These Middle Stone Age tools for grinding wild grains were found in a cave site in Mozambique (Grady Semmens, University of Calgary)

A Canadian archeologist exploring a cave in Mozambique has found the earliest evidence of prehistoric humans using and processing wild grains for food.

Julio Mercader of the University of Calgary found dozens of stone tools dating back more than 100,000 years ago containing traces of starch from wild sorghum.

Mercader said the discovery means that early humans of this period had a more sophisticated diet than previously believed.

"This happened during the Middle Stone Age, a time when the collecting of wild grains has conventionally been perceived as an irrelevant activity and not as important as that of roots, fruits and nuts," said Mercader.

Wild sorghum is the ancestor of the chief cereal crop now consumed in sub-Saharan Africa, where it's milled and prepared as porridge, baked goods and sorghum beer.

The stone tools were found during a 2007 excavation of a limestone cave near Lake Niassa in northwestern Mozambique. Mercader and colleagues from Mozambique's University of Eduardo Mondlane also found the remains of plants and animal bones showing what the humans who lived there ate.

Other plant foods found in the cave include the African wine palm, pigeon peas, wild oranges, false banana (actually ensete, a root crop), and star grass, also called the African potato.

The grains of sorghum starch on the stone grinders and scrapers show that wild grain was brought into the cave and processed for food, Mercader said.

"The inclusion of cereals in our diet is considered an important step in human evolution because of the technical complexity and the culinary manipulation that are required to turn grains into staples," said Mercader.

The use of sorghum by these cave-dwelling humans could be one of the earliest examples of this change in diet, he said.