In recent decades, a number of sites have vied for the title of earliest human-controlled fire. At Koobi Fora and Chesowanja, both in Kenya, small patches of reddened soil were found in areas containing stone tools up to 1.5 million years old. To try to prove that Early Stone Age campfires caused the discoloration, researchers in the 1980s and ’90s used techniques such as magnetic susceptibility analysis and thermoluminescence dating. The first tool detects burned earth by gauging fluctuations in its magnetic field; the second determines how long ago an object was heated by measuring the photons it emits when baked in a lab. Although these methods showed that burning had occurred, the evidence is simply too sparse to convince most archaeologists that humans — not wildfires or lightning — were responsible.
Another promising site is a South African cave called Swartkrans, where archaeologists in the ’80s found burned bones in a section dating between 1 million and 1.5 million years ago. In 2004, Williams College chemist Anne Skinner analyzed the bones using electron spin resonance, which estimates the temperature to which an artifact has been heated by measuring molecular fragments called free radicals. She determined that the bones had reached at least 900 degrees — too hot for most wildfires, but consistent with a campfire. But since the cave has a gaping mouth and a downward-sloping floor, naysayers argue that the objects might have washed in later after being burned outside.
Until the Wonderwerk Cave find, Gesher Benot Ya’aqov, a lakeside site in Israel, was considered to have the oldest generally accepted evidence of human-controlled fire. There, a team of scientists found traces of numerous hearths dating to between 690,000 and 790,000 years ago. A wide range of clues made this site convincing, including isolated clusters of burned flint, as if toolmakers had been knapping hand axes by several firesides. The team also found fragments of burned fruit, grain and wood scattered about.
Then came Wonderwerk. The ash-filled sediment that Goldberg and Berna found came from a spot approximately 100 feet from the entrance to the tunnel-like cave, too far to have been swept in by the elements. The team also found circular chips of fractured stone known as pot-lid flakes — telltale signs of fire — in the same area. These clues turned up throughout the million-year-old layer of sediment, indicating that fires had burned repeatedly at the site.
Does that mean fire drove the evolution of H. erectus? Is the cooking hypothesis correct? The occupants who left these ashes at Wonderwerk lived nearly a million years after the emergence of H. erectus. Goldberg and Berna point out that it’s unclear whether the cave’s inhabitants knew how to start a fire from scratch or depended on flames harvested from grass fires outside the cave. If they were eating barbecue, it may have been only an occasional luxury. Whether that could have had an impact on human development remains an open question.
Finding the answers will require more digging. At Wonderwerk, team members plan to probe deeper, analyzing sediments up to 1.8 million years old, for evidence of fire. And they are using their cutting-edge detection methods at other early H. erectus
sites as well. “If you don’t look, you’re not going to find it,” Goldberg says.