By Bruce Bower
Although separated by several thousand kilometers, southern and eastern Africa were, in a sense, a stone’s throw from each other ancient times. New evidence suggests that human ancestors in southern Africa fashioned teardrop-shaped stone hand axes 1.6 million years ago, nearly twice as long ago as many researchers thought and about the time such tools are known to have first appeared in eastern Africa.
Ryan Gibbon and his colleagues dated hand axes and related stone implements, collectively known as Acheulean artifacts, using measures of the relative decay of radioactive forms of aluminum and beryllium in quartz grains from the soil and gravel bearing the artifacts. Exposure to cosmic radiation causes quartz to produce these substances, which then decay after enough sediment covers the quartz.
Acheulean finds have been dated to 1.7 million years ago in Ethiopia. Less-advanced stone tools have been dated to as early as 2.5 million years ago in eastern Africa.