By Heather Pringle, Science Now, April 2, 2012
For millions of years, our ancestors were spread thinly across the African continent. They dined on raw meat, huddled in the cold, and slept fitfully in the dark, fearing the approach of large predators. But eventually, early humans reached a crucial turning point: They learned to make fire. Kindling and controlling a small blaze, they cooked their dinners, socialized around warm hearths, and frightened off large, menacing carnivores. Now, a new study suggests that humankind tamed the flame at least 200,000 years earlier than previously thought.
"I think this research establishes as well as possible that by 1 million years ago, early humans were able to control fire," says one of the paper's authors, archaeologist Michael Chazan of the University of Toronto in Canada.
The new evidence comes from a site known as Wonderwerk Cave in South Africa. Back in the 1980s, a now-retired South African archaeologist, Peter Beaumont, found what appeared to be wood ash and charcoal in a layer of sediment dated to 1.7 million years ago. But early claims for fire are often controversial because it is difficult to distinguish between natural fires and those set by humans. The most widely accepted previous claim came from the site of Gesher Benot Ya'aqov in Israel, where archaeologists found traces of what appeared to be fire dating to 790,000 ago.
Other researchers challenged Beaumont's finding. The ash and burnt bits, critics suggested, could have blown into the cave from a forest fire or resulted from the spontaneous combustion of bat guano--an event that is rare but has been documented elsewhere. So when Chazan and his team began working at Wonderwerk Cave, they took a new look at the evidence.
The hearthlike feature that Beaumont spotted turned out to be the product of natural geologic processes. But Chazan and his team found other visible signs of burning in a younger layer they dated by geological methods to 1 million years ago. Then team member Francesco Berna examined thin sections from the layer microscopically and analyzed their molecular composition by passing an infrared beam through the samples and studying the resulting spectrum. The presence of angular, sharp-edged bone fragments and well-preserved plant ashes, he says, indicates that neither wind nor water had buffeted and transported the particles; the burning took place in the cave. Moreover, Berna detected no trace of guano or any sign of the high-temperature combustion that phosphate-rich guano would produce. Instead, his studies showed that the bone mineral had been heated to between 450 and 700 degrees Celsius, the temperature of a small campfire. "Before this, I didn't believe that humans used fire so early," Berna says. "But there's no other explanation."
Such a small, low-temperature fire would be ideal for cooking. And the new finding, published online today in Proceedings of the National Academy of Sciences, fits well with earlier research conducted by Harvard University primatologist Richard Wrangham on the anatomy and behavior of Homo erectus, an extinct human ancestor who roamed Africa as early as 1.9 million years ago. The relatively small molars of Homo erectus and its reconstructed small gut suggest that it dined regularly on soft, nutrient-packed cooked food. And the fact that Homo erectus slept on the ground suggests that it had already begun building campfires to deter predators.
Still, the new Wonderwerk Cave study is likely to stir controversy. Wil Roebroeks, an archaeologist at Leiden University in the Netherlands, says that Berna and his colleagues failed to report any signs that the sediments in the thin sections had been heated or burned. And this, Roebroeks notes by e-mail, "suggests that the fire did not burn on the sample spot, and that the charred material indeed underwent some transport from a nearby fire."
But Wrangham thinks that the new microscopic way of looking for fire holds great promise, particularly for turning up data that may have been overlooked by previous archaeologists in several key early sites in Africa. The new study at Wonderwerk, Wrangham concludes, "is an exciting breakthrough."