Some non-human primates occasionally walk upright on two legs. So how are humans different? Watch this video to find out. NOTE: This video is silent.

Walking

One Step at a Time

The earliest humans climbed trees and walked on the ground. This flexibility helped them get around in diverse habitats and cope with changing climates.

Silhouette of Sahelanthropus tchandensis. Image courtesy of Karen Carr Studios

From at least 6 to 3 million years ago, early humans combined apelike and humanlike ways of moving around. Fossil bones like the ones you see here record a gradual transition from climbing trees to walking upright on a regular basis.

7–6 million years ago

Sahelanthropus may have walked on two legs.

Becoming bipedal

By 6 million years ago

The oldest evidence for walking on two legs comes from one of the earliest humans known, Sahelanthropus. Walking upright may have helped this species survive in the diverse habitats near where it lived—including forests and grasslands.

Leg support

6 million years ago

Silhouette of Orrorin tugenensis for femur. Image courtesy of Karen Carr Studio.

The upper part of this thigh bone is similar in size to those of other large apes. But the angled part more closely resembles that of modern humans. It formed a strong bridge with the hip to support the body’s weight, suggesting Orrorin tugenensis walked upright.

Strong knee

4.1 million years ago

Every time you take a step, you briefly stand on one leg—putting stress on your leg bones. The wide area of bone just below this the knee joint in Australopithecus anamensis is a result of that stress. It provides strong evidence that this individual walked upright.

Mostly bipedal

By 4 million years ago

Silhouette of Australopithicus afarensis with vertebrae

Fossils from around this time period come from early human species that lived near open areas and dense woods. Their bodies had evolved in ways that enabled them to walk upright most of the time, but still climb trees. As a result, they could take advantage of both habitats.

Curved spine

2.5 million years ago

The curve of your lower back absorbs shock when you walk. It is uniquely human. You can see a similar curve in the spine of this early human, Australopithecus africanus, who walked upright in a way very similar to modern humans.

Hip support

1.95 million years ago

The size and broad shape of the hip bones of Homo erectus are similar to a modern human’s, showing that this early human species had given up climbing for walking.

Fully bipedal

By 1.9 million years ago

Silhouette of Homo erectus pelvis. Image courtesy of Karen Carr Studios

The pelvis and thigh bones (outline to right) of Homo erectus are similar to modern humans, and show that this early human was able to walk long distances. That ability was a big advantage during this time period. East Africa’s environments were fluctuating widely between moist and dry, and open grasslands were beginning to spread.

Long leg

1.89 million years ago

The long thigh bones of Homo erectus enabled its owner to take long strides and therefore to walk farther and faster than earlier humans.

Compare a Chimp with an Early and Modern Human

Modern chimpanzees occasionally walk upright, but their skeletons are not adapted for regular walking on two legs. Early humans evolved skeletons that supported their bodies in an upright position. Modern humans have bodies adapted for walking and running long distances on two legs.

Skull

Drawings of skulls of a chimpanzee (left), early human (middle), and modern human (right). Image courtesy of Karen Carr Studios

The spine of a chimpanzee connects with the skull at the back, holding the head at an angle.

The spine of early humans connected with the skull underneath, stabilizing the head when walking upright.

Your spine connects with your skull underneath and near the center, holding your head firmly upright.

Upper Leg Bone

Drawings of upper leg bones of a chimpanzee (left), early human (middle), and modern human (right). Image courtesy of Karen Carr Studios

Because the connection between the upper thigh and hip bones is short in chimpanzees, the hip muscles cannot contract effectively to provide support for upright walking.

The connection between the upper thigh and hip bones was longer in early humans than in chimpanzees, and its base thicker. The hip muscles could provide support for walking.

The connection between your upper leg and hip bones is long. Its base is strong and able to withstand the stresses of walking and running.

Lower Knee

Drawings of lower knee bones of a chimpanzee (right), early human (middle), and modern human (left). Image courtesy of Karen Carr Studios

The chimpanzee knee joint is lightly built, so chimpanzees cannot rest their weight on one leg at a time to walk for long periods.

The early human knee joint was strong, enabling this early human to regularly support its weight on one leg at a time during walking.

Strong knee joints help support your body’s weight on one leg at a time while walking long distances.

Benefits and Costs of Walking

Scene illustrating a few of the benefits of upright walking. Image courtesy of Karen Carr Studios

Benefits

As environments changed, walking on two legs helped early humans survive by:

making it easier to pick fruits and other food from low-lying branches;

freeing hands for carrying food, tools, or babies;

enabling early humans to appear larger and more intimidating;

helping early humans cover wide, open landscapes quickly and efficiently.

Costs

Silhouette of a modern human with back pain

Does your back ever hurt?

Back pain and other skeletal problems are relatively common in modern humans, an unfortunate side effect of walking upright. Distributing all our weight on just two limbs can have painful consequences, like lower back pain, slipped disks, arthritis in hips and knees, and collapsed foot arches.

Try it!

Press your fingers beneath the top bones of both of your hips.

Now stand on one leg at a time.

Can you feel the muscles contract?

These are the muscles that support your body during walking. They attach to the areas that curve inward above the hip socket.