The timing and intensity of the seasons shapes life all around us, including tool use by birds, the evolutionary diversification of giraffes, and the behaviour of our close primate relatives.

Some scientists suggest early humans and their ancestors also evolved due to rapid changes in their environment, but the physical evidence to test this idea has been elusive – until now.

After more than a decade of work, we’ve developed an approach that leverages tooth chemistry and growth to extract information about seasonal rainfall patterns from the jaws of living and fossil primates.

We share our findings in a collaborative study just published in Proceedings of the National Academy of Sciences.

Teeth are environmental time machines

During childhood our teeth grow in microscopic layers similar to the growth rings found in trees. Seasonal changes in the world around us, such as droughts and monsoons, influence our body chemistry. The evidence of such changes is recorded in our teeth.

That’s because the oxygen isotope composition of drinking water naturally varies with temperature and precipitation cycles. During warm or dry weather, surface waters accumulate more heavy isotopes of oxygen. During cool or wet periods, lighter isotopes become more common.

These temporal and climatic records remain locked inside fossilised tooth enamel, which can maintain chemical stability for millions of years. But the growth layers are generally so small that most chemical techniques can’t measure them.

To get around this problem, we teamed up with geochemist Ian Williams at the Australian National University, who runs the world-leading Sensitive High Resolution Ion Microprobe (SHRIMP) facilities.