The first land plants appeared around 470 million years ago, during the Ordovician period, when life was diversifying rapidly. They were non-vascular plants, like mosses and liverworts, that didn't have deep roots.
About 35 million years later, ice sheets briefly covered much of the planet and a mass extinction ensued. Carbon dioxide levels probably fell sharply just before the ice arrived – but nobody knew why.
Tim Lenton of the University of Exeter, UK, and colleagues think the mosses and liverworts are to blame.