Evolution

Dark Phase Dating

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Science  24 Dec 2010:
Vol. 330, Issue 6012, pp. 1725
DOI: 10.1126/science.330.6012.1725-b

Molecular phylogenies indicate that grasses that transform CO2 through the C4 photosynthetic pathway developed around 30 million years ago, long after the appearance of the first C3 grasses 60 million years ago or earlier. Fossil and isotopic records do not show the presence of C4 grasses until around 20 million years ago, however, leaving their date of origin poorly constrained. It has been suggested that the C4 pathway evolved in response to the rapid decrease of atmospheric CO2 from 1000 ppm to 500 ppm that occurred between 30 and 25 million years ago, as C4 photosynthesis confers a competitive advantage over C3 photosynthesis in low-CO2 conditions. Urban et al. present evidence that C4 grasses already were abundant in southwestern Europe 34 million years ago, before the concentration of atmospheric CO2 began to fall in the early Oligocene. The authors measured the carbon isotopic composition of morphologically indistinguishable C3 and C4 pollen grains in order to determine to which metabolic group they belonged, taking advantage of the large difference in the carbon isotopic signatures that characterize the two photosynthetic pathways. The results thus indicate that factors other than decreasing atmospheric CO2 concentrations must have driven the evolution of C4 photosynthesis.

Geology 38, 1091 (2010).

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