For the first time, scientists have figured out how the toddlers of humanity's earliest ancestors walked and climbed trees to escape from predators.
Modern babies are usually able to walk by 15 months - but although our Australopithecus afarensis are known to have been somewhat bipedal, it wasn't clear how well they could walk and at what age.
Scientists were divided on how much time our ancestors spent in the trees, and there is much debate about how humans first began to walk upright.
Now, analysis of the almost complete skeleton of a female toddler from 3.3 million years ago has confirmed key details about how they walked.
"For the first time, we have an amazing window into what walking was like for a two-and-a-half-year-old, more than three million years ago," said the study's lead author, Dr Jeremy DeSilva.
"Walking on two legs is a hallmark of being human. But, walking poorly in a landscape full of predators is a recipe for extinction," explained Dr DeSilva.
The foot bones of the fossil show that the child was already walking on two legs at the time, but also spending a lot of time in the trees - hanging onto her mother as she foraged for food.
According to the study published in Science Adventures, the skeleton offers evidence that as homo sapiens evolved, it was the adults who developed the ability to walk while infants took longer to be able to come down from the trees.
"If you were living in Africa three million years ago without fire, without structures, and without any means of defence, you'd better be able get up in a tree when the sun goes down," said Dr 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," added Professor Zeresenay Alemseged, the study's senior author who discovered the skeleton in 2002.
The fossil is the same species as the famous Lucy fossil and was found in the same vicinity in Dikika in Ethiopia.