Though our forebears cooked with fire some 800,000 years ago, the consensus so far was that people hadn’t used fire to heat-treat materials until around 25,000 years ago. The new finding pushes that back at least 45,000 years.The international team of researchers published their findings this week in the journal Science. The work by lead author Kyle Brown, an American doctoral student at the University of Cape Town in South Africa, rewrites the history book for the use of fire as an engineering tool.Scientists had been puzzled by the fine-grained and often reddish coloured silicate blades and axes excavated from prehistoric sites at Pinnacle Point on the South African coast. Researchers spent six years looking for the same type of silcrete rock that they found in tools at Pinnacle Point, a Middle Stone Age site on the South African coast. It wasn’t until they happened across such a piece of silcrete embedded in ash that they realised that they could get the deep red, glossy and brittle silcrete such as the one found in tools by putting regular silcrete in fire. They estimate that stones were heated to around 300 degrees Celsius, probably by cooking them under fire in a process that could last for up to 40 hours. This resulted in stones that were easier to shape into tools by using other rocks, and that could be used as knives, hunting weapons or for exchange for other goods.