To an astronaut today, the Earth looks like a vibrant blue marble from space. But 700 million years ago, it would have looked like a blinding white snowball. This seems an unlikely cradle for life, yet new evidence suggests the frozen ocean featured restricted ice-free oases that provided a lifeline for our earliest complex ancestors.
During the Cryogenian period, from 720 million to 635 million years ago, the Earth was buried by massive ice sheets that marched from the poles to the tropics. Surface temperatures were as low as -50°C.
