The foot bones of a toddler who died 3 million years ago show that baby pre-humans could both walk upright like modern humans and scamper up trees like apes, researchers said Wednesday.

The thumb-sized fossils come from a skeleton discovered in Dikika, Ethiopia, in 2002 and they have delivered priceless insights into how modern humans evolved from our distant ancestors, the researchers said.

“To have a fossil this complete, and to have the fossil of a child, gives us a brand-new window into of what it was like 3 million years ago,” said Jeremy DeSilva, a paleontologist at Dartmouth University whose team examined the tiny foot bones.

“Skeletons are rare and skeletons of kids are even rarer.”

A 3.32 million-year-old Australopithecus afarensis foot from Dikika, Ethiopia, superimposed over researcher Jeremy DeSilva's toddler's footprint. Jeremy DeSilva

Different teams of experts have looked at various parts of the precious fossil skeleton, which belonged to an Australopithecus afarensis that would have been about 2 ½ years old when she died.

It took years to remove the foot bones from the sandstone it was embedded in, DeSilva told NBC News.

“This is done grain by grain under the microscope,” he said.

More like apes or more like humans?

The bones have been dated to about 3.3 million years ago and identified as the same species as “Lucy”, the most famous example of an Australopithecus. The toddler bones date to about 200,000 years earlier than Lucy’s, however.

DeSilva’s analysis of the foot shows that baby Australopithecines had features that were similar to both modern humans and modern apes, he reports in the journal Science Advances.

“This is a very humanlike foot,” he said. “What is curious is it still retains some apelike features. The toes are slightly curved and longer than a modern human’s.”

To DeSilva, this suggests that juvenile Australopithecines clung to their parents much as baby apes do today, and probably could climb trees quickly.

Paleontologists who study Australopithecus argue about whether they were more like apes or more like modern humans.

“Our field has been divided into two camps, with one group saying Lucy and her kind … still climbed trees … and maybe that means they didn’t walk as well as we do today,” DeSilva said.

"There is another group that says, ‘Look at these bones. They did walk as we do and these apelike features are just an echo of the past from when their ancestors were still in the trees.'"

That’s because, in part, the bones of adult Australopithecus have a heavy heel, made for slamming into the ground, just like modern humans have.

But the baby foot doesn’t have such a heavy heel.

A partial skeleton of a 3.32 million-year-old fossil of an Australopithecus afarensis child. Zeray Alemseged

“What this baby’s foot allows us to do is introduce a third possibility,” DeSilva said.

“That is that the adults were very good walkers, but it is the kids that are climbing up into those trees,” he added.

That could be for play, just as modern children clamber all over the monkey bars while their parents stay on the ground.

“But you also don’t see leopards roaming around playgrounds,” DeSilva pointed out. “Three million years ago, this environment was filled with predators.”

"If you were living in Africa 3 million years ago without fire, without structures and without any means of defense, you'd better be able get up in a tree when the sun goes down.”

Also, having infants and toddlers that could hold on tight, without having to be carried, would make it much easier for the parents to move around. “Another thing they didn’t have 3 million years ago was strollers,” DeSilva said.

Modern human babies are born with feet already made for walking, but Australopithecus could have developed strong walking feet in another way, DeSilva said — by using them that way.

“Bones are living tissues. They grow and respond and they change shape depending on what you do to them,” he said. “Even though humans and Lucy’s kind as adults have chunky heels, we developed them in completely different ways. We would never have known that without this fossil.”

DeSilva, who has twins who were themselves toddlers when he started studying the fossil, says he feels a special attachment to it.

“This was a toddler who died and it’s really hard to think about when you have your own toddlers,” he said. “And yet it is giving us this absolutely extraordinary amount of information about our past and about why we are the way we are today.”