Fallback foods, preferred foods, adaptive zones, and primate origins.

Appreciation has grown for the impact of tropical forest seasonality and fallback foods on primate diets, behaviors, and morphology. As critically important resources in times of shortage, seasonal fallback foods may have an outsized role in selecting for form and function while the diversity of preferred plant foods has played an equally prominent role in shaping primate evolution. Here, hypotheses of primate origins are examined in the context of food choice models developed by Marshall and Wrangham [2007] and related to the broader concepts of adaptive zones and radiations. The integrated evolution of primate diet and positional behavior is consistent with a growing reliance on angiosperm products--not prey--as preferred and seasonal fallback foods, temporally and phylogenetically coordinated with evolutionary phases of the angiosperm adaptive radiation. Selection for an incisor oriented but non-specialized heterodont dentition, in contrast with most other orders, attests to the universal role of a highly varied vegetation diet as the primates' primary food resource, with diverse physical properties, phenology and high seasonality. A preference by plesiadapiforms for eating small protein- and lipid-rich seeds may have predisposed the primates and advanced angiosperms to diversify their evolving ecological interdependence, which established the primate adaptive zone and became realized more fully with the rise of the modern euprimate and angiosperm phenotypes. The "narrow niche" hypothesis, a recent challenge to the angiosperm co-evolution hypothesis, is evaluated further. Finally, I note support for visual predation as a core adaptive breakthrough for primates or euprimates remains elusive and problematic, especially considering the theoretical framework provided by the Marshall-Wrangham model, updated evidence of primate feeding habits and the counterpoint lessons of the most successful primate predators, the tarsiiforms.