Hammer time in the Stone Age.

Battered quartz and limestone spheres, each about the size of a tennis ball, litter Stone Age archaeological sites dating from about 1.8 million to 40,000 years ago. For more than a century, investigators have granted that the stones served as a major class of prehistoric tools and assigned them all sorts of speculative titles, including bone smashers, club heads, plant grinders, and bolas, which some hunters still tie to thongs and throw to trip up and bring down game.

But new evidence suggests that human ancestors produced the ubiquitous stone balls through repeatedly using chunks of stone to hammer out other tools, such as sharp-edged scrapers and choppers, assert anthropologists Nicholas Toth and Kathy D. Schick, both of Indiana University in Bloomington.

Toth and Schick traveled to Zambia in central Africa, where angular pieces of quartz are the most common raw material in many areas. In field experiments, they found that after about four hours of hammering to remove pieces (called flakes) from the edges of stone tools, quartz stones assumed a round shape without any predetermined intent to produce a sphere. Quartz proves highly susceptible to gradual chipping and wear during prolonged battering, the scientists contend.

The use of quartz tools increased sharply between approximately 1.8 million and 1.2 million years ago at Olduvai Gorge in Tanzania, at sites both close to and far from local quartz deposits, Toth says. During the same period, the number of quartz spheres found at various Olduvai sites also increased dramatically, he points out.

Early toolmakers at Olduvai apparently carried quartz hammers with them from one place to another, and they may have returned frequently to sites where such implements received regular use, Toth theorizes.

"Sometime around 1.7 million years ago, opportunistic toolmakers evolved into dedicated toolmakers whose existence relied on flake-stone technology," he argues.