Origins of biotic seed dispersal by vertebrates and evolution of different seed sizes are conspicuous features in angiosperm evolution. In the Cretaceous, angiosperms remained small seeded, and biotic seed dispersal was rare. In the Early Tertiary, average seed size increased drastically, and biotic seed dispersal by vertebrates became common. Later in the Tertiary, along with climate cooling and the opening of vegetation, average seed size dropped. Fossil data suggest a positive correlation between seed size and biotic seed dispersal. This article examines three hypotheses: (1) the coevolution hypothesis, which suggests that evolution of large seeds and biotic dispersal were driven by interactions between plants and frugivorous vertebrates; (2) the recruitment hypothesis, which suggests that vegetation change altered recruitment conditions that favored large seeds and in turn promoted biotic dispersal; and (3) the life‐form hypothesis, which suggests that large seeds and biotic dispersal evolved as coadapted traits along with evolution of large plant life‐forms. The hypotheses are complementary rather than mutually exclusive. Evidence suggest that changes in vegetation structure (open vs. closed) are probably a primary driving mechanism of seed size and dispersal evolution, but life‐forms, seed sizes, and biotic dispersal systems have evolved as coadapted sets of traits in response to these environmental changes.