Scientists working at an archaeological site in South Africa say they have evidence that people living there 70,000 years ago used fire to heat up rocks so they would make better stone tools.
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.
"Generally people have accepted, archaeologists accept that heat treatment began around 25,000 years ago in Europe," said Brown. "So we're pushing this back 30,000-40,000 years and putting it in Southern Africa rather than in Europe.
Brown calls himself an experimental archaeologist. So to find out how ancient people made their tools, he tried to make stone tools like the ones he was finding. But they weren't coming out right.
"We spent a lot of time looking for [stones we could work to replicate the ancient tools] in field surveys, and we couldn't find anything that looked like what we were seeing."
The researchers knew that ancient tool makers had heated up stone before working it into tools, though not remotely as long ago as 70,000 years, the age of the site where they were working.
"So it was kind of almost out of desperation that we decided to try this because we knew that in recent times people were doing it, and lo-and-behold, it worked."
What worked was a controlled heating and cooling of the stone that was a lot more sophisticated than just sticking some rocks in a fire. First, says Brown, you have to get the fire hot enough.
"To get a temperature of about 350 degrees Celsius beneath that fire, you actually have to have quite a large fire, or at least use a lot of hardwood fuel."
And you don't just drop the rocks into the flames. To even out the temperature, he buried the stone under the fire, beneath a layer of sand. And like any kind of cooking, timing is critical.
"So this fire, you build it up, slowly over time, about 12 hours, you have to wait about, you know, five hours for it to do its thing underneath the fire, and then however long it takes to cool back down to the ambient temperature outside," Brown explained. "So this whole process can take 20 to 40 hours to do."
The ability to figure out how to heat-treat stone to make a better tool shows a level of sophistication that Brown says demands a reassessment of people we might otherwise dismiss as primitive savages.
"These aren't the kind of cavemen that you see in cartoons. They were able to have these big genius moments, where they could connect seemingly unrelated events and put them together. Somebody discovers, 'Aha!, I've got this tool that I accidentally burned in the fire, and I pulled it out and flaked it, I can do this again.' So yeah, these are pretty sophisticated people we're talking about."
And what were these stone tools used for? Kyle Brown of the University of Cape Town says killing or butchering game for sure; they know that from marks on the animal bones they found near the tools, and probably also for woodworking.