Four million years of African herbivory.

The vast African continent is an ecological stage with ever-changing scenery and casts of characters that included human ancestors. The mammals that came and went upon this stage have been enlisted to help answer a compelling question relevant to our own evolutionary story: How do land animals respond to long-term environmental change? The paper by Cerling et al. (1) is a milestone in this quest. It synthesizes a huge amount of information on stable carbon isotopes to reconstruct the dietary history of three major herbivore groups over the last 4.3 million y. The study focuses on Kenya’s Turkana Basin, a prime source of vertebrate fossils and geological information relating to human evolution. Results from the fossil record are interpreted using an even larger sample of isotopic data from modern eastern and central Africa ecosystems, making this study a unique comparison of past and present herbivore adaptations on a changing continent. The power of carbon isotopes is that they can record what an individual animal ate during its lifetime via differing ratios of C and C in their tissues, providing information on trophic links between primary producers and consumers. This insight on ancient diets is possible because of a rapidly expanding collaboration between geochemists, ecologists, and paleobiologists. Although organic molecules are not well preserved in most vertebrate fossils, decades of research have established that the original biochemical signal is retained in the δC values of preserved tooth enamel. The method presented in this paper focuses on Artiodactyla (Cetartiodactyla), Perissodactyla, and Proboscidea (APP) and classifies modern and fossil herbivores in these groups using δC ratios that signal a diet of grass (C4), dicots (C3), or a mix of the two, expressed as a ratio of grazers: mixed feeders:browsers (G:M:B). This ratio is examined through time in the Turkana Basin and compared with G:M:B for the same groups in modern Central and East Africa. Before discussing the implications of the paper by Cerling et al. (1), it is worth considering how stable isotopes record animal diet at different scales. The δC ratios represent an individual’s diet during the time of tooth formation. If the individual is a selective C3 or C4 feeder, the enamel records this faithfully; if it eats a mix of leaves and grass, then the isotopic ratio falls in an intermediate range. The δC data points are compiled by taxonomic group and averaged over space (modern ecosystems) or time (fossil-bearing intervals, ∼10 y in this case) to represent larger-scale patterns of G:M:B. The number of isotopic values per time interval from the 919 Turkana Basin fossil teeth is variable but provides a sound basis for inferring general