Selam’s discovery was first announced in 2006: After first spotting her in 2000, it took the paleontologist Zeresenay Alemseged and his team more than half a decade just to unearth her skeleton intact. It has taken another 12 years for a related team to image and reveal the chunks of rock that contained her foot bones.
On Wednesday, the latter team published the first results of their work, in the journal Science Advances.
Their analysis matters because it gets to one of the most important questions in piecing together humans’ origin story: When did we learn to walk on two feet? Australopithecus, whose dozens of subspecies roamed Africa before the Ice Age, seems to provide a key phase in that story.
And Selam’s skeleton is presented in a way like very few other early-human fossils. “The bones are still in anatomical association,” said Kim Congdon, an anthropologist at Touro University Nevada who was not connected to the new paper. In other words, Selam’s foot bones still connect as they connected in life.
“Mostly, when we find fossils, they’re scattered. Lucy’s skeleton was scattered over a wide area. Nothing was found with one bone connecting to the next—but in this skeleton, her foot is still held together, which allows us to really see how these bones in life were oriented,” said DeSilva, an author of the new paper.
Take Selam’s big toe, which is somewhat larger and bendier than a modern-day human child’s. Anthropologists have long argued that the shape of a primate’s big toe implies a kind of evolutionary trade-off: A large, curved big toe makes it easy to climb trees; a short, stubby one makes it easier to walk on two feet. Compare a human’s foot to a chimpanzee’s, for instance. Chimpanzees have a long, grasping big toe, positioned on the foot in roughly the same place as an opposable thumb. Chimps also “almost sprint up trees,” DeSilva said.
Humans, meanwhile, have short, stubby big toes. We’re adept at walking, but when it’s time to ascend trees, we have to lift ourselves slowly and carefully. “It appears that as you acquire the adaptations for upright walking, you necessarily lose some of the anatomies that are good for climbing,” DeSilva told me.
These chimp-human differences are more than happenstance. Chimpanzees are modern humans’ closest living relatives, and we share a common ancestor 7 million years in the past. “Humans and chimps also have the same 26 foot bones—they’re just shaped a little bit differently. It’s those subtle differences that make all the difference in how we use our foot,” he said.
The new paper argues that Selam’s toe was somewhere in between. It wasn’t as long as a chimp’s toe, but it had more grasping ability than a modern human’s. DeSilva and his colleagues argue that young Australopithecus like Selam had long, big toes because they were climbing around a lot—even if they weren’t as skilled at the arboreal life as chimpanzees.