Bipedalism

Bipedalism refers broadly to any mode of terrestrial locomotion performed on two limbs. It is not a single, uniform behavior but rather a spectrum encompassing walking, running, hopping, and even waddling gaits. The critical functional distinction is between obligate bipeds, which are anatomically committed to two-legged locomotion (e.g., birds, humans), and facultative bipeds, which normally move on four legs but can adopt bipedal locomotion under certain circumstances such as high-speed flight from predators (e.g., certain agamid and iguanid lizards). A further subcategory, habitual bipedalism, describes species for which bipedal locomotion is the primary mode but whose anatomy may retain vestigial quadrupedal capabilities.

Among vertebrates, bipedalism has evolved independently multiple times: in the archosaur lineage leading to dinosaurs and birds, in the primate lineage leading to hominins, in macropod marsupials (kangaroos), in various lizard families, and sporadically in other groups. Each instance involves a distinct suite of anatomical modifications, demonstrating convergent functional solutions to the biomechanical challenges of balancing, propelling, and steering on only two support points.

Bipedalism is considered the ancestral locomotor condition for Dinosauria. The earliest dinosauriforms of the Middle Triassic — including taxa such as Marasuchus lilloensis and the silesaurid Silesaurus opolensis — already show strong bipedal tendencies in the form of elongated hindlimbs relative to forelimbs, a parasagittal (fully erect) hindlimb posture, and a well-developed fourth trochanter on the femur indicating a substantial caudofemoralis longus muscle. The first true dinosaurs, including Eoraptor lunensis, Herrerasaurus ischigualastensis, and Coelophysis bauri, were all obligate bipeds.

The functional explanation for why bipedalism arose in this lineage has been debated. An older hypothesis proposed that bipedalism evolved to free the forelimbs for use as predatory weapons. However, Persons & Currie (2017) argued persuasively that bipedalism arose primarily for enhanced cursoriality (high-speed running ability). Their key evidence centers on the musculus caudofemoralis longus (CFL), a large tail-to-femur retractor muscle retained in the diapsid reptile lineage. Because the CFL provides a greater source of hindlimb propulsion than is generally available to the forelimbs, cursorial animals possessing this muscle are biomechanically predisposed to shift weight posteriorly and adopt a bipedal stance at high speeds. This hypothesis is supported by the observation that extant lizards capable of facultative bipedalism consistently adopt it during maximum-speed sprinting, and that mammals — which lost the CFL during the Permian, possibly in the context of adaptation to a fossorial (burrowing) lifestyle — have rarely evolved bipedalism.

The tail of bipedal dinosaurs was far more than a passive counterweight. Gatesy (1990), in a landmark study published in Paleobiology, demonstrated that the caudofemoralis longus served as the principal femoral retractor — the primary muscle responsible for pulling the hindlimb backward during the power stroke of locomotion — in theropod dinosaurs and other archosaurs. The CFL originates from the centra and transverse processes of multiple caudal vertebrae and inserts onto the fourth trochanter of the femur, a prominent ridge on the medial surface of the bone. A well-developed fourth trochanter in fossil taxa is therefore a reliable indicator of a powerful CFL and, by extension, of tail-driven hindlimb propulsion.

In early theropods such as Coelophysis, the CFL was massive relative to body size, and the tail comprised a substantial fraction of total body mass. Over theropod evolution, a progressive reduction in tail length, loss of the fourth trochanter, and diminution of CFL attachment sites document a shift from tail-based to knee-based locomotion — a transition that reached its culmination in crown-group birds, which retain only a vestigial pygostyle and rely primarily on knee extensors (e.g., femorotibialis) for propulsion.

Bishop et al. (2021) advanced understanding of tail function further with the first fully predictive, three-dimensional, muscle-driven gait simulations of an extinct terrestrial vertebrate (Coelophysis). Their simulations, published in Science Advances, unexpectedly revealed that the tail underwent pronounced lateral flexion (side-to-side swinging) during running. Rather than being a static counterbalance, the tail acted as a passive inertial damper, regulating whole-body angular momentum about the vertical (yaw) axis. This dynamic role is functionally analogous to the swinging arms of running humans: both are largely passively driven, primarily modulate fluctuations in vertical angular momentum, and are coordinated with other body movements to minimize required muscular effort. When the simulated tail was forced to swing in the opposite phase, required muscle effort increased by a factor of at least 2.6, demonstrating that proper coordination of tail movement was critical to locomotor economy.

If the ancestral dinosaur was bipedal, then all quadrupedal dinosaurs represent secondary reversals to four-legged locomotion. These transitions occurred independently in at least four major lineages: Sauropodomorpha (sauropods and their ancestors), Thyreophora (stegosaurs and ankylosaurs), Ceratopsia (horned dinosaurs), and Ornithopoda (iguanodontians and hadrosaurs, though many retained facultative bipedality). In Ornithischia alone, at least three — and potentially more — independent transitions to obligate quadrupedality have been documented.

These transitions are strongly correlated with increases in body size. As body mass increases, the mechanical demands of supporting weight on two legs grow disproportionately (scaling roughly with the cube of linear dimensions, while muscle cross-sectional area scales with the square). Larger animals therefore face increasing difficulty maintaining the speed and agility advantages of bipedalism, and the forelimbs are progressively recruited back into a weight-bearing role. Anatomical signatures of this transition include elongation and robusticization of forelimb elements, development of broad, columnar manus (hand) structures, and modifications to the shoulder girdle for weight support.

Some taxa appear to have been facultative bipeds/quadrupeds, switching between gaits depending on speed or behavior. Ornithopods such as Iguanodon and hadrosaurs likely walked quadrupedally at slow speeds but could shift to bipedal locomotion for faster movement or foraging at different heights. Evidence for this includes the disparity in forelimb versus hindlimb length, forelimb joint range of motion, and trackway evidence showing both bipedal and quadrupedal prints from the same ichnogenus.

Although both dinosaurs and humans are bipedal, their locomotor strategies differ profoundly. Human bipedalism is characterized by an upright (orthograde) trunk posture, achieved through an S-shaped spinal curvature, a short and broad pelvis, anteroposteriorly flattened femoral condyles that permit full knee extension and locking, and a centrally positioned foramen magnum that balances the head atop the vertebral column. Humans lack a tail and rely instead on trunk musculature, arm swing, and vestibular reflexes for balance.

Dinosaur bipedalism, by contrast, was characterized by a horizontal (pronograde) trunk posture. The torso was held roughly horizontal, balanced at the hip joint, with a long muscular tail extending posteriorly as a counterweight to the head, neck, and trunk anterior to the hips. The limbs were held in a fully erect, parasagittal alignment directly beneath the body — a posture shared with all dinosaurs and distinguishing them from the semi-sprawling or sprawling postures of most other reptiles. Locomotion was digitigrade (walking on the toes) rather than plantigrade (walking on the full foot sole) as in humans.

The center of mass in bipedal dinosaurs was located near the hip joint, maintained there by the balance between cranial and caudal body segments. In humans, the center of mass is higher and situated near the sacrum, supported by the pelvis acting as a structural basin.

Among living animals, birds are the most species-rich group of obligate bipeds, having inherited this locomotor mode directly from non-avian theropod ancestors. Avian bipedalism differs from that of their Mesozoic forebears in several key respects: a dramatically shortened tail consolidated into a pygostyle, a forward shift of the center of mass, a more crouched hindlimb posture with strongly flexed knee, and a transition to knee-driven rather than tail-driven propulsion.

Humans are the only habitually striding bipeds among extant primates, though many other primates (chimpanzees, gorillas, gibbons, macaques) exhibit facultative bipedalism of varying frequency and competence. According to Britannica, the evolution of the human striding gait probably occurred gradually over approximately 10 million years, with direct fossil evidence of bipedalism extending back at least 4 million years (Australopithecus) and indirect evidence potentially further.

Kangaroos and wallabies (Macropodidae) represent an independent evolution of bipedal locomotion via saltation (hopping), employing elastic energy storage in tendons for highly efficient high-speed movement. Various lizards (e.g., Basiliscus, Chlamydosaurus, some agamids) exhibit facultative bipedalism during sprinting, providing living analogs for the hypothesized early stages of dinosaurian bipedal evolution.

Bipedalism is widely regarded as a key innovation underpinning the evolutionary success of Dinosauria. By conferring energy-efficient high-speed locomotion through the erect, parasagittal hindlimb combined with the CFL-powered tail, and by liberating the forelimbs for functions other than locomotion, bipedalism enabled dinosaurs to exploit ecological niches unavailable to their sprawling or semi-erect archosaur contemporaries during the Triassic. It set the stage for subsequent adaptive radiations: the diversification of theropods into a vast range of predatory and omnivorous niches, the secondary evolution of quadrupedalism in giant sauropods and armored ornithischians, and ultimately the evolution of powered flight in the avian lineage.

In the hominin lineage, bipedalism similarly served as a transformative innovation, freeing the hands for tool use, food carrying, and gestural communication, and enabling efficient long-distance travel across open landscapes — a shift that profoundly shaped the trajectory of human cognitive and cultural evolution.

Several questions about bipedalism in the dinosaur lineage remain active areas of research. These include the precise phylogenetic point at which obligate bipedalism arose within Dinosauromorpha; the degree to which facultative bipedalism in taxa like Silesaurus represents an ancestral or secondarily derived state; the biomechanical constraints that determined when and why lineages reverted to quadrupedalism; and how the dynamic role of the tail in locomotion varied across the extraordinary diversity of bipedal dinosaur body plans — from small, cursorial coelurosaurs to giant tyrannosaurs. The development of predictive musculoskeletal simulation methods (as exemplified by Bishop et al. 2021) is opening new frontiers in addressing these questions, enabling researchers to reconstruct gaits and test hypotheses about locomotor function in anatomies with no living equivalent.