The heavy bombardment
The Moon's craters are the receipt — Earth got worse
See it on the timeline →In depth
The Apollo lunar samples clustered strikingly around 3.9 billion years, giving rise to the idea of a "Late Heavy Bombardment" — a brief, violent episode triggered by the giant planets shifting orbit. The interpretation is contested: more recent work suggests the samples may be dominated by a single large impact, and that the bombardment was rather a steadily declining tail. Either way, Earth's surface was repeatedly sterilised during this period. That the first traces of life appear immediately afterwards makes the timing conspicuous.
For children
Look at the Moon on a clear night: all those round patches are asteroid craters. Earth was hit at least as hard, but weather and erosion have wiped the traces away. Some of the largest impacts probably boiled off part of the ocean.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org