Horizon: Snowball Earth

An examination of the remarkable theory that life evolved as a result of one particularly severe ice age which covered not just part but the whole of the Earth - even the Tropics - in ice up to a kilometre thick. The theory resolves long-standing anomalies, including signs of glaciation in places near the Equator, supposedly beyond old ice margins. It's a story that has taken scientists from Africa to Antarctica. Dilly Barlow narrates.