Scientists have uncovered the first archeological evidence that Homo erectus, an ancestor of modern humans, used fire.

Traces of ash and burnt bone about a million years old were discovered in Wonderwerk Cave in South Africa by an international team led by researchers at the University of Toronto and Hebrew University, reported a paper to be published this week in the Proceedings of the National Academy of Sciences. The study described the ability to control fire as "a crucial turning point in human evolution."

"When we found this, I was very surprised," said Michael Chazan, an archeologist at the University of Toronto who co-led the project.

That's because there was previously very little archeological evidence for the use of fire by humans or human ancestors more than 400,000 years ago. That was about the time that modern humans and Neanderthals arose.

Nevertheless, paleoanthropologists — scientists who study the anatomy of human and pre-human fossils — had earlier suggested that Homo erectus may have cooked its food, based on its size and the size of its molar teeth.

The recent discovery "puts the theoretical and archeological evidence a bit more in sync," Chazan said.

A fragment of burned bone appears in a slice of sediment from the cave, viewed under a microscope. (P. Goldberg) The study used material — including minerals and sediment mixed in with archeological artifacts - that was excavated previously from Wonderwerk Cave. It was encased in plastic, and then cut into thin slices for examination under the microscope.

The researchers discovered that it contained the ash of plant matter such as grass, brush and leaves, but no big pieces of wood. It also included burned bone fragments and stone tools that had been exposed to fire.

The fact that the ash had a lot of angular edges suggested that the burning had taken place inside the cave — if the ash had blown in from outside, the edges would have been rounded and worn down by the elements, Chazan said.

It was challenging to figure out how old the ash was, he added, because radiocarbon dating doesn't work on material that old.

The researchers used a combination of two different geological methods. The results were consistent with the type of stone tools found with the ash, which were known to be made by Homo erectus.

Different use for fire?

Chazan said a lot of questions remain, such as what Homo erectus was using the fire for. The evidence doesn't prove that fire was used to cook meat. And it's very different and subtle compared to more recent evidence of fire use by ancient humans, which typically includes a large build-up of ash and charcoal, along with burnt stone tools.

"Could it be that what we're seeing is a different way of using fire than in later periods?" Chazan asks.

The researchers hope to gain more information by going back to the cave to do another excavation, so they can see where the burned sediment is in relation to the stone tools. They are also running experiments to see what type of evidence fires from grass and leaves would leave behind on materials such as bones and stone tools.