Toddler's fossil foot opens window into human evolution

Why an ancient toddler was likely able to scamper up trees better than her parents
Zeresenay Alemseged holds the skull of a three-year-old Australopithecus afarensis at the National Museum of Ethiopia in Addis Ababa, Ethiopia. The discovery sheds light on the contentious debate about how well the species walked and climbed. (Max Planck Institute for Evolutionary Anthropology/Associated Press)
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The fossilized foot of a toddler who lived three million years ago in what is now Ethiopia didn't just pound the ground. Scientists say her bare feet were made for climbing trees. 

Our distant cousin, Australopithecus afarensis, walked on two legs with a brain that was chimp sized.

Researchers have long debated whether members of same species as the famed fossil Lucy regularly climbed trees on top of walking on two legs or if the climbing was just an evolutionary hangover from an earlier ancestor. 

Jeremy DeSilva, an associate professor of anthropology at Dartmouth College in Hanover, N.H. didn't think the early human ancestors climbed much. That was until he saw a fossilized foot from partial skeleton of a 3.3-million-year-old female. It's thought that she died before the age of three.

The human story

"The story of human evolution is so much more complicated and so much more interesting than we ever could have imagined," DeSilva said. "It's a real humbling experience to work with these remains and constantly have to be nimble about your own scientific ideas. It can be really emotional to work with these things. They're fossilized humans. These are our ancestors. These bones are telling the human story, which is my story and your story. And so it is it's just wonderful."

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/University of Chicago)

Her young age sets her apart from the few other fossils of adults from her species, including Lucy, who walked the Earth 200,000 later than the toddler. 

In this week's issue of the journal Science Advances, DeSilva, Zeresenay Alemseged, a professor of organismal biology and anatomy at the University of Chicago and their team describe the nearly complete foot found in Ethiopia's Dikika  region.

The discovery offers clues to how our early ancestors grew, changed and developed — a unique window into an ancient juvenile. 

DeSilva said there's good evidence from fossilized footprints and adult fossils from the same time period that they were adapt at walking on two legs. From a distance, we wouldn't be able to tell their gait apart from how we now put heel to toe.

In the toddler's foot, there are signs that while her big toe doesn't stick out to the side like in an ape, it's surface was very curved.

Left block of images shows the 3.32 million-year-old foot from an Australopithecus afarensis toddler from different angles. The right block of images compares the child's foot with the fossil remains of an adult Australopithecus foot (top). ( Jeremy DeSilva and Cody Prang)

"That would suggest to us that she was moving her toe and could wiggle her toe and grasp with her toe significantly more than a modern human toddler could."

The greater mobility of the toe means she could likely scamper up trees faster and more often that adults to escape lurking leopards and other predators at a time the land was wetter with wooded areas, he said. And she likely played up high just as kids now do on juggle gyms. 

A life and death adaptation

The scientists also suspect the mobile toe was an important adaptation to carry infants. While chimpanzees and gorillas can just carry their babies on their back, early human ancestors who walked on two legs needed to find a way to stop youngsters from sliding off in an age when strollers and sling inventions weren't for sale. 

"Actively carrying your kid can be quite energetically expensive unless the kid can grab onto you. If the kid can grab onto you pretty well then you can save some energy and that could be the difference between life and death," DeSilva said. 

He continues to marvel at the beauty of the well-preserved and intact specimen that took years to prepare for examination. 

Modern day humans and our early ancestors both have 26 bones in our feet. But DeSilva says there are subtle differences in shape. "Curved joints show motion," he says. Think of our hip and shoulder joints. The fossil evidence points to this individual being able to grasp with the digit, based on the researcher's analysis.  

The foot fossil presented a rich opportunity because the foot bones were all stuck together and didn't need to be reconstructed from scattered sites. For now, it means scientists can't see what the interior structures and joint patterns are really like, but DeSilva and his team plan to analyze CT scans to learn more. 

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