Were Our Ancestors Sleeping in Trees 3 Million Years Ago?

It happened fast—that’s all we know.

Perhaps the young girl fell out of a tree or was struck by an illness. Maybe she drowned. But 3.3 million years ago, a roughly 3-year-old Australopithecus afarensis died in modern-day Ethiopia.

From her misfortune has sprung a wealth of knowledge. She fossilized quickly, likely because she tumbled into a stream bed or rushing floodwaters. The movement of rocks and water were kind to her skeleton, leaving it largely intact and whole. And soon after her skull was spotted sticking out of a cliff wall in 2000, anthropologists realized they had something unprecedented on their hands.

The girl is the most complete juvenile skeleton of an early-human ancestor ever discovered. Her skull, neck, vertebrae, rib cage, and lower body are almost entirely preserved. Even her brain left a cast of its shape on the rock. Scientists call her Selam, after the Amharic word for “peace.”

“The presentation is remarkable—it’s like nothing I’ve ever seen before,” said Jeremy DeSilva, a professor of anthropology at Dartmouth College. “Adult bones are larger, denser, and more easily discovered. Kid bones are often quite fragile, and they don’t preserve. Every so often, we find a fragmentary piece of a kid’s mandible, or some teeth. But this discovery is just extraordinary.”

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.

“These kiddos would be scrambling up trees if they got spooked by a predator, or they’d be climbing up on their moms to be carried,” DeSilva told me. “In the absence of strollers and Baby Björns, moms had to carry their kids. And if the kids can grab onto you a little bit, when you pick them up, that reduces the energy needed to carry them.”

“I don’t think they’re climbing like chimpanzees do,” he added. “They don’t have the anatomies for it—but they do have the anatomy to climb slightly better than we do.”

He and his colleagues argue that Selam’s toe helps resolve a long-running puzzle about this era of human ancestors. By 3.3 million years ago, Australopithecus adults seem to have had very “humanlike” feet. Their feet were well adapted for walking. Even young kids would have been bipedal. But even adult Australopithecus still have curved, apelike big toes. Hence the debate: Perhaps Australopithecus was both climbing trees and walking around. Perhaps large toes were—like the modern human appendix—a mostly useless, vestigial feature that had not yet been lost to evolution’s dynamo.

DeSilva proposed a third choice: “The adults look the way they do because those very adults were once kids, and their bones were once growing. Now, bones were living tissue, and they’re going to respond to what you’re doing—so if you’re climbing a whole lot, those bones are going to respond and they’re going to curve.”

Australopithecus also probably climbed trees at night, though not to hunt. As the sun set on the savanna, family units or larger social groups would have avoided predators by climbing into trees at night to sleep. “They’re likely slowly going up into the trees, passing the babies up, and building night nests,” DeSilva told me. “But they’re much better suited for living on the ground and walking like we do.”

Michelle Drapeau, an anthropologist at the University of Montreal who was not involved in this research, told me that she agreed that Selam’s longer toe probably allowed for “a little more movement than there is in modern humans.”

But she doubted whether big toes helped Australopithecus babies cling to their mothers. Researchers have found that modern-day monkeys hold on to their mothers by flexing their smaller toes. Why would Australopithecus children be any different? “You don’t really need this prehensile big toe to grab to your mom’s hair,” she said.

The many overlapping interpretations of the big toe point to the difficulty of tracking human evolutionary history—particularly of “primitive” traits, features that once existed but are now lost.

“When you look at traits that you know have changed from the ancestor, then you know there was natural selection, because the trait became different. But if a trait is still there, you don’t know if it [remained] because there was no reason to get rid of it, or because it was still important,” Drapeau said.

All of which didn’t make the paper’s interpretation unreasonable, she added: “It’s just not the only one.”

And note this entire discussion follows from just one of Selam’s features. There are dozens more hypotheses and theories to be gleaned from the foot. The new paper also asserts that Selam’s foot had an arch; Drapeau wasn’t so sure about that interpretation, either.

The paper also finds that Selam’s heel bone wasn’t as well developed and bony as a human adult’s. This is less of a surprise, as modern-day human children have softer, smaller heel bones that enlarge and harden as they grow. Since Australopithecus adults seem to have large heel bones, this suggests that Australopithecus kids developed similarly to how human children do today.