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Scientists discovered the big toe that let toddlers climb trees like apes 3 million years ago

Jeremy DeSilva/Cody Prang
The 3.32 million year old foot of an Australopithecus afarensis toddler shown in different angles.
By Annabelle Timsit
Published Last updated This article is more than 2 years old.

More than three million years ago, an ancestor of our Homo Spaiens species, called Australopithecus afarensis, was already walking around on two legs. But the feet of children during that period retained some apelike traits, perhaps giving them an edge when it came to scaling trees, according to a study published in the journal Science Advances on Wednesday (July 4).

A group of anthropologists examined a foot belonging to the “world’s oldest toddler”—a partial skeleton of a 3.3 million-year-old A. afarensisfemale child, which was originally excavated in 2002 in Dikka, Ethiopia. The Dikika toddler’s foot is crucial because it gives scientists a window into how the A. afarensis species grew up, and what characteristics were crucial to its evolution.

Jeremy DeSilva & Cody Prang
Partially-complete skeleton of Selam, a 3.3-million-year-old female A. afarensis that died before the age of four.

“Fossils of kids are unbelievably rare,” said Jeremy DeSilva, the lead study author and a paleoanthropologist at Dartmouth College, “and it means we’ve had very little physical evidence for this incredibly important stage in our life.” The best-known representative of the A. afarensis species is Lucy, a 3.2-million-year-old adult fossil excavated in 1974 in Hadar, Ethiopia.

When it comes to the toddler skeleton, “bone for bone, this foot looks very human-like,” DeSilva told Quartz. But there is a crucial difference. The skeletal structure of the base of the child’s big toe suggests that kids from that species probably spent more time in the trees than adults. In modern-day humans, the bone at the base of the big toe is flat, and angled in a way that it lines up with other toes.

Because this structure would allow the child to have a better grip, anthropologists believe that at two-and-a-half years old, the Dikika child was already walking on two legs, but she was still spending time in the trees, or climbing onto her mother’s back.

Jeremy DeSilva/Cody Prang
The 3.32 million year old foot of an Australopithecus afarensis toddler shown in different angles.

“I suspect what we have here a kind of scenario where the adults are mostly on the ground, walking and foraging for food, and the kids are walking right along with them, but every once in a while the kids run up and play, they climb up a tree, they get spooked by a predator or a sound in the bushes, and run up a tree real quick, or they run up on a caretaker, like mom,” DeSilva said.

There doesn’t appear to be any relationship between the A. afarensis‘s propensity for tree-climbing and the same behavior in today’s adventurous tree-climbing kids. But the behavior may stem from a common urge. “Kids today absolutely explore different forms of locomotion (more so than adults) and incorporate climbing into their play behavior. It is likely that Australopithecus did the same,” DeSilva explained.

But more than 3 million years ago, scaling trees wasn’t all fun and games, according to DeSilva: “While kids climbing trees today is done for fun, in Australopithecus kids, living on a landscape full of predators, it may have been the difference between life and death.”

Why did our ancestors stop climbing trees?

How did humans evolve from the tree-climbing Australopithecus child to our current tendency to keep our feet on the ground? “There does seem to be one more step: the completely abandonment of the trees,” DeSilva said. “And my suspicion is that it required controlled fire; that it wasn’t until we began to control fire that we could safely continue to live on the ground at night and keep those predators away.”

It will probably take years, and many more fossil excavations, to fully answer that question. But the A. afarensis child’s foot is a big step in the process of understanding our ancestors’ history. As DeSilva explained, “the wealth of information that is contained in this skeleton is going to occupy our field for decades, and allow us to ask questions and answer questions that were previously off limits to our science.”

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