Australopithecus afarensis: The Climber That Walked Upright

Australopithecus afarensis is one of the best-known early human ancestors, a species that walked upright in eastern Africa between roughly 3.9 and 2.9 million years ago while still retaining a body built partly for climbing trees. Its most famous representative, the partial skeleton nicknamed “Lucy,” was discovered in 1974 at Hadar, Ethiopia, and remains one of the most complete early hominin fossils ever found. What makes A. afarensis so compelling is how thoroughly it sits between the ape world and our own, blending a small, chimp-sized brain with legs and feet that carried it across open ground on two feet.

How Lucy Got Her Name and Her Species

In the 1970s, a wave of fossil discoveries in the Afar Triangle of Ethiopia and at Laetoli in Tanzania produced the first substantial evidence of hominins older than three million years. The Hadar site yielded Lucy (specimen A.L. 288-1), roughly 40 percent of a single skeleton, along with the remarkable A.L. 333 locality, where remains of at least thirteen individuals were found together in what researchers informally call the “First Family.” At Laetoli, a trail of footprints preserved in volcanic ash offered direct evidence of upright walking. These fossils were ultimately grouped into a new species, Australopithecus afarensis, which at the time represented the most ancient and primitive hominin taxon known.¹

Lucy herself has been precisely dated using single-crystal argon-argon analysis of volcanic ash layers surrounding the fossils. Her remains come from sediments dated to about 3.18 million years ago, while the First Family locality dates to roughly 3.20 million years ago.² That places them squarely in the mid-Pliocene, an epoch when much of eastern Africa was a patchwork of forests, woodlands, and more open grasslands.

Walking Upright on Short Legs

The defining feature that sets hominins apart from other apes is habitual bipedalism, and A. afarensis is the earliest species for which the evidence is overwhelming. The Laetoli footprints, dated to about 3.66 million years ago, show a foot striking heel-first and pushing off the toes in a pattern that statistical analysis has found to be generally modern in aspect, though less strongly expressed than in living humans.³ More recent work comparing how arched footprints record motion found that the Laetoli tracks show a similar heel strike to modern humans but a different pattern of propulsion, suggesting the push-off phase was not yet fully like ours.⁴

Lucy herself stood only about 1.1 meters tall with notably short legs relative to her body. Her pelvis was very broad compared to modern humans, which might seem like a disadvantage for walking. But biomechanical modeling suggests the opposite. A wide pelvis coupled with short limbs could actually improve walking efficiency by allowing greater stride length through axial rotation. This would have been especially useful for a short-legged biped carrying an infant or traveling in a mixed-size foraging group.⁵ Computer simulations that built a full three-dimensional musculoskeletal model of Lucy’s lower body, complete with 52 muscles and 10 joints, confirmed that upright bipedal walking at her body size would have been energetically appropriate, producing smooth locomotion rather than an awkward shuffle.⁶

A Climber That Walked

The debate over whether A. afarensis spent significant time in trees ran for decades, and it is now mostly settled in favor of a mixed locomotor strategy. The shoulder blades tell the story most clearly. A study of juvenile and adult A. afarensis scapulae found that they display several traits characteristic of apes that hang from branches, and the juvenile shoulder blades look much like those of young African apes. Because changes in scapular shape through ape development are tied to shifts in how the animals move, the presence of these same features in australopith fossils supports the idea that climbing was a substantial part of the species’ movement repertoire.⁷

This combination makes ecological sense. A. afarensis lived in habitats that ranged from closed woodland to more open settings. Being able to walk efficiently between food patches on the ground while still climbing capably into trees for fruit, safety from predators, or nighttime sleeping would have been a powerful survival strategy. The species was not a committed ground-dweller like later Homo, nor was it a committed tree-dweller like a chimpanzee. It split the difference, and its anatomy reflects the compromise.

A Small Brain with a Slow Growth Pattern

If you expected an upright-walking hominin to have a notably large brain, A. afarensis would disappoint you. Adult brain volumes cluster around 400 to 550 cubic centimeters, roughly the size of a chimpanzee’s. A detailed study using synchrotron and conventional CT scanning of eight fossil crania found that the brain’s surface features, visible as impressions on the inside of the skull, reveal an organization that is ape-like with no features clearly derived toward the human pattern.⁸

But the same study turned up something far more interesting than brain size alone. By comparing the skull volumes of two infant specimens with known ages at death to adult volumes, researchers found evidence of protracted brain growth in A. afarensis. In great apes, the brain reaches adult size relatively quickly. In humans, brain growth stretches over many years, which is tied to the long childhood dependency period during which children learn from their social group. A. afarensis appears to have already begun moving in that direction, even while retaining an ape-like brain structure. That slow growth pattern may have been one of the earliest steps toward the extended childhood learning period that characterizes our own species.⁹

What They Ate

Diet is one of the areas where A. afarensis turns out to be more flexible than early researchers assumed. Carbon isotope analysis of tooth enamel from 20 specimens at Hadar and Dikika, spanning roughly 3.4 to 2.9 million years ago, shows that the species consumed a significant amount of foods from C4 or CAM photosynthetic pathways. In practical terms, that means grasses, sedges, or animals that ate those plants. This diet differed from that of its likely ancestor, Australopithecus anamensis, which leaned more heavily on the fruits and leaves of trees and shrubs that use the C3 pathway.¹⁰

Microscopic wear patterns on the teeth add nuance to this picture. Molar microwear textures in A. afarensis show low complexity, meaning the teeth were not being used to crack hard, brittle items like nuts or seeds. At the same time, the scratch patterns are not strongly directional enough to indicate a diet dominated by tough, fibrous foods like grass blades. The textures most closely resemble those of grass-eating gelada baboons and certain leaf-eating monkeys, though with enough differences to suggest A. afarensis was eating something softer or chewing it in a less constrained way.¹¹ A separate analysis of wear patterns on the outer surface of the cheek teeth found similarities to gorillas living in relatively open environments, consistent with a diet based mainly on succulent fruits with tougher seasonal fallback resources when preferred foods were unavailable.¹²

The canine teeth, though reduced compared to apes, also tell us something. A. afarensis canines have a distinctive wear facet along the back edge that superficially resembles the “honing” facets that other primates use to sharpen their canines into weapons. Detailed examination shows these facets are not the same thing at all. Instead, the wear blunted and dulled the back edge of the canine so that it could mesh smoothly with the lower premolar, functioning more like a chewing tooth than a display weapon.¹³ This shift away from large, sharp canines is one of the signature changes that separate hominins from other apes.

Hands Built for Gripping

One of the more surprising findings about A. afarensis involves its hands. Despite a body that was clearly still adapted partly for climbing, the hand proportions show an increased thumb-to-hand ratio that is fully human in character. That means A. afarensis had the basic anatomy for a pad-to-pad precision grip, the kind of pinch between thumb and fingertips that allows for fine manipulation of small objects.¹⁴ Having the anatomy for a precision grip does not automatically mean the species was making tools in any systematic way, but it does mean the hands were capable of more than just gripping branches.

Evidence from Dikika, Ethiopia, strengthens this picture. Cut marks and percussion marks on animal bones from sediments dated to before 3.39 million years ago show that someone was using stone tools to strip flesh and crack bones for marrow. The only hominin known from that time and place is A. afarensis.¹⁵ No shaped tools were found at the site, so the tools may have been unmodified sharp rocks rather than deliberately crafted implements. Even so, this pushes back the evidence for stone-tool-assisted meat eating by about 800,000 years compared to previously known sites and ties the behavior directly to this species.

Sexual Dimorphism and What It Might Mean

Males and females of A. afarensis differed noticeably in body size, but how much they differed has been argued about for decades. One influential analysis, using simulation techniques calibrated against modern humans, chimpanzees, and gorillas, concluded that skeletal size dimorphism in A. afarensis was most similar to that of living humans. The researchers argued that this level of dimorphism implies the species was not characterized by the extreme size differences seen in gorillas, and that the most likely reproductive strategy was principally monogamy.¹⁶

That conclusion has been challenged. A more recent study comparing postcranial dimorphism across Australopithecus species found that dimorphism differs significantly among A. afarensis, A. africanus, and modern humans, with results suggesting that intense sexual selection maintained high dimorphism in both fossil species.¹⁷ If dimorphism was substantially greater than in modern humans, the monogamy interpretation weakens, and the species may have lived in social groups with more male-male competition. The honest answer is that we do not yet have a consensus on this, and inferring social behavior from bone size is inherently uncertain.

Where A. afarensis Came From

A. afarensis did not appear out of nowhere. Its likely ancestor is Australopithecus anamensis, an older species known from sites in Kenya and Ethiopia dating to roughly 4.2 to 3.9 million years ago. The two species show progressive changes in the front teeth, jaw structure, and molar shape that suggest selection for altered diet or food processing over time.¹⁸ The pattern of these changes, where traits shared between the two are found only with geologically younger specimens, supports the idea that A. anamensis gradually evolved into A. afarensis rather than the two species splitting off from a common ancestor.¹⁹

Fossils from the Woranso-Mille site in Ethiopia, dated between 3.57 and 3.8 million years ago, fill what was once a frustrating gap in the record between the two species. These specimens show dental features closer to A. anamensis but with intermediate characteristics that blur the line between the two. The researchers who described them noted it is particularly difficult to draw a boundary between the two species given this morphological overlap, which is exactly what you would expect if one was gradually becoming the other.²⁰

Not Alone in the Pliocene

For a long time, A. afarensis was treated as the only hominin species in eastern Africa during its time range. That picture has changed. Fossils from the Burtele and Lomekwi sites show statistically significant differences in facial shape from A. afarensis and have been assigned to the species Australopithecus deyiremeda and Kenyanthropus platyops, respectively. The differences between these specimens and A. afarensis, and between each other, are consistent with at least three contemporary hominin species living in the middle Pliocene of eastern Africa.²¹ Not everyone agrees that these represent genuinely separate species rather than variants within a single diverse lineage, but the trend in paleoanthropology over the past decade has been toward recognizing more hominin diversity in this period rather than less.

Living Through Environmental Upheaval

A. afarensis was not living in a static environment. High-resolution pollen data from Hadar show that between 3.4 and 2.9 million years ago, the species experienced a major biome shift involving up to 5°C of cooling and a 200 to 300 millimeter per year increase in rainfall, occurring just before 3.3 million years ago. This shift corresponded with a global change visible in deep-sea oxygen isotope records.²² The fact that the species persisted through this substantial environmental variability suggests it was not narrowly specialized for one habitat type. Its mixed locomotor strategy, flexible diet, and ability to exploit both tree-based and ground-level resources likely helped buffer it against shifting conditions.

The broad pelvis of A. afarensis females also has implications beyond locomotion. Early hominin pelvic evolution produced a wide, platypelloid (side-to-side flattened) birth canal quite different from the rounder canal of modern humans. This shape was maintained across hominins for several million years and was not replaced by the narrower modern form until Homo sapiens appeared roughly 200,000 years ago. For A. afarensis, the wide pelvis served bipedal walking while also accommodating birth of relatively large-headed infants, though the obstetric pressures were far less extreme than in later species with much bigger brains.

Thermoregulation is another area where A. afarensis differed from later hominins. Modeling work on heat regulation during movement suggests that endurance running, the kind of sustained jogging that Homo erectus likely used for persistence hunting, would have required locomotor efficiency, sweating capacity, and hairless skin area similar to what modern humans have. A. afarensis almost certainly lacked that package, meaning sustained running across open ground in the heat was probably not part of its behavioral toolkit. Its relationship with open habitats was more cautious: foraging on the ground, retreating to trees, but not yet the long-distance strider that later species would become.