In 1879, Charles Darwin wrote to his botanist friend Joseph Hooker describing the sudden appearance of angiosperms in the fossil record as an 'abominable mystery'. Though he held that evolutionary change must be slow and cumulative, flowering plants seemed to have diversified in the Late Cretaceous with almost no intermediate forms. Researchers have been working to resolve the puzzle ever since.

The earliest fossil records of angiosperms (enclosed-seed flowering plants) date to around 130–125 million years ago in the Early Cretaceous. Archaefructus sinensis, found in China, ranks among the oldest known angiosperm fossils; this plant, thought to have been partly aquatic, remains debated as to whether it bore true flowers in the strict sense. More definitive evidence comes from pollen records: tricolpate pollen grains characteristic of angiosperms appear abruptly in Early Cretaceous sediments.

Why did they diversify so rapidly? A large part of the answer lies in insect-plant co-evolution. The enclosed seed offers a radical advantage in seed protection and dispersal. Colourful flowers and sweet nectar attract insects, birds, and mammals as pollinators, providing a reproductive efficiency unavailable to wind-dependent ferns and conifers. Unlike their competitors, angiosperms could co-evolve with specific pollinators, a specialisation that diversified both groups simultaneously.

The ecological consequences of the angiosperm revolution were profound. Coniferous forests gradually gave way to broadleaf forests, deciduous or evergreen. Deep, carbohydrate-rich root systems enriched soil microbial life. Fruit-eating birds and mammals diversified rapidly to fill new ecological niches. The evolution of our own primate ancestors depended significantly on the abundant fruit that angiosperms provided.

Today angiosperms comprise roughly 300,000 described species and account for about ninety percent of Earth's terrestrial vegetation. The grains, vegetables, and fruits that sustain human civilisation all belong to this revolutionary group.