Biology
The Evolution of Human Bipedalism in Paleoanthropology
Quick fact
The oldest unequivocal evidence of habitual bipedalism is the 4.4-million-year-old skeleton of Ardipithecus ramidus, nicknamed 'Ardi,' whose pelvis and foot bones already show a mix of walking upright and climbing trees.
Why this is interesting
You walk on two legs every day without thinking. But why did our ancestors first rise from all fours—and what pushed them to make this the very first step in becoming human?
Read the full explanation
Understanding The Evolution of Human Bipedalism in Paleoanthropology
Bipedalism means moving on two legs. For humans, this involves a straight spine, a bowl-shaped pelvis, and legs that line up under the body. When paleoanthropologists find fossils, they look for clues like the position of the hole in the skull where the spinal cord enters (the foramen magnum)—if it's located underneath the skull, the animal held its head upright, a sign of bipedality. The hip bones and the angle of the thigh bone also reveal whether a creature walked upright. The first major fossil to show such adaptations was 'Lucy' (Australopithecus afarensis), who lived about 3.2 million years ago. But even older fossils, like 'Ardi' (Ardipithecus ramidus) from 4.4 million years ago, suggest that bipedalism began much earlier. The story is not a simple line from ape to human; it's a branching tree of hominins experimenting with upright walking in different ways.
A deeper explanation
The emergence of bipedalism is best understood as a response to changing environments. Around 6–7 million years ago, African forests shrank and savannas expanded. One classic hypothesis (the savanna hypothesis) argued that our ancestors left the trees to forage on open grasslands, where standing upright gave them a better view of predators and allowed them to carry food. However, new fossil discoveries suggest that bipedalism may have first evolved while hominins still spent time in trees. The skeleton of Ardipithecus shows a grasping foot and long arms, indicating a life in the trees, yet the pelvis allowed upright walking. This has led to a revised view: bipedalism could have been an efficient way to move between scattered patches of trees and to feed on low-hanging fruits. Anatomically, bipedalism required a suite of changes: a shorter, broader pelvis to support the internal organs, a curved lower spine, and a foot with arches to absorb shock. These changes did not come all at once, and they came with costs—back pain, hernias, and a narrower birth canal. The fact that our ancestors persisted with bipedalism for millions of years before brains expanded suggests that walking upright was the first major evolutionary shift that made us human, setting the stage for later changes like tool use and encephalization.