Ancient hominin foot fossil adds insights to mobility over 3 million years ago

Scientists report on a nearly complete hominin foot from Dikika, Ethiopia, and its implications for Australopithecus afarensis.

A rare juvenile foot fossil of our early hominin ancestor, Australopithecus afarensis, exhibits several ape-like foot characteristics that could have aided in foot grasping for climbing trees, a new study shows. This finding further challenges a long-held assumption that A. afarensis, a hominin that lived over 3 million years ago, was exclusively bipedal (using only two legs for walking) and only occasionally climbed into trees. 

“For the first time, we have an amazing window into what walking was like for a 2½-year-old, more than 3 million years ago,” says lead author, Jeremy DeSilva, an associate professor of anthropology at Dartmouth College, who is one of the world’s foremost authorities on the feet of our earliest ancestors. “This is the most complete foot of an ancient juvenile ever discovered.”

The tiny foot, about the size of a human thumb, is part of a nearly complete 3.32-million-year-old skeleton of a young female Australopithecus afarensis discovered in 2002 in the Dikika region of Ethiopia by Zeresenay (Zeray) Alemseged, a professor of organismal biology and anatomy at the University of Chicago and senior author of the study. Alemseged is internationally known as a leading paleontologist on the study of human origins and human evolution. Although this 3.32-million-year-old fossil was announced in a previous 2006 study, many of the skeleton’s elements, including the partial foot known as DIK-1-1f, were encased in sediment and therefore had to be carefully uncovered. Many of these structures have now been exposed after additional preparatory work through 2013. 

“Placed at a critical time and the cusp of being human, Australopithecus afarensis was more derived than Ardipithecus (a facultative biped) 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” explained Alemseged.

Given that the fossil of the tiny foot is the same species as the famous Lucy fossil and was found in the same vicinity, it is not surprising that the Dikika child was erroneously labeled “Lucy’s baby” by the popular press, though this youngster lived more than 200,000 years before Lucy.

In studying the fossil foot’s remarkably preserved anatomy, the research team strived to reconstruct what life would have been like years ago for this toddler and how our ancestors survived. They examined what the foot would have been used for, how it developed and what it tells us about human evolution. The fossil record indicates that these ancient ancestors were quite good at walking on two legs. “Walking on two legs is a hallmark of being human. But, walking poorly in a landscape full of predators is a recipe for extinction,” explained DeSilva.

At 2½ years old, the Dikika child was already walking on two legs, but there are hints in the fossil foot that she was still spending time in the trees, hanging on to her mother as she foraged for food. Based on the skeletal structure of the child’s foot, specifically, the base of the big toe, the kids probably spent more time in the trees than adults. “If you were living in Africa 3 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,” added 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 noted.

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Left block of images: The 3.32 million-year-old foot from an Australopithecus afarensis toddler shown in different angles. Right block of images: The child’s foot (bottom) compared with the fossil remains of an adult Australopithecus foot (top). Jeremy DeSilva & Cody Prang

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The 3.32 million-year-old Australopithecus afarensis foot from Dikika, Ethiopia, superimposed over a footprint from a human toddler. Jeremy DeSilva

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The Dikika foot is one part of a partial skeleton of a 3.32 million-year-old skeleton of an Australopithecus afarensis child. Zeray Alemseged

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Article Source: AAAS and Dartmouth College news releases.

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