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Tool manufacturers know that sometimes you have to heat-treat a material to make it harder or stronger.

Ancient toolmakers learned that trick, too. And archaeological research from South Africa pushes back the date of the earliest use of heat treatment at least 45,000 years, to more than 70,000 years ago.

Kyle S. Brown, a doctoral student at the University of Cape Town, and colleagues report finding stone tools that show signs of being heated to about 600 degrees Fahrenheit. Heat-treating in this way, most likely by burying under a fire, made the stone easier to knap, or shape into a tool by striking with another stone.

Archaeologists were studying several sites on the South African coast, with artifacts dating from 72,000 to 164,000 years ago that would have been made by modern humans from the African Middle Stone Age. Mr. Brown, an archaeological knapper who tries to replicate ancient tools, said they noticed that blades found at the site, made from a stone called silcrete, did not match silcrete obtained from outcroppings in the area. “We realized we were missing something,” he said.

They experimented by heat-treating some of the stone themselves. “When we pulled it out of the fire and flaked it, it did look like the kind of stone we were finding at our site,” Mr. Brown said. Their findings are published in Science.

The researchers had to show that the tools they found were intentionally heated to improve workability, not accidentally through a bushfire or other means. They found tools in areas where there was no evidence of burning. And they conducted tests on some of the artifacts, including one that showed that flaked surfaces had a glossiness that occurs only when the stone has been heated, proving that the stones were heated first and then worked into tools.

Mr. Brown said that the consensus among archaeologists had been that systematic heat treatment first occurred in Europe about 25,000 years ago. The current work, he added, “is almost indisputable evidence” for heat treatment 72,000 years ago, and perhaps as early as 164,000 years ago, although researchers need more samples from the earlier period to be sure.