Earth’s first few hundred million years brought relentless impacts, intense volcanism and an atmosphere almost entirely devoid of oxygen—yet genetic analysis places the common ancestor of all life alive today around 4.2 billion years ago, suggesting biology gained a foothold remarkably soon after the planet became habitable.

A genetic reconstruction dates the last universal common ancestor to about 4.2 billion years ago, implying life emerged before that complex anaerobic ecosystem. The post Earth’s first few hundred million years brought relentless impacts, intense volcanism and…