The end of the Mesozoic, around 66 million years ago, closed with one of the best-documented mass extinctions on record. A thin sedimentary layer found around the world — the K-Pg boundary, anomalously rich in the rare element iridium — preserves the trace of an extraterrestrial cause: an asteroid roughly 10–15 km across.

Walter and Luis Alvarez proposed this iridium anomaly in 1980; the 180-km-wide Chicxulub crater off Mexico's Yucatán Peninsula, dated to the same age, was found in 1991. The impact threw billions of tons of dust, sulphate and vapour into the atmosphere; global wildfires followed, sunlight was largely blocked for months, and photosynthesis collapsed. The food chain failed from its base upward.

All non-avian dinosaurs, the great marine reptiles, ammonites and many plankton groups were wiped out. Surviving small mammals began to fill the empty ecological niches; tens of millions of years later, one of those lineages would lead to the Homininae and, eventually, to humans. Most of the extinction is concentrated in the short interval following the impact, though Deccan volcanism in India also played a role — the event cannot be reduced to a single cause.