Ecological polarities of African Miocene apes

Abstract

Humans have been considered ecologically unspecialized, and our evolution a compromise path through a maze of conflicting influences. Generalist ecological roles contrast with ecological polarities such as competitor, breeder or tolerator. Ecological polarities can be approximated in fossil mammals by relative size of canines for competitors, incisors for breeders, and molars for tolerators. Considered in this way, early Miocene apes of Kenya were generalists, but show a greater range of ecological polarity than modern apes, including ecological polarities found in living apes, macaques, and vervets. As many as six primates in a single Miocene aleosol show diverse ecological polarities, implying competitive exclusion. Middle Miocene monkeys and apes were more molarized and marginally more tolerant than early Miocene primates, so more like humans in that respect. This adaptive shift of 15 ​Ma was at a time of climatic aridity and open vegetation indicated by associated paleosols. A 20 ​m.yr record of Kenyan paleoprecipitation from paleosols indicates that 15 ​Ma was unusually dry after exceptionally wet paleoclimate of 16 ​Ma. This new Kenyan paleosol record of paleoclimate is from the same localities as the fossil apes, and uninfluenced by whole ocean mixing, salinity and ice volume effects which compromise similar marine isotopic proxies of global change. Molarization of apes at 15 ​Ma ago was only one of a series of adaptations selected by at least 9 dry alternating with wet episodes which shifted forest to shrubland ecotones over the past 20 ​m.yr. Our evolutionary lineage ran a gauntlet of Neogene climatic and vegetation changes in Africa by adopting a generalist ecological role.