A 1.5-million-year-old set of hand bones, unearthed from a lake bed in Kenya, are the first to suggest an ape-like cousin of humans could use tools.
While the owner of the hands was a relative of modern humans called Paranthropus boisei, it was not a direct ancestor of ours.
But, according to a study led by researchers in the US and Kenya and published in Nature, the individual may have used tools, an ability some archaeologists doubted possible in this ancient species.
Tool use is a key milestone in the evolution of humans, study lead author and Stony Brook University anthropologist Carrie Mongle said.
"It represents a turning point in our behavioural and cognitive complexity."
Fossils in a Kenyan lake bed
The hand bones were part of a collection of fossils discovered in 2019 near Lake Turkana in Kenya, where several other ancient human-like species have previously been found.
Over the next two years, researchers found teeth, hand and foot bones, and skull fragments, all within metres of each other.
The team dated the sediment around the bones to estimate their age, calculating them to be around 1.52 million years old.
The skull fragments and teeth allowed the researchers to identify the bones as belonging to Paranthropus boisei. It was an ape-like species and a relative of the Homo group, in which our species Homo sapiens belongs.
This makes P. boisei a cousin, many times removed, of modern humans.
And while a handful of P. boisei skulls have been found before, the 2019 fossil discovery was the first to contain hand bones.
Ancient hands may have gripped tools
The shape of P. boisei's hand bones suggest it had some of the dexterity human hands have.
"Their finger proportions and broad thumb suggest they could have formed a 'precision grip,'" Dr Mongle said.
This means that P. boisei could have been capable of the nimble movements needed to make and use simple stone tools.
But other hand bones were more gorilla-like, according to the researchers, suggesting P. boisei also had a powerful grip that may have come in handy for climbing.
Michael Petraglia, an archaeologist at Griffith University who wasn't involved in the study, said P. boisei's status as a tool-user has been debated in the past.
For a long time, it was thought that species in the Homo genus might be the only tool users.
"These authors are showing the hand morphology of Paranthropus was slightly different than what we see in the genus Homo, but nevertheless potentially a tool user as well," Professor Petraglia said.
"There's still debate about the evolution of the hand and tool use, but this is an important piece of that."
Professor Petraglia pointed out that while this research showed P. boisei was capable of making tools, there were no tools found at this site.
"I wish there were stone tools because that really would be much more of a smoking gun."
Previously, P. boisei fossils have been found at sites with stone tools, but early Homo toolmaking human remains also appeared in those places, making it harder to tell who the tools belonged to.
Andy Herries, an archaeologist at La Trobe University who also wasn't involved in the research, agreed the find made P. boisei a likely toolmaker.
"They've got a power grip [and] could probably hit one stone against another in a fashion to make certain types of tools."
He said P. boisei may have used tools differently to the early Homo humans who were living at the same time, such as Homo erectus and Homo habilis.
"If we've got multiple toolmakers on the landscape at the same time, perhaps they're making different things."
Professor Herries added it would have been helpful to see more environmental information from this study to give researchers a better idea of P. boisei's ecosystem and who they may have shared it with.
What was Paranthropus boisei like?
Researchers only know about P. boisei from fragments collected around the African continent.
"The one thing that's really missing from the record is a good articulated Paranthropus skeleton. That would be great to find," Professor Herries said.
He added that it would have looked quite ape-like, and was probably highly suited to an environment with lots of tough vegetation to eat.
"It seems P. boisei is not really adapting too much over time. It's highly specialised and it remains that way."
Professor Petraglia said P. boisei may have used its big molars and strong grip to strip and eat vegetation in a wooded environment.
"Paranthropus was around for a long time, from 3 million years ago to about 1 million years ago, so 2 million years," he said.
But P. boisei's specialist features may eventually have been its undoing, as the environment changed and only the more generalist Homo species were able to adapt and survive.
Professor Petraglia said it was long-thought that tool use was confined to Homo sapiens and our closest relatives.
But observations from fossils to modern chimpanzees were undoing that assumption.
Dr Mongle said she hoped to glean more about P. boisei from the fossil find.
"I would love to use CT scans to study the internal structure of these bones to learn more about how exactly P. boisei was using its hands."
