The Great Divergence: When Did Diversity on Land Exceed That in the Sea?

Between 85% and 95% of all living macroscopic species are found on land; the rest are mainly marine. We argue that the extraordinary diversity on land is geologically recent, dating from the mid-Cretaceous period, similar to 110 million years ago. We suggest that the ability and necessity to be rare-that is, to maintain populations at low density-are made possible by the low cost of mobility of consumers on land, and that rarity is critical to the attainment of high-terrestrial diversity. Increasing productivity beginning in the mid-Cretaceous led to an increase in the survival of populations at low density and to an increase in the intensity of selection for that ability as well as for high mobility and habitat specialization. The pre-eminence of terrestrial, as compared to marine, diversity is therefore an historical phenomenon that is best explained by selection-related changes in mobility, dispersibility, and the evolution of partnerships.