C4 eudicots are not younger than C4 monocots

C-4 photosynthesis is a plant adaptation to high levels of photorespiration. Physiological models predict that atmospheric CO2 concentration selected for C-4 grasses only after it dropped below a critical threshold during the Oligocene (similar to 30 Ma), a hypothesis supported by phylogenetic and molecular dating analyses. However the same models predict that CO2 should have reached much lower levels before selecting for C-4 eudicots, making C-4 eudicots younger than C-4 grasses. In this study, different phylogenetic datasets were combined in order to conduct the first comparative analysis of the age of C-4 origins in eudicots. Our results suggested that all lineages of C-4 eudicots arose during the last 30 million years, with the earliest before 22 Ma in Chenopodiaceae and Aizoaceae, and the latest probably after 2 Ma in Flaveria. C-4 eudicots are thus not globally younger than C-4 monocots. All lineages of C-4 plants evolved in a similar low CO2 atmosphere that predominated during the last 30 million years. Independent C-4 origins were probably driven by different combinations of specific factors, including local ecological characteristics such as habitat openness, aridity, and salinity, as well as the speciation and dispersal history of each clade. Neither the lower number of C-4 species nor the frequency of C-3-C-4 intermediates in eudicots can be attributed to a more recent origin, but probably result from variation in diversification and evolutionary rates among the different groups that evolved the C-4 pathway.