By Griet Scheldeman
Science reporter, BBC News
The heat treatment makes the stones more brittle
Early modern humans in South Africa were using "heat treatment" to improve their stone tools about 72,000 years ago, according to new research.
This technique may bridge a gap between the use of fire to cook food 800,000 years ago and the production of ceramics 10,000 years ago.
Evidence for this innovation was found at Pinnacle Point, a Middle Stone Age site on the South African coast.
The researchers have published details in the journal Science.
"We found that as early as 165,000 years ago, but definitely 72,000 years ago, people are doing more than just using fires for cooking, heat, light or protection," lead researcher Kyle Brown, from Arizona State University, US, told BBC News.
"I think heating stones is the dawn of human engineering.
"One of the things that makes us uniquely human is that we can take the things in our landscape and adapt them. We can engineer them to fit our needs."
Evidence suggests that about 800,000 years ago, people were able to control fire to cook their food.
By about 10,000 years ago, humans were using fire to make ceramics and to extract iron and copper from their ores.
Until now, there was nothing to link these two different uses of fire, the researchers explained.
"These early modern humans seem to have been able to make a big mental jump," Kyle Brown commented.
The heat treatment makes the stones more brittle, making it easier to chop off clean flakes. This produces a fine cutting edge - like a modern day razor blade - good for cutting animal skins or making clothing.
However, the breakable edge is not very strong, making it unsuitable for use as a chopping tool in wood working, digging or stone shaping.
The stone tools found at the Pinnacle Point site were made of silcrete, a hard and resistant material that looks like another mineral called quartzite - but is not as coarse.
When the archaeologists searched a 50km radius around the site, they could not find the right type of stone required to recreate the ancient tools.
The stones had a different colour and texture; they were a deeper red, with a high gloss and were more brittle.
Mr Brown explained: "What we had spent six years looking for had always been right under our noses.
"Taking that step of putting the stone in the fire and pulling it out
I could imagine what the first people must have thought when they did that and realised that what they just did was going to change their lives."
Mr Brown, who is an experimental archaeologist, tried to recreate the "heat treatment technology" for himself.
"The fire requires a significant amount of fuel, which you need to gather in advance, together with the stones," he explained.
"Then you bury the stone in sand two centimetres below the fire, and, gradually, over 12 hours, build up the fire. You keep it at about 300 degrees for roughly five hours. Then you gradually let it cool down on its own so the stone does not crack.
"This can take 10 or even 20 hours. So you need to schedule your time, knowing that you need to be around the fire for 40 hours."
Establishing this sequence is crucial, Kyle Brown explained.
"As you can imagine, there are several ways in which materials can be unintentionally burnt on an archaeological site."
At around the same time as the tools were being fired in South Africa, humans had begun harvesting shellfish, making pigments and grinding ochre.
"These are highly sophisticated people who spread from Africa to eventually colonise the rest of the world," Mr Brown explained.
Heat treatment technology appears to have been known about 165,000 years ago, but it was not used routinely until about 70,000 years ago. It is unclear why the technology suddenly took off.
"Perhaps," the archaeologist speculated, "heat treatment could have become popular because good silcrete stones were scarce. Or it could have been a way to produce high quality material to be traded for other goods".