麻豆传媒

A human-ape toddler who lived 3.3 million years ago had slightly ape-like feet that she could use to grasp the branches of trees.

The finding suggests that young hominins spent more time climbing than adults did. It also indicates that, long after hominins started walking upright on two legs, they retained some ability to grasp with their feet.

鈥淭hey don鈥檛 have the grasping ability of chimpanzees,鈥� says team member of Dartmouth College in Hanover, New Hampshire. 鈥淏ut it appears to us, based on the anatomy of the foot, that they鈥檝e got more grasping ability than a modern human does.鈥�

DeSilva and his colleagues have described the almost complete foot of an Australopithecus afarensis, which lived 3.3 million years ago in what is now Ethiopia. That is at least 4 million years after our ancestors split from those of chimpanzees, but 3 million years before our own species appeared.

Meet Selam the toddler

The study is the latest development in a story that began in 2000, when discovered a virtually complete A. afarensis skeleton in Dikika, Ethiopia. The skeleton belonged to a little girl, who was about two and a half years old when she died.

Alemseged described the Dikika Child in 2006. She has been named 鈥淪elam鈥�, which means 鈥減eace鈥� in a local language.

However, Selam鈥檚 foot was embedded in rock. It has taken 鈥測ears of painstaking cleaning and preparation鈥� to get it out, says Alemseged, now at the University of Chicago, Illinois.

The discovery is remarkable because foot bones are hard to find. 鈥淚f you see a femur lying there鈥� it doesn鈥檛 look like a rock and your eye will snap to it,鈥� says DeSilva. Whereas the small bones from inside the foot 鈥渓ook like rocks鈥�. What鈥檚 more, in Selam鈥檚 foot the bones are still connected.

Selam鈥檚 preserved foot includes most of the interior bones, but no toe bones. Alemseged, DeSilva and their colleagues have now found that it was surprisingly flexible. 鈥淭his little girl had more mobility in her foot than the adults did, especially with the big toe,鈥� says DeSilva. This would have allowed her to grip things with her feet..

Australopithecus children may have done a lot of clambering around in trees, either for play or to stay safe from predators. 鈥淭he features that make it a climber seem to be more accentuated in the juvenile than in the adults,鈥� says Alemseged. The youngsters may also have spent a lot of time clinging to their mothers.

As the Australopithecus children grew up, they probably spent less time in the trees. 鈥淭hat鈥檚 what you see in gorillas and chimpanzees,鈥� says Alemseged.

Between ape and human

This fits with existing evidence that australopithecines like Selam possessed 鈥渁 mosaic of characters鈥�, says Alemseged. 鈥淭he upper part of the body 鈥� the arms, the fingers and even the brain, which is very small 鈥� is more ape-like, by and large.鈥� But below the waist the skeleton becomes 鈥渧ery human-like鈥�.

In line with this, previous studies found that Australopithecus retained adaptations for living in trees. The most famous Australopithecus fossil, Lucy, had powerful arms from years of climbing trees, and it has even been suggested that she died after falling out of a tall tree.

Australopithecines represent a halfway house between our ape ancestors and humans. they mostly lived in the trees, but could walk on two legs at times. Then by 2 million years ago, the first members of our own genus Homo arose. They were 鈥渙bligate bipeds鈥�, firmly adapted to life on the ground.

鈥淢ost people have agreed that A. afarensis spent some time in the trees and a lot of time on the ground,鈥� says of City University of New York. But a few researchers held out, arguing they were obligate bipeds like the later Homo species. 鈥淭his is the nail in the coffin for the 100-per-cent-bipedal argument. At a very young age these creatures were climbing around in the trees.鈥�