Early Human Lessons: Hot Rocks Make Sharper Tools

Early humans may have used heat to transform silcrete (left) into sharp tools (right). i i

hide captionEarly humans may have used cooking methods to transform silcrete (left) into sharp tools (right). And the ability to heat and carve rocks into tools shows they were thinking in complex and symbolic ways.

Erich C. Fisher/Arizona State University
Early humans may have used heat to transform silcrete (left) into sharp tools (right).

Early humans may have used cooking methods to transform silcrete (left) into sharp tools (right). And the ability to heat and carve rocks into tools shows they were thinking in complex and symbolic ways.

Erich C. Fisher/Arizona State University
Deep down in layers of rock, paleoanthropologists dig up clues to ancient toolmaking. i i

hide captionDeep down in layers of rock, paleoanthropologists dig up clues to ancient toolmaking.

Simen Oestmo
Deep down in layers of rock, paleoanthropologists dig up clues to ancient toolmaking.

Deep down in layers of rock, paleoanthropologists dig up clues to ancient toolmaking.

Simen Oestmo
The southern tip of Africa was home to early human masters of fire, say paleoanthropologists. i i

hide captionThe southern tip of Africa was home to the first human masters of fire, say paleoanthropologists. The discovery of silcrete tools in ancient campsites indicates that pyrotechnology may have occurred at least 20,000 years earlier than previously thought.

Erich C. Fisher/Arizona State University
The southern tip of Africa was home to early human masters of fire, say paleoanthropologists.

The southern tip of Africa was home to the first human masters of fire, say paleoanthropologists. The discovery of silcrete tools in ancient campsites indicates that pyrotechnology may have occurred at least 20,000 years earlier than previously thought.

Erich C. Fisher/Arizona State University

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, someone 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 — pyroengineering is born. "It's a real critical step in the evolution of technology," says Kyle Brown, a graduate student in archaeology at the University of Cape Town in South Africa. "Somewhere around 70,000 years ago it became very common."

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 it's thought to have been a more modern discovery; no one knew when or where the practice started.

So Brown tried heating some silcrete rocks himself, for 10 hours at a time.

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

His neighbors thought he was crazy. But Brown discovered that cooked rock was much easier to flake. "The stone becomes harder and stiffer," Brown says. "It basically becomes more brittle, which is great if you are breaking something you want it to break more easily."

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 early humans must have done this intentionally. He doesn't think the flakes were 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 will fracture 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.

"The striking ones from elsewhere in southern Africa have to do with the manufacture of symbolic objects," he says. "So you have the manufacture of marine shell beads or engraved ochre fragments ... that indicate that people were operating in a very symbolic way and that they possessed modern language in the way that modern humans do today."

This explosion of modern behavior — including pyroengineering tools — seems to have occurred earlier in southern Africa than in Europe or Asia. And it's possible pyroengineering 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.

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