Never underestimate moss. When the simple plants first arrived on land, almost half a billion years ago, they triggered both an ice age and a mass extinction of ocean life.

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 ensuedMovie Camera. 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.

It’s not the first time that plants have been fingered as a cause of glaciation. Researchers already suspect that the rise of vascular plants in the Devonian period, some 100 million years later, triggered another ice age. The plants’ roots extracted nutrients from bedrock, leaving behind vast quantities of chemically altered rock that could react with CO2 and so suck it out of the atmosphere. » Read more: First land plants