How and when did early humans start walking upright? For clues, researchers have been looking at feet—and, more specifically, at toes.

Bipedalism was a critical step in human evolution, and one that affected so many subsequent evolutionary changes in our lineage, from social behavior to the development of material culture,

says Sergio Almécija, a senior research scientist in the American Museum of Natural History – Division of Anthropology and an author of a new study out this week in the journal Proceedings of the National Academy of Sciences that analyzes toe joints in human ancestors, modern humans, apes, and monkeys.

Knowing how and when human features evolved in the foot are details scientists have long sought to help piece together the story of our species.

Almécija and Peter Fernandez, a researcher at Stony Brook University, have been investigating the shape of special joints, called metatarsophalangeal joints, between the long bones of the foot (the metatarsals) and the adjacent toe bones (pedal proximal phalanges), for several years. In 2015, Almécija, Fernandez and collaborators showed that, contrary to other primates, the metatarsophalangeal joints in humans face upwards, allowing extension of the joint—a functional requirement for bipedalism. On the other hand, living apes and other primates have downward-facing joints, which give feet a grasping ability similar to hands.

In the study published this week, the researchers looked at fossil primates and fossil hominins, or members of the human group, in their analyses and modeling. They found that the first instance of the metatarsophalangeal joints facing upward appeared in the lateral toes closest to the outside edge of the foot about 4.4 million years ago in the hominin Ardipithecus ramidus.

The big toe, however, maintained its downward-facing joint until the appearance of Homo, the human genus, much later, about 2.2 million years ago.

This suggests that while adapting to bipedal locomotion, early hominin feet may have retained some grasping capability, with a fully bipedal-adapted big toe evolving relatively recently,

says Almécija.

Note: This article is reprinted from materials provided by American Museum of Natural History