Why Be Bipedal?

I've been thinking about hominins lately, because of a class I'm in at the American Museum of Natural History on human origins.

When a primate fossil is found, researchers try to figure out what group of primates it belongs to. One of the key features for determining whether or not a fossil belonged to a hominin, is to tell if it was bipedal, ie. walked on two legs.Ways to tell are to see where the foramen magnum exits the skull (directly underneath for bipedals, back for quadrupedal), and if the femurs are angled in or not ( angled in femurs are a sign of bipedalism). It's funny that you can tell how an animal walked by its head, but it makes sense because the heads of quadrupeds stick out in front of their bodies while the heads of bipeds sit directly above their bodies.

There are countless theories about why hominins and their descendants (us!) gave up living in trees and walking around on all fours for standing upright. The thing is, bipedalism isn't completely beneficial. Bipedals are much more prone to lower back and knee problems, and they also tend to be weaker than quadrupeds.

A Man Running by Eadweard Muybridge

Hominins are bipedal, but the question is why? Why bother being bipedal in the first place?
Some benefits our early ancestors had due to being bipedal were: the ability to see over tall grass, the ability to carry children etc., more cooling regulation because less of the body is directly exposed to the sun, more energy efficient movement, and the ability to make and use tools and weapons. Some sexual selection may have also played a role.

The most intriguing theory of all of these, in my opinion, is energy efficient movement. Walking on two legs takes less energy than doing the same thing on four. Also, hominins have less hair and sweat more than other animals, allowing them to cool off while moving rather than having to stand in the shade for awhile. Freed hands allow hominins to carry provisions for long journeys. All these abilities allow hominins to persistence hunt, or run down an animal till it dies of exhaustion. This method of killing, still used today by some tribes in Africa, allowed hominins to obtain high caloric meat even without advanced weapons.

Some scientists get in heated arguments over the specific reason for bipedalism, but the truth is that all of these factors probably played a role, and we may never know exactly what happened.