Our human ancestors walked on two feet, but their children still had a backup plan

Dikika foot fossils

More than three million years ago, our ancient human ancestors stood on two feet and walked upright. But new analysis of a child's foot from a 3.3 million-year-old fossil skeleton shows that our ancestors, at least the youngest ones, still had adaptations to help them climb trees like their apelike cousins.

The tiny foot, about the size of a human thumb, is part of a nearly complete 3.3 million-year-old skeleton of a young female Australopithecus afarensis. Zeresenay (Zeray) Alemseged, PhD, a professor of organismal biology and anatomy at the University of Chicago and senior author of the study, discovered the fossil in 2002 in the Dikika region of Ethiopia.

“The skeleton continues to provide new, really amazing insight in terms of the many adaptations that our early human ancestors had,” Alemseged said. “Now to identify features that are slightly apelike in an otherwise very humanlike creature is a bit of a surprise.”

The fossil, known as “Selam,” is from the same species as the famous Lucy fossil, and was found in the same vicinity. The popular press erroneously labeled it “Lucy’s baby,” though Selam lived more than 200,000 years before Lucy.

In studying the fossil foot’s remarkably preserved anatomy, the research team reconstructed what life would have been like years ago for the toddler, and how our ancestors survived. They examined how the foot would have been used, how it developed, and what it tells us about human evolution.

“For the first time, we have an amazing window into what walking was like for a two-and-a-half-year-old more than three million years ago,” said Jeremy DeSilva, an associate professor of anthropology at Dartmouth College and lead author of the study, published in Science Advances. “This is the most complete foot of an ancient juvenile ever discovered.”

The fossil record indicates that these ancient ancestors were quite good at walking on two legs. “Otherwise, if you are a bad upright walker, you are going to get eaten by leopards and go extinct,” DeSilva said.

Denis Geraads and Zeray Alemseged
Denis Geraads (left) and Zeray Alemseged (right) examining the small block of sandstone containing the Dikika foot. Upper right: the Dikika foot after some cleaning of the sandstone. Lower right: the Dikika foot today. (Credit: Zeray Alemseged and Jeremy DeSilva)

At two-and-a-half years old, Selam was already walking on two legs too, but there are hints in her fossil foot that she was still spending time in the trees and hanging onto her mother as she foraged for food. Based on the skeletal structure of the child’s foot, specifically a more movable big toe which allowed them to grasp branches and climb, the kids probably spent more time in the trees than adults.

This lasting ability to climb gave the younger, more vulnerable juveniles a backup plan to walking.

“If you were living in Africa three million years ago without fire, without structures, and without any means of defense, you’d better be able get up in a tree when the sun goes down,” DeSilva said.

The new findings give researchers greater insight into the workings of evolution, not just in human ancestors but in general.

The 3.32 million-year-old Australopithecus afarensis foot from Dikika, Ethiopia superimposed on a footprint from a human toddler. (Credit: Jeremy DeSilva)
The 3.32 million-year-old Australopithecus afarensis foot from Dikika, Ethiopia superimposed on a footprint from a human toddler. (Credit: Jeremy DeSilva)

“These findings are critical for understanding the dietary and ecological adaptation of these species and are consistent with our previous research on other parts of the skeleton, especially the shoulder blade,” Alemseged said.

“Placed at a critical time on the cusp of being human, Australopithecus afarensis was more derived than Ardipithecus, a facultative biped that walked on two legs some of the time, but not yet an obligate strider like Homo erectus. The Dikika foot adds to the wealth of knowledge on the mosaic nature of hominin skeletal evolution.”

Adapted from a press release provided by Dartmouth College

Zeray Alemseged, PhD

Zeresenay "Zeray" Alemseged is the Donald N. Pritzker Professor in Organismal Biology and Anatomy at UChicago. His research is interested in human evolution in general, specifically in the events surrounding the origin and diversification of early hominin. In 2000, he discovered the 3.3 million-year-old skeleton of a young female Australopithecus afarensis, the most complete skeleton of a human ancestor discovered to date.

Learn more about Prof. Alemseged