Cave deposits support human migration hypothesis

New data from Soreq Cave, Israel, have allowed scientists to reconstruct changes in seasonal rainfall over a key period of early human migration (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1903139116). When rain drips into a cave, it carries organic matter with it. Under a fluorescent microscope, these deposits appear as annual bands in stalactites and stalagmites—much like growth rings in trees. The researchers used oxygen isotopes from samples taken within individual bands to construct a picture of seasonal rainfall variability in the area surrounding the cave. Because of the complexities of evaporation and precipitation, minerals deposited during a rainy season, generally winter, typically have lower levels of oxygen-18. In the modern climate, this means that the isotope ratio differs greatly between summer and winter. However, in the Soreq Cave stalagmites, the seasons have similar isotopic records, signifying that winters and summers both saw heavy rainfall, says Feng He, a paleoclimatologist at the University of Wisconsin–Madison. By comparing this geochemical record with output from climate models, the researchers were able to link their observations to a northward expansion of the summer monsoon during the last interglacial period, 125,000 years ago. Crucially, He says, the timing of the expanded monsoon aligns with archaeological evidence of early human migration out of Africa, suggesting that the wetter climate may have helped enable this exodus.