Were our ancestors sleeping in trees 3 million years ago?

The discovery of Selam was first announced in 2006: after spotting it for the first time in 2000, it took more than half a decade for the paleontologist Zeresenay Alemseged and his team to find his intact skeleton. It took another 12 years for a team related to the image and to reveal the pieces of rock that contained his foot bones.

On Wednesday, this last team published the first results of their work, in the journal Science Advances.

Their analysis is important because it addresses one of the most important questions to reconstruct the original history of humans: When did we learn to walk on our two feet? Australopithecus whose dozens of subspecies traveled Africa before the ice age, seems to provide a key phase in this story.

And the skeleton of Selam is presented in a way as very few other early human fossils. "The bones are still in anatomical association," says Kim Congdon, an anthropologist at Touro Nevada University who was not connected to the new journal. In other words, Selam's foot bones always connect as they connected in life.

"Most of the time, when we find fossils, they are scattered." Lucy's skeleton was scattered over a wide area – nothing was found with a bone in contact with the next – but in this skeleton, his foot is still held together, allowing us to really see how these bones in life were oriented, "said DeSilva, an author of the new paper. Take the big toe of Selam, which is a little bigger and more curved than a modern human child. Anthropologists have long argued that the shape of the big toe of a primate involves a kind of evolutionary compromise: A big big curved toe facilitates the climbing of trees; a short and stocky makes walking on two feet easier. Compare the foot of a human to that of a chimpanzee, for example. Chimpanzees have a long, tenacious toe, positioned on the foot at about the same place as an opposable thumb. The chimpanzees also "almost sprinted the trees," said DeSilva.

Humans, on the other hand, have short, stocky big toes. We are walking enthusiasts, but when it's time to climb trees, we have to get up slowly and carefully. "It seems that when you acquire adaptations for the right walk, you necessarily lose some of the anatomies that are good for climbing," DeSilva said.

These differences between humans and chimpanzees are more than just chance. Chimpanzees are the closest relatives of modern humans, and we share a common ancestor 7 million years ago. "Humans and chimpanzees also have the same 26-foot bones, they just have a slightly different shape.It is these subtle differences that make everything the difference in the way we use our foot," he said.

The new article argues that Selam's toe was somewhere in between.This was not as long as a chimpanzee toe, but he had more input capacity than a modern human DeSilva and his colleagues argue that the young Australopithecus as Selam had long, big toes because they were climbing a lot, even though they were not in the same position. they were not as skilled in arboreal life as chimpanzees.