The age of the grasses and clusters of origins of C-4 photosynthesis

At high temperatures and relatively low CO2 concentrations, plants can most efficiently fix carbon to form carbohydrates through C-4 photosynthesis rather than through the ancestral and more widespread C-3 pathway. Because most C-4 plants are grasses, studies of the origin of C-4 are intimately tied to studies of the origin of the grasses. We present here a phylogeny of the grass family, based on nuclear and chloroplast genes, and calibrated with six fossils. We find that the earliest origins of C-4 likely occurred about 32 million years ago (Ma) in the Oligocene, coinciding with a reduction in global CO2 levels. After the initial appearance of C-4 species, photosynthetic pathway changed at least 15 more times; we estimate nine total origins of C-4 from C-3 ancestors, at least two changes of C-4 subtype, and five reversals to C-3. We find a cluster of C-4 to C-3 reversals in the Early Miocene correlating with a drop in global temperatures, and a subsequent cluster of C-4 origins in the Mid-Miocene, correlating with the rise in temperature at the Mid-Miocene climatic optimum. In the process of dating the origins of C-4, we were also able to provide estimated times for other major events in grass evolution. We find that the common ancestor of the grasses (the crown node) originated in the upper Cretaceous. The common ancestor of maize and rice lived at 52 +/- 8 Ma.