Orangutan swings through a rope playground using his upper limbs

From four legs to two

Why did humans evolve to be bipedal?

Most animals get around fine on four, six or eight legs. So what made us bipedal, walking and running on just two? It is unlikely that we will ever know the real answer, but by studying our closest living relatives, the great apes, alongside fossil data, we can begin to build a picture of how our common ancestor may have moved.

Evolution of Man Illustration

‘The Evolution of Man’, 1879

Wellcome Library, London

One prevailing current theory is that we evolved from an ancestor that moved around using quadrupedal knuckle-walking, much like our African ape relatives (chimpanzees, bonobos and gorillas) do today. Later, our ancestors stood up and began to move around on two legs. Various reasons for this have been proposed, including improved fighting ability, improved carrying ability or reaching food on low branches from the ground.

Recent fossil evidence, however, suggests that we spent more time in the trees than previously thought. Orang-utans are the most arboreal (tree-dwelling) of the great apes, and recent studies show that they use a human-like form of straight-legged bipedalism to move around on the very thin branches in the trees to obtain food. So, we might even have been using some form of bipedalism before we came down to the ground.

Lead image:

Shankar S/Flickr CC BY


About this resource

This resource was first published in ‘Exercise, Energy and Movement’ in January 2012 and reviewed and updated in August 2016.

Genetics and genomics, Physiology, History
Exercise, Energy and Movement
Education levels:
14–16, 16–19, Continuing professional development