Stone tools hint that our first human ancestors lived all over Africa

An ancient stone tool, but made by whom?

M. Sahnouni

By Colin Barras

A collection of unusually old stone tools found in Algeria is challenging our ideas about early human evolution. The find suggests one of two scenarios: either the first humans expanded rapidly from their small East African homeland, or humans emerged simultaneously across a vast region of Africa.

Our hominin ancestors diverged from the ancestors of chimps at least 7 million years ago, but it wasn’t until the last 3 million years that “true” humans in our Homo genus evolved. The evidence suggests they did so in East Africa. The earliest human-like fossils there date back about 2.8 million years.

But Mohamed Sahnouni at the National Research Centre on Human Evolution (CENIEH) in Spain and his colleagues suspect humans didn’t remain confined to East Africa for long. At Ain Boucherit in northeast Algeria, they have discovered primitive stone tools, alongside animal bones that are scratched in a way that suggests the animals were skinned, disembowelled and butchered.

Working out how old the discoveries are is tricky, says Sahnouni, because there is no volcanic ash at the site that can be chemically dated. Instead, they exploited the fact that the Earth’s magnetic field has flipped at known times. A record of these flips is recorded in magnetic minerals trapped in the ancient sediment. This, plus other evidence including the presence of fossil animals that went extinct before 2 million years ago, suggests the oldest signs of hominin activity at Ain Boucherit are about 2.4 million years old.

“We don’t know whether or not they hunted, but the evidence clearly shows that they were successfully competing with carnivores for meat,” says Sahnouni.

Mystery people

But who were “they”? There are no human fossils at Ain Boucherit, so the toolmaker’s identity is unclear. Hominin evolution 2.4 million years ago was in flux. Successful earlier hominins, including Australopithecus, were beginning to disappear, and early species of Homo were taking over. Sahnouni suspects the Algerian tools were made by one of these early Homo species.

“If I had a line-up and I had to pick one, that would be the one I’d pick,” agrees Jessica Thompson at Emory University in Atlanta, Georgia, who wasn’t involved in the study.

Thompson is reasonably happy with the idea that the stones are tools, but is less convinced that the animal bones are covered in cutmarks. She says researchers have found that it’s difficult to identify unambiguous cutmarks on ancient bones, because natural processes might scratch the surface of bones in a similar way. There are several ancient sites with bones that may have cutmarks on them, but all such claims are disputed.

Thompson also says the oldest stone tools might not be quite 2.4 million years old, because that date assumes the soil and sediment at Ain Boucherit accumulated at a steady rate.

Pan-African origins

If humans were in Algeria 2.4 million years ago, we might have to rewrite parts of our evolutionary story.

Sahnouni’s team argues that the simplest explanation is that humans evolved in East Africa and then spread rapidly to reach North Africa. However, he says we can’t rule out an alternative: that earlier hominins had spread across a vast region of Africa and all gradually evolved into humans. “The evidence from Algeria shows that the cradle of humankind was not restricted to only East Africa,” he says. “Rather the entire African continent was the cradle of humankind.”

A similar argument was put forward in August by Thompson and others, in relation to the appearance of our species – Homo sapiens – about 300,000 years ago. But Thompson says this idea reflects the fact that H. sapiens was dramatically different from earlier humans. We seem to have been the first African human species with the cultural and technological knowhow to adapt rapidly to different environments, allowing H. sapiens populations across Africa to connect and interact. She thinks earlier hominins would have struggled to achieve this multi-regional contact.

Either way, Thompson says the Algerian finds offer a more complete picture of our evolution. “If you start going into new places, you’re going to find things,” she says. “I’m a huge advocate of us needing to get the hell out of East Africa and start looking at other places.”

Journal reference: Science, DOI: 10.1126/science.aau0008