THE indelible marks made by cosmic rays that bombard the Earth can reveal whether our ancestors spent time and effort looking for the best raw materials for stone tools or simply used what came easily to hand.
Cosmic rays are high-energy particles thought to come from supernova explosions. These produce a shower of secondary particles when they collide with atomic nuclei in the upper atmosphere. Both primary and secondary particles can slam into the nuclei of atoms in the Earth’s crust to form stable radioactive isotopes. For instance, cosmic rays interact with the oxygen in flint and form beryllium-10.
These isotopes can shed light on whether prehistoric humans mined or searched for high-quality flint to produce their tools, says Elisabetta Boaretto, of the Weizmann Institute of Science in Rehovot, Israel.
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First, Boaretto’s team established that the level of Be-10 in flint collected from depths ranging from the surface to 2 metres below ground dropped with the depth from which it was taken. This is because most cosmic rays can only penetrate about 50 centimetres into the ground before hitting an atom (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0402302101).
Next, they compared these baseline Be-10 levels with those in 300,000-year-old flint artefacts from two caves in Israel. One set had a wide range of Be-10 concentrations, suggesting the flint came from many different sources. But tools from the other cave had consistently low levels of the isotope, suggesting the flint was either mined or was collected from a single exposed outcrop of buried rock. “It shows maybe they found a good source and invested energy in collecting this material, not just picking up what’s available,” Boaretto says.
Rainer Wieler, an earth scientist at the Swiss Federal Institute of Technology in Zurich, says the isotope method is “a really good approach to study these tools”. But he cautions that the evidence for mining by prehistoric humans comes from very few artefacts.