Increasing climate variability has been implicated as a driving force for the origins of our species (Homo sapiens) over 300,000 years ago, our genus (Homo) several million years earlier, and our more ancient great ape ancestors. The variability selection hypothesis posits that the landmark evolutionary features of humans, such as upright bipedal walking, large brains, and refined cognitive ability, arose in response to complex environmental processes in Africa. This influential theory has received support from environmental indicators including sediment sequences and changes in the composition of ancient animal communities. Yet these methods yield information on the scale of thousands of years or more, making it difficult to understand how climate patterns directly impacted ancient humans and their evolutionary kin on the time scale of an individual’s life.
The fossil record consists predominantly of teeth, which contain unique information about childhood development and concurrent environmental and dietary chemistry. For example, when individuals drink water, naturally occurring oxygen variants (isotopes) are quickly incorporated into the minerals of growing teeth and bone. Oxygen isotope values (δ¹⁸O) in water vary with temperature, altitude, precipitation, and evaporation cycles. In seasonal environments surface water is enriched in the heavy isotope (¹⁸O) during periods of warm or dry weather, raising δ¹⁸O, while the opposite occurs during cool or wet periods. Because teeth do not remodel during life, and tooth enamel is rarely modified after burial, these faithful climate records can be recovered from fossil teeth thousands and even millions of years later—facilitating tests of whether ancient climate variation directly impacted our evolutionary history.