Each summer, archaeologist Sarah Hlubik treks rutted dirt tracks to a dry riverbed in Kenya, following, approximately, in the footsteps of ancient hominins who camped there about 1.6 million years ago. Those early people likely butchered animals and fashioned stone tools, and Hlubik, a graduate student at Rutgers University in New Brunswick, New Jersey, thinks they did so around a flickering campfire. On her summer expeditions, she collects bits of burned bone and soil samples in the hope of proving her case.
Her question is a basic one about a crucial early technology: When did humankind first put fire to work for them, using it regularly for heat and cooking? Hlubik and other archaeologists who sift through the long-cold ashes of fires past cannot say for sure. It probably wasn’t as early as 2 million years ago—but it almost certainly occurred by 300,000 years ago. That leaves a big gap, with plenty to investigate.
It’s a deceptively hard question to answer. “In order to find early fire, we have to work really, really hard,” says Michael Chazan, director of The Archaeology Centre at the University of Toronto. Evidence of fire is ephemeral: Its traces, in the form of ashes or baked soils, are usually eroded away by wind or water. Even when signs of potential burning are present, it can be surprisingly hard to understand their exact origin. Researchers recently determined that the “burned” darkened wood and reddened sediments found at a site in northern Germany, now a coal mine called Schöningen, were really colored by water exposure and soil decomposition, not ancient flames. Even if evidence for flames is more certain, it can be tricky to tell if the fire was the result of a natural blaze or a human-made spark, or if people harvested it from a nearby wildfire for their own use. Even harder to decipher is whether people were using those fires “regularly,” and whether that means every week, year, or decade.