limited amount of food to eat? Would you stay where you lived until more food arrived? Would you allow your family to starve to death to avoid taking a dangerous step? Probably not. Well, some early primates would have agreed.

Anthropologists have proposed several good reasons primates developed the ability to walk on two legs. These include the need to free the hands for tool use and for carrying children, food, and other belongings; sexual dimorphism in food gathering; changes in climate and habitat as they moved from jungle to savanna (changes that favored a more elevated eye-position); and the need to reduce the amount of skin exposed to the tropical sun.

Small primates (chimps, baboons, gibbons, and bonobos) are omnivorous but don’t always have meat in their diets. They usually eat the food that’s far more plentiful in their world: plant matter, which includes fruit. These primates inhabited the African countryside for millions of years, spending most of their days in trees eating, and sharing with other members of their groups, the fruits that grew there.

But there were times when insufficient food grew to support the numbers of primates living in any one area. Even swinging from tree to tree for additions to their diets, eventually these resources would falter. And most of the trees they lived in only grew fruit in one or two seasons; during other times, primates needed to obtain food supplies elsewhere. Thus, these groups would seek out other trees that had more fruit, and more aggressive individuals would kill smaller animals since these primates tended to be competitive with regard to their food supply. They would find that taking fruit from another group was an easier method of increasing their food supply than seeking out more productive trees.

Those primates whose food was stolen regularly soon found a need to devise a method of preventing this from happening in the future. A few eventually developed the ability to walk upright on the savanna now and then, in order to keep a watch for bands of more aggressive primates. As with other animals, such as the meerkat and the prairie dog, this also allowed them to keep an eye on their offspring.

Thus, primates came down from the trees where it was safe, but where food was limited, in order to travel to other trees. Many primate groups still kept watch on other groups in order to steal what they had, or to overpower them and take their food. So it became dangerous to travel across open ground, even when one was hidden by the tall grasses. In addition, other groups might hide in these grasses, and pounce on individuals attempting to reach other trees.

So some primates made an effort to rise above the grasses to see if any enemies might be nearby. Those who could do this tended to stay alive longer than their peers, and thus had more offspring. Over generations, some of these offspring inherited the parent’s ability to rise above the grasses, and their offspring inherited it from them. After a few thousand years, some primates were able to rise and stand for short periods of time. This led to a genetic preference for standing more and more upright, for longer and longer periods.

Some of these primates still only walk on two legs for short periods of time, or use their fists to assist them, but humans went further and rarely walk on four limbs, preferring two, even though our skeletons still object to this mode of transport, which often causes intolerable stress on the spine.