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LINDA WERTHEIMER, host:

This is MORNING EDITION from NPR News. I'm Linda Wertheimer.

STEVE INSKEEP, host:

And I'm Steve Inskeep. It's taken almost 200,000 years for humans to evolve from cave-dwelling stone throwers to net surfing twitterers. Is that evolution or devolution? Anyway, along the way, were a few eureka moments when somebody figured out how to make life a little easier. Scientists in South Africa believe they've discovered evidence of just such a moment, as NPR's Christopher Joyce reports.

CHRISTOPHER JOYCE: So, it's the southern tip of Africa, about 70,000 years ago, and some humans have built a fire. Maybe it's to keep warm, or to cook up some gazelle steaks. To kill time, they do a little flint knapping - whacking one rock with another to chip off razor-sharp flakes. They use the flakes to cut meat or make spear points.

After the fire dies down, one of them drags a big stone out of the embers and tries whacking that, and discovers that it makes really good flakes: it chips predictably, and the flakes are symmetrical and sharp. Eureka — pyro engineering is born.

Mr. KYLE BROWN (Graduate Student, University of Cape Town): It's a real critical step in the evolution of technology. Somewhere around 70,000 years ago, it became very common.

JOYCE: Kyle Brown is a graduate student in archaeology at the University of Cape Town in South Africa, and he makes his own stone tools. The campfire scenario is based on Brown's own eureka moment.

Here's how it happened: Brown had noticed that many ancient tools found in South Africa were made from silcrete, a kind of rock notoriously difficult to work with. It's known that heating rock can improve stone for making tools, but no one knew when or where the practice started. So Brown tried heating some silcrete himself, for 10 hours at a time.

Mr. BROWN: I did a lot of the early experiments with a fire pit in my yard, just sitting up making sure that the fire stayed hot and had enough wood on it.

JOYCE: What'd your neighbors think of you sitting out there all night long…

(Soundbite of laughter)

JOYCE: …cooking rocks?

Mr. BROWN: They think I'm pretty crazy.

JOYCE: Crazy or not, Brown discovered that the cooked rock was much easier to flake.

Mr. BROWN: The stone becomes harder and stiffer. It basically becomes more brittle.

JOYCE: The flakes were sharper, too, and they had a certain glossiness on the surface that the 70,000-year-old stone tools also had.

Writing in the journal Science, Brown says the practice must have been intentional. The flakes weren't made first and then dropped into a fire. He tried that, and it didn't work. Further tests suggested that cooking the rock first - to about 600 degrees Fahrenheit - apparently alters its crystalline structure and gives it something knappers call rebound hardness so it fractures more predictably.

Anthropologist Grant McCall from Tulane University, who studies and makes stone tools himself, says it seems to be part of the flowering of what scientists call behavioral modernity - things that modern humans did that set them apart from their ancestors.

Professor GRANT MCCALL (Anthropology, Tulane University): The striking ones from elsewhere in southern Africa have to do with the manufacture of symbolic objects. So you have the manufacture of marine shell beads and engraved ochre fragments and things like that that indicate that people were operating in a very symbolic way.

JOYCE: This explosion of modern behavior - including pyro engineering tools -seems to have occurred earlier in south Africa than in Europe or Asia. And it's possible pyro engineering began even further back. The scientific team has found the same kind of tools at other South African sites that appear to date back 165,000 years. That's long before so-called modern behavior emerged elsewhere, making the discovery even more striking.

Christopher Joyce, NPR News.

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