The First Steps: Tracing the Origins of Human Bipedalism

When we think about human evolution, it is easy to imagine a simple progression from an ape-like ancestor to modern humans. In reality, human evolution was much more complex. Different traits evolved at different times, and some of the earliest members of our lineage looked more like apes than like modern humans. One of the earliest and most important changes was bipedalism, or the ability to walk upright on two legs.

Our Last Common Ancestor with Chimpanzees

Humans and chimpanzees share a common ancestor, known as the last common ancestor (LCA). The human and chimpanzee lineages are thought to have separated approximately 8 million years ago, although molecular-clock estimates place the split somewhere between 6.5 and 9.3 million years ago.

The Earliest Known Hominins

A hominin is a species belonging to the human branch of the evolutionary tree after it split from the chimpanzee branch. The period between approximately 8 and 4 million years ago is especially important because it contains some of the earliest known members of this lineage. One of the most significant developments during this period was bipedalism. Evidence of upright walking appears surprisingly early in the fossil record and gives scientists a minimum age for the chimp-human split. If a fossil shows evidence of bipedalism, the human and chimpanzee lineages must have already split by that point.

Sahelanthropus tchadensis is one of the earliest known candidates for a hominin. It was discovered in Chad in 2002 and dates to approximately 7.2–6 million years ago. Fossils include a cranium, pieces of the mandible, and several teeth. Despite its early appearance in the human lineage, Sahelanthropus had a very small brain, estimated at approximately 350 cubic centimeters (cc), much closer to the size of a chimpanzee's brain than a modern human's. It also had chimp-like incisors, reduced canines, and relatively large cheek teeth in comparison to chimpanzees. So why do scientists think it may have been bipedal? One important clue is the position of the foramen magnum, the opening in the skull through which the spinal cord passes. Its position provides information about how the head was positioned relative to the spine and can therefore provide evidence about upright posture. In the case of the Sahelanthropus tchadensis, the foramen magnum is perpendicular to the eye sockets, similar to modern humans. Evidence from femur and ulna fossils found at the Sahelanthropus site also suggests that these early hominins were already terrestrial bipeds while still retaining important climbing abilities. In other words, the emergence of bipedalism did not necessarily mean that our ancestors immediately stopped climbing trees.

Another early hominin, Orrorin tugenensis, was discovered in Kenya and dates to approximately 6 million years ago. Thirteen fossils representing at least five individuals have been found. The strongest evidence for bipedalism comes from the top of the femur, or thigh bone. Its shape resembles the femur of a human biped and differs from the anatomy associated with chimpanzee locomotion.

The genus Ardipithecus provides additional evidence for early bipedalism. Two species were discovered in Ethiopia: Ardipithecus kadabba, which lived approximately 5.4 million years ago, and Ardipithecus ramidus, which lived approximately 4.4 million years ago.

The most famous Ardipithecus specimen is known as "Ardi." This fossil was extremely fragmented and took many years to uncover and reconstruct. Ardi was an adult female estimated to have been about four feet tall and had a combination of ape-like and bipedal characteristics. Her brain size, facial features, and foot were relatively chimp-like, while her foramen magnum and pelvis showed evidence associated with bipedalism. Her arms and legs were also approximately the same length. This combination of traits is important because it shows that bipedalism did not appear at the same time as all of the characteristics we associate with modern humans. Early hominins could walk on two legs while still retaining many characteristics useful for climbing.

Why Did Our Ancestors Become Bipedal?

Although bipedalism was an important evolutionary development, walking on two legs also came with disadvantages. Compared with moving on all fours, bipedalism can be slower and less stable, makes pregnancy and birth more difficult, and may have reduced climbing abilities. However, walking upright also offered several potential advantages. Bipedalism freed the hands, allowed our ancestors to see farther, helped with thermoregulation, and may have made carrying objects more efficient. 

One possible explanation for the evolution of bipedalism involves changes in the African environment. Dense forests shrank into wide-open grasslands and patchy woodlands. Walking upright allowed hominins to travel long distances between scattered food sources much more efficiently than quadrupedal apes. Furthermore, biomechanical studies show that human walking uses about 75% less energy than a chimpanzee's four-legged knuckle-walking. This drastic energy savings allowed early hominins to forage across much larger territories.

Conclusion

The origins of bipedalism mark one of the earliest major changes in human evolution. Fossils such as Sahelanthropus, Orrorin, and Ardipithecus show that our ancestors were walking upright millions of years ago, even while retaining many ape-like traits. Although scientists are still investigating exactly why bipedalism evolved and where these early hominins fit on our family tree, one thing is clear: walking on two legs was an important first step in the long evolutionary journey toward becoming human.