Dark Phase Dating

摘要

Molecular phylogenies indicate that grasses that transform CO_2 through the C_4 photosynthetic pathway developed around 30 million years ago, long after the appearance of the first C_3 grasses 60 million years ago or earlier. Fossil and isotopic records do not show the presence of C_4 grasses until around 20 million years ago, however, leaving their date of origin poorly constrained. It has been suggested that the C_4 pathway evolved in response to the rapid decrease of atmospheric CO_2 from 1000 ppm to 500 ppm that occurred between 30 and 25 million years ago, as C_4 photosynthesis confers a competitive advantage over C_3 photosynthesis in low-CO_2 conditions.