A toddler who lived more than 3 million years ago is helping researchers today understand what daily life looked like for our ancient human ancestors, and is helping solve an anthropological debate that has plagued scientists for decades.

“For almost 40 years, our field has been divided about the locomotion [of the species Australopithecus afarensis],” Jeremy DeSilva, associate professor of anthropology at Dartmouth, told IFLScience in an interview. “To some, this species walked on two legs like humans do, but did not climb much at all. To others, A. afarensis climbed quite a bit and walked in a biomechanically less efficient way than humans do.”

As it turns out, both parties were correct. DeSilva says the species still retained features for climbing not because they used to climb, but because adults would climb as children while their bones were still growing and developing. The study is published in Science Advances. 

Preserved fossils of this species are extremely rare to come by and have made research on their evolution trying. Then, in 2002, archaeologists found a nearly complete skeleton of a female A. afaransis in the Dikiki region of Ethiopia. The 2.5-year-old was the same species as the famous Lucy skeleton, but lived 200,000 years before her.

The Dikiki child’s fossilized foot (known as DIK-1-1f) was encased in sediment. After careful removal, the team reconstructed its anatomy in order to see what the foot would have been used for and how humans have evolved. Turns out, they were “quite good” at walking on two legs – a hallmark of being a human, but a detriment in a landscape rife with predators. They believe the Dikiki child was still spending time in trees and hanging on to her mother’s back while foraging for food like apes do today. 

The base of her thumb-sized toe is more curved and angled than in modern human children, suggesting she had more grasping ability, which allowed her to scramble into trees to escape predators. But that’s not the only advantage to having a flexible toe. Carrying a child would have been an energetic burden to its mother, but the ability to cling to her back would have been a survival advantage. Evolutionary theory predicts that scientists should find fossils of ancestors with a mixture of human and ape-like features, and that’s exactly what the Dikiki child embodies.

“The Dikika head is ape-like with a small brain, but one that is growing slowly like human brains do. Her hyoid bone is ape-like, suggesting she does not yet have spoken language, and her shoulders are gorilla-like, indicating she spent time in trees. But, her foot is quite human-like, and possesses anatomies found only in the foot bones of humans,” said DeSilva.

“In other words, she walked on two legs, like we do. She displays a mosaic of human and ape-like anatomies, and is a wonderful contribution to the human family,” he said.