Julio Mercader reports in a short Science paper that the MSA stone artifacts from Ngalue cave, Mozambique, preserve thousands of grains of sorghum starch, along with a few other grasses and palm pith.
The role of starchy plants in early hominin diets and when the culinary processing of starches began have been difficult to track archaeologically. Seed collecting is conventionally perceived to have been an irrelevant activity among the Pleistocene foragers of southern Africa, on the grounds of both technological difficulty in the processing of grains and the belief that roots, fruits, and nuts, not cereals, were the basis for subsistence for the past 100,000 years and further back in time. A large assemblage of starch granules has been retrieved from the surfaces of Middle Stone Age stone tools from Mozambique, showing that early Homo sapiens relied on grass seeds starting at least 105,000 years ago, including those of sorghum grasses.
This is another of those very interesting technical developments in archaeology. The use of grass seeds may not be surprising in itself. Some think that australopithecines were eating grass seeds for a substantial amount of their diet; some (notably Clifford Jolly and Jonathan Kingdon) have suggested that grass seeds were one of the resources that prompted the evolution of bipedality. The dental reduction in early humans doesn't argue strongly against seed consumption; they are an important part of the diet for many recent hunter-gatherers including Australians. But it's nice to see a direct confirmation that humans were gathering seeds relatively intensively.
How intensive? Well, there is a slight problem:
It is not clear why the tools should be mostly coated with grass starches and not so much with other types of starch. It is possible that high-starch–bearing grass refuse built up considerably in the cave’s main chamber at times of human occupation, thus coating both tools that were used in the processing of grass seeds and others that were not.
Hmmm. On the one hand, that means pretty intensive grass collection. On the other, if such a substantial fraction of the actual sedimentary debris in the cave was composed of anthropogenic plant waste, it's probably not possible to get an accurate picture of the importance of the seeds as a fraction of the diet. It's a data point: these people, living around this cave, used a lot of Sorghum grasses and processed seeds in some way with stone tools.
It makes me wonder about what non-stone implements they may have used. Winnowing baskets?
Mercader J. 2009. Mozambican grass seed consumption during the Middle Stone Age. Science 326:1680-1683. doi:10.1126/science.1173966