The darling of plant research may be millions of years older than botanists thought.

Arabidopsis thaliana is a dainty weed used by plant biologists around the globe, but the details of its evolutionary history are controversial. Sarah Mathews and Mark Beilstein of the Arnold Arboretum of Harvard University in Jamaica Plain, Massachusetts, and their colleagues re-examined its fossil record, including several fossils that other studies had overlooked.

The team determined that the genus Arabidopsis split from the genus Brassica, which includes broccoli and cabbage, about 43 million years ago — two to three times earlier than previously thought. The results suggest that the Arabidopsis genome has been changing more slowly than expected, and that members of the order Brassicales, which includes Arabidopsis, coevolved with butterflies that are able to detoxify the plants' defensive chemicals.

Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0909766107 (2010)