In the last 800,000 years, Earth has chilled and thawed its way through eight ice ages, each lasting tens of thousands of years. But why? Why didn’t Earth just freeze the one time and stay that way?

You’re glad it didn’t, because the periods between glaciations are when you get things like agriculture, city-states, plumbing, sunbathing, and the Nintendo Switch. Human civilization happened because something reversed a cooling trend about 20,000 years ago.

A new study, published in Nature Geoscience, has a hypothesis what that something was: plants. Or, more specifically, a complicated process in which plants wear down certain kinds of rocks, and how those rocks remove carbon dioxide from the atmosphere as they wear down—leaving just enough CO2 out there to trap solar warmth, and gradually bring summer back.

Over the course of its 4.5 billion year history, Earth has trended hot. Ice ages are relatively rare, and could happen for many reasons. For instance, the Earth’s tilt relative to the sun might shift, so the poles receive less heat. Or the orbit might swing further from the sun. Continental drift might mess with the ocean currents, or uplift a mountain range to interrupt atmospheric flow. The most recent ice age, known as the Pleistocene, began about 2.6 million years ago and was actually a series of many, 10- to 20 thousand year-long smaller ice ages interrupted by so-called interglacials—relatively short warm periods like the present.

Read more at Wired