In Synch with the Weather

摘要

Cooling climates in Africa over the past 10 million years have led to substantial environmental changes that might have influenced hurrian evolution. Demonstrating a specific relation, however, has been difficult because the human fossil record is sparse, and uncertainties in dating of both these fossils and climate records make correlation at the necessary resolution (within 100,000 years) problematic. To help tackle some of these issues, Joordens et al. examined strontium isotopes from nsn tosshs in sediments trom Lake Turkana that also contain human fossils, focusing on an interval around 2 million years ago. Rivers feeding the lake drain rocks with different Sr isotope compositions, and thus the lake Sr chemistry varied during monsoon wet and dry periods. The record, which extends over about 150,000 years, implies that this variation primarily reflects the precession of Earth's orbit, which varies over about 21,000 years. The sediments were deposited during a well-known magnetic field reversal, allowing an accurate time-climate sequence to be constructed. The authors were able to place 12 hominid fossils into the finely calibrated sequence. Although the age span is too brief to establish a larger relation between climate change and human evolution, the approach could lead to longer records here and elsewhere.