Geochemistry

Shifting Grasses

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Science  25 Nov 2005:
Vol. 310, Issue 5752, pp. 1247
DOI: 10.1126/science.310.5752.1247d

One of the major ecological changes in the recent past in East Africa was a great expansion of grasslands from about 10 to 15 million years ago (Ma) to the present. This change had a pronounced effect on the evolution of many African species, including humans. Feakins et al. reveal some important details about this expansion by analyzing carbon isotope ratios in organic compounds derived uniquely from African terrestrial plants preserved in a marine core in the Gulf of Aden. Because grasses photosynthesize using the C4 pathway, they produce a diagnostic shift in carbon isotopes in plant material when compared to C3 plants—mostly trees and shrubs. The record, although discontinuous, shows that although some grasses were present by 9 Ma, the major expansion occurred after about 3.4 Ma. Interestingly, the detailed record shows dramatic oscillations in the abundance of grasses, likely tied to Milankovitch cycles, beginning about 3.8 Ma, before the onset of glacial cycles. Evolving African mammalian species would have to have adapted to these shifts. — BH

Geology 33, 977 (2005).

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