The interplay between evolution, paleoecology, and environmental change is examined in a geochemical study of a group of Eocene planktonic foraminifera. The hantkeninids, which are well-known biostratigraphic index fossils, underwent spectacular long-term evolution in the middle and upper Eocene (49.0-33.7 Ma), a time when major global climate and oceanic changes were occurring. We use oxygen and carbon isotope analysis of their shell calcite to investigate how their habitat changed as they evolved. The hantkeninids originated in a deep-water oxygen-minimum environment, but migrated into fully oxygenated near-surface waters as global temperatures decreased and water-column stratification declined. This change in depth ecology coincided with pronounced morphological evolution, involving changes in chamber shape and degree of inflation, and modification of the primary aperture. These developments are considered to be adaptations to a near-surface habitat.