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).(Credit: Jeremy DeSilva & Cody Prang)
A new study published on Wednesday in Science Advances revealed that more than three million years ago, our ancient human ancestors were standing on two feet but their toddler-aged children still had a backup plan for climbing trees.
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.
The researchers discovered that this infant possessed many of the structures necessary to walk on two legs that have been found in adult specimens, but it also retained into adulthood a convexity of the medial cuneiform, a bone important for joint movement, such as that involved in climbing.
"For the first time, we have an amazing window into what walking was like for a two and half year old, more than 3 million years ago," said the paper's lead author Jeremy DeSilva, an associate professor of anthropology at Dartmouth College. "This is the most complete foot of an ancient juvenile ever discovered."
Zeresenay Alemseged who discovered the fossil said, "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."
A facultative biped is an animal that is capable of walking or running on two legs, often for only a limited period, in spite of normally walking or running on four limbs or more.
"The Dikika foot adds to the wealth of knowledge on the mosaic nature of hominin skeletal evolution," said the paper's senior author Alemseged with the University of Chicago.
At two-and-half-year-old, the Dikika child was already walking on two legs, but it exhibited several ape-like foot characteristics that could have aided in foot grasping for climbing trees.
They're hints that she was still spending time in the trees, hanging on to her mother as she foraged for food.
According to the researchers, 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 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," said 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," said Alemseged.