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How does potassium-argon dating help determine the age of Homo erectus fossils?

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Side-view illustration of a Homo erectus individual, depicted with a robust, upright body covered in hair, walking with one arm bent. A scale bar beside the figure shows measurements in inches and centimeters. — Homo erectus Artist's rendering of *Homo erectus*, which lived from approximately 1,700,000 to 200,000 years ago.

Potassium-argon dating is a vital radiometric technique used by paleoanthropologists to determine the age of Homo erectus fossils by measuring the decay of radioactive isotopes within volcanic materials found in the same geological deposits as the specimens.1

The method relies on the radioactive decay of potassium-40 into argon-40. Because argon is a gas that escapes during volcanic activity, any argon-40 found in a rock sample after it cools is assumed to have accumulated through the decay of potassium-40 over time. By calculating the ratio of these isotopes, scientists can determine the age of the volcanic rock or ash layer.23

When H. erectus fossils are discovered in association with these datable volcanic layers—such as those found at Koobi Fora—researchers can establish a precise chronological framework for the species. This allows scientists to reconstruct the evolutionary timeline of H. erectus, which is estimated to have flourished throughout much of the Pleistocene epoch. In instances where radiometric dating cannot be applied directly to a fossil, researchers may use these dated layers to assign a relative age to the site's contents.14