Conventional wisdom holds that early humans survived on a diet of meat, fruits, vegetables, nuts, and the occasional tuber. Our love affair with cereals supposedly came later, about 20,000 years ago. But a new study hints that wild cereals were part of the human diet more than 100,000 years ago.
Making cereals palatable is hard work. They have to be roasted in a fire or pounded into flour and cooked. Because the process is energy-intensive and requires specialized tools, many archeologists assumed that humans didn't begin consuming mass quantities of cereal until the advent of farming about 10,000 years ago. Then in 2004, researchers reported finding a residue of barley and wheat on a 23,000-year-old grinding stone in Israel. The new study indicates that cereal consumption is "a lot older than that," says author Julio Mercader, an archeologist at the University of Calgary in Canada.
Two years ago, Mercader and colleagues excavated a cave in Mozambique called Ngalue. They uncovered an assortment of stone tools in a layer of sediment deposited on the cave floor 42,000 to 105,000 years ago. The tools can't be directly dated, but Mercader presumes that the ones buried deepest in the layer are at least 100,000 years old. Other researchers had identified tubers as an important food source during the Stone Age, so Mercader decided to check for starch residue on 70 stone tools from the cave, including scrapers, grinders, points, flakes, and drills.
About 80% of the tools had ample starchy residue, Mercader reports  today in Science. The starches came from the African wine palm, the false banana, pigeon peas, wild oranges, and the African potato. But the vast majority--89%--came from sorghum, a grass that is still a dietary staple in many parts of Africa.
According to Mercader, the findings suggest that people living in Ngalue routinely brought starchy plants, including sorghum, to their cave. He doesn't have definitive evidence that they ate the grass but says it seems likely. "Why would you be bringing sorghum into the cave unless you are doing something with it?" he asks. "The simplest explanation is that it would be a food item."
Curtis Marean, an archeologist at Arizona State University in Tempe, isn't sure. "Grasses can be used for many things," he notes, such as bedding or kindling. Even if Ngalue's residents were dining on sorghum more than 100,000 years ago, Marean doubts that it was a major food source. "The processing costs of wild grasses are so high," he says, "and most African environments have a diversity of far more productive foods for hunter-gatherers."
Huw Barton, an archeologist at the University of Leicester in the United Kingdom, is skeptical as well. He points out that Mercader found sorghum residue on tools that likely wouldn't be used to process cereals, such as drills. "That doesn't make any sense to me," he says.
Still, says Robin Torrence, an archeologist at the Australian Museum in Sydney, the study is "tantalizing, because it expands the role of plants beyond the roots and tubers that previous scholars ... predicted would have been staples of early hominid diets."