Although there were no machines for detecting them systematically before 1935, cosmic rays have left their footprints in the geological record by creating radioactive elements such as the carbon-14 used in carbon-dating.
There are many different isotopes of carbon and one that we have probably heard of is in the form of carbon-14, known for radio-carbon dating, but where does this radioactive isotope come from?
Unlike archaeological radiocarbon dating based on the fixed rate of decay of the carbon-14 isotope, there is simply no fixed rate of decay of basic vocabulary, which would allow us to date ancestral proto-languages.