A History of Grass

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Science  05 Sep 2003:
Vol. 301, Issue 5638, pp. 1291
DOI: 10.1126/science.301.5638.1291c

Grasslands are thought to have spread widely during the late Cenozoic, starting about 10 to 15 million years ago, and this expansion has been suggested to have had major effects on ecology and evolution: a radiation of ruminants and decline of other grazers, and eventually the rise of humans. Grasses use a different photosynthetic process (C4) than trees and shrubs (C3); the C4 pathway is favored at lower atmospheric CO2 levels and cooler temperatures, conditions that became prevalent later in the Cenozoic. Assessing the expansion of grasses and whether there was an abrupt change in ecosystems is critical for testing these proposed links.

Because the C4 and C3 pathways produce different stable carbon isotope signatures in plants, a fingerprint of the abundance of grasses can be recovered from the geologic record. Fox and Koch examined paleosoils from the Great Plains in the United States (now mostly grassland) spanning the period since about 23 million years ago. Their data imply that grasslands were about 20 % of the plant biomass until about 5 million years ago, and then expanded to modern levels by about 2.5 million years ago. This record differs from those suggesting an earlier expansion, and implies a local, not global, climate control of grassland extent. — BH

Geology 31, 809 (2003).

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