Human Walking: The Origins of Bipedalism in Hominids

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Evolutionary Context and Appearance of Bipedalism

A Rare Locomotion in Mammals

Bipedalism, that is, locomotion on two hind limbs, is a rare trait in mammals. Although most terrestrial mammals have been quadrupeds since the Triassic (−251 to −199.6 million years), some groups have developed bipedal locomotion. In hominids, it becomes dominant.

The First Bipedal Hominids

The first primates appeared about 70 million years ago. The oldest putative representative of the human lineage, Sahelanthropus tchadensis (~7 Ma), discovered in Chad, is known from a skull whose position of the occipital foramen suggests an erect posture; limb bones (femur and ulnas) attributed to the same site were interpreted in 2022 as evidence of habitual bipedalism, but this reading remains hotly contested (see dedicated chapter below). Orrorin tugenensis (~6 Ma), in Kenya, has a femur whose morphology is also compatible with erect walking. Later, Australopithecus afarensis (between about 3.85 and 2.95 Ma) presents well-established characters of bipedalism, confirmed by the Laetoli fossil footprints (Tanzania), dated to 3.6 million years. These footprints show a plantigrade walk close to that of modern humans.

Bipedalism in Other Animals

A Capacity Shared by Several Groups

Bipedalism is not an exclusively human trait. Birds (descendants of theropods), some mammals (meerkats, kangaroos, bears) and many primates (chimpanzees, bonobos, etc.) can adopt it temporarily. In hominids, it becomes permanent.

Sahelanthropus tchadensis: Bipedalism as Early as 7 Million Years?

A Femur at the Heart of a Scientific Controversy

Discovered in 2001 in the Djourab desert, in Chad, the skull of Sahelanthropus tchadensis, nicknamed "Toumaï," was presented as early as 2002 as belonging to a bipedal hominin, based on the advanced position of the occipital foramen, where the vertebral column inserts. A partial femur and two ulnas (forearm bones), found at the same site but not formally associated with the skull, remained apart from the debate for a long time. In 2020, a first study of this femur concluded a morphology incompatible with habitual bipedalism. In 2022, a team from the CNRS and the University of Poitiers published in Nature a detailed analysis of the femur and ulnas, concluding on the contrary that the femur presents several traits characteristic of a selection for bipedalism, while retaining indices of arboreal life.

An Interpretation Called into Question

This interpretation has since been called into question. In 2024, a team including some of the authors of the 2020 study published a critical response in the Journal of Human Evolution, estimating that the femur presents no trait exclusively associated with habitual terrestrial bipeds like australopithecines. A new comparative reanalysis, published in early 2026 in Science Advances, reaches similar conclusions and emphasizes the very fragmentary nature of the remains (only the diaphysis of the femur is preserved, without the proximal and distal ends, usually the most informative). The debate therefore remains open: Sahelanthropus could have been an occasional rather than habitual biped, or its status as a biped could remain undemonstrable in the current state of the fossils.

An Open Debate and a Methodological Difficulty

Note: this case illustrates a recurring difficulty in paleoanthropology: rare and incomplete fossils can be the subject of opposing interpretations by equally qualified teams, without a consensus emerging quickly.

Danuvius guggenmosi: A Miocene Great Ape Between Suspension and Upright Walking

"Extended Limb Clambering"

Described in 2019 from remains discovered at the Hammerschmiede site, in Bavaria (Germany), Danuvius guggenmosi is a fossil great ape dated to about 11.62 million years ago. Unlike Sahelanthropus or Orrorin, it is known from relatively complete skeletons, including vertebrae, limb bones and phalanges. Its anatomy combines extended hips and knees, comparable to those of a biped, with elongated forelimbs typical of suspensory great apes. The researchers proposed for this novel mode of locomotion the term extended limb clambering: Danuvius probably moved by walking upright on branches while holding onto upper branches with its arms.

A Common Ancestor Between Suspension and Bipedalism?

This discovery suggests that the last common ancestor of great apes and humans may have been neither a strict quadruped nor a pure suspensory animal, but an animal capable of combining both modes, with an upright trunk inherited from arboreal life. This hypothesis remains debated: as early as 2020, other paleoanthropologists contested certain elements of the interpretation (hip mechanics, orientation of the distal tibia, functional length of the lumbar column), recalling that the remains of Danuvius, although rich, also remain fragmentary.

A Contribution to the Arboreal Verticality Hypothesis

Regardless, Danuvius provides a concrete fossil argument to the arboreal verticality hypothesis developed later in this article.

The Major Theories on the Origin of Human Bipedalism

The Main Explanatory Hypotheses

Several hypotheses seek to explain the origin of bipedalism in hominids:

• Savanna theory: walking upright would allow seeing above tall grasses and moving in open environments. However, paleoenvironmental data indicate that Australopithecus evolved rather in wooded environments.
• Freeing of the hands: standing upright frees the arms to carry food. Experiments have shown a significant increase in bipedalism in chimpanzees when they carry objects.
• Use of tools: close to the previous one, this theory suggests that free hands served to manipulate tools, stimulating a joint evolution of posture and dexterity.
• Squat feeding: proposed by Kingdon, it evokes an arboreal or ground feeding favoring the vertical posture.
• Aquatic theory: Niemitz defends a semi-aquatic origin, where bipedalism would have been advantageous for moving in shallow waters. Human thermoregulation would reinforce this hypothesis.

Limitations of Current Hypotheses

Speculative Scenarios

All these theories remain speculative. Fossils are rare and partial, making it difficult to reconstruct the evolutionary scenario, as illustrated by the debates around Sahelanthropus and Danuvius presented above. It is likely that several selective pressures – ecological, behavioral, dietary – jointly favored bipedalism.

Toward a Synthesis: Arboreal Verticality

The Arboreal Heritage

Vertical posture is frequent in arboreal monkeys, useful for climbing or suspending. It is plausible that the common ancestor of hominids retained this verticality when moving to the ground. Human bipedalism would then be a terrestrial extension of an arboreal adaptation: this is precisely what the anatomy of Danuvius guggenmosi suggests, whose upright trunk associated with suspensory forelimbs offers a plausible model for this transition.

Bipedalism: A Multifactorial Process

A Complex Evolutionary Transition

Human bipedalism results from a slow evolutionary transition. It allowed the freeing of the hands, the broadening of the diet, the use of tools... but also imposes biomechanical constraints. This complex evolution illustrates the deep interaction between environment, behavior and anatomy in human history.