Here’s what you’ll learn when you read this story:
- Ardi is the oldest known partial skeleton of a hominin and shows foot features that are transitioning from vertical climbing to bipedal walking.
- While Ardi has the primitive grasping big toe of the more apelike human ancestors that came before her, other parts of her feet are more evolved.
- It is Ardi’s talus, a bone in the ankle, that lies somewhere between the morphology of the same bone in apes and humans.
Humans didn’t start out as expert two-legged walkers. Our earliest primate ancestors were far more apelike than we are today, and for a long time they relied on the trees. One of the biggest shifts in our evolution was learning to move efficiently on two feet—but pinning down exactly when that transition began has been a longstanding puzzle.
A 4.4-million-year-old female skeleton known as Ardi may be one of the clearest clues yet. She lived about a million years before the famous Lucy and belonged to the species Ardipithecus ramidus. Ardi’s remains are helping scientists understand how early hominins moved—and how bipedalism first took hold.
Ardi was discovered in Ethiopia in 1994 and remains the oldest known partial hominin skeleton. Her species had previously been identified from older finds, and Ardi provided the first chance to examine a much more complete individual. Biological anthropologist Thomas Prang of Washington University in St. Louis and his team recently revisited her anatomy. Their conclusion: Ardi could walk upright, but she still carried major adaptations for life in the trees.
As Prang explained in a study published in Communications Biology, the fossil record doesn’t match the idea that the last common ancestor of humans and chimpanzees was simply a “generic” tree-dwelling ape. Instead, the evidence points to humans emerging from an African ape-like ancestor with a more complex mix of behaviors.
That mix is exactly what makes Ardi so important. Her body is a blend of old and new traits. She had a grasping big toe suited for gripping branches, like earlier, more apelike ancestors. Yet other parts of her feet, along with features of her pelvis and skull base, hint that her species was already adapting to life on the ground and upright walking.
One bone in particular stands out: the talus. This ankle bone is the second-largest bone in the back of the foot and acts as a key weight-transfer point between the lower leg and foot. In African apes, the talus is crucial for vertical climbing. It allows dorsiflexion (bending the foot backward) and inversion (tilting it sideways), movements that help climbers keep their center of mass close to the tree and avoid dangerous backward falls.
Because talus shape is tied to how an animal moves, Prang’s team analyzed Ardi’s talus in detail. They also estimated her body mass by measuring the width of the talar trochlea—the joint surface at the top of the talus. Some researchers have argued that this joint alone isn’t a perfect guide to locomotion, since different species show overlap. Still, when you compare forefoot length, body mass, and talar width across apes, monkeys, and humans, clear patterns emerge.
In general, animals with longer forefeet relative to body size tend to have wider talar trochleae. Humans, built for bipedal propulsion, fit this pattern. So do many apes and monkeys, depending on how much they climb or walk on the ground.
Ardi’s measurements landed in a fascinating middle zone. Her talar trochlea was wide for her estimated body mass—more like a biped’s—yet parts of the talus were shaped similarly to chimpanzees and other African apes. Some talus features common in apes and monkeys suggest plantigrade quadrupedalism, meaning walking on all fours with the soles flat on the ground. Ardi shared a few of these traits, but her foot also showed improvements that would later become essential for pushing off the ground during upright walking.
In short, Ardi wasn’t a full-time tree climber anymore, but she wasn’t a modern walker either. Her talus had ape-like foundations with early hominin upgrades. Taken together, the researchers argue that this doesn’t fit a simple evolutionary path from a generalized arboreal ancestor. Instead, it suggests a stage where our ancestors combined climbing, ground movement on four limbs, and emerging bipedal abilities.
Over time, that pathway appears to have led from grasping, climbing feet to feet designed for stable weight-bearing and forward propulsion. Ardi captures a crucial moment in that transition—one where the shift to two-legged walking was underway, but the trees still mattered.
