The first 700 million years of Earth’s 4.5-billion-year existence are known as the Hadean period, after Hades, or, to shed the ancient Greek name, Hell.

That name seemed to fit with the common perception that the young Earth was a hot, dry, desolate landscape interspersed with seas of magma and inhospitable for life. Even if some organism had somehow popped into existence, the old story went, surely it would soon have been extinguished in the firestorm of one of the giant meteorites that slammed into the Earth when the young solar system was still crowded with debris.

Scars on the surface of the Moon record a hail of impacts during what is called the Late Heavy Bombardment. The Earth would have received an even more intense bombardment, and the common thinking until recently was that life could not have emerged on Earth until the bombardment eased about 3.85 billion years ago.

Norman H. Sleep, a professor of geophysics at Stanford, recalled that in 1986 he submitted a paper that calculated the probability of life surviving one of the giant, early impacts. It was summarily rejected because a reviewer said that obviously nothing could have lived then.