C 4 photosynthesis is a plant adaptation to high levels of photorespiration. Physiological models predict that atmospheric CO 2 concentration selected for C 4 grasses only after it dropped below a critical threshold during the Oligocene (∼30 Ma), a hypothesis supported by phylogenetic and molecular dating analyses. However the same models predict that CO 2 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 CO 2 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. © 2011 The Author.