https://www.pnas.org/doi/10.1073/pnas.2123366119

Significance
Environmental variability may have spurred unique adaptations among
Miocene apes and later hominins, but this hypothesis has been impossible
to test on the scale relevant to individual lifespans. We establish that
oxygen isotope compositions in modern primate teeth record annual and semiannual seasonal rainfall patterns across a broad range of environments
in equatorial Africa. We then document annual dry seasons experienced by
the large-bodied Early Miocene ape Afropithecus turkanensis, which may
explain its novel dental adaptations and prolonged development. By
revealing real-time historical and prehistoric environmental variation on a near weekly basis, we demonstrate that extraordinary behavioral and
ecological variability can be recovered from modern and fossil African primates.

Abstract
Variability in resource availability is hypothesized to be a significant
driver
of primate adaptation and evolution, but most paleoclimate proxies cannot recover environmental seasonality on the scale of an individual lifespan. Oxygen isotope compositions (δ18O values) sampled at high spatial
resolution in the dentitions of modern African primates (n = 2,352 near
weekly measurements from 26 teeth) track concurrent seasonal
precipitation, regional climatic patterns, discrete meteorological events,
and niche partitioning. We leverage these data to contextualize the first
δ18O values of two 17 Ma Afropithecus turkanensis individuals from
Kalodirr, Kenya, from which we infer variably bimodal wet seasons,
supported by rainfall reconstructions in a global Earth system model. Afropithecus’ δ18O fluctuations are intermediate in magnitude between
those measured at high resolution in baboons (Papio spp.) living across a gradient of aridity and modern forest-dwelling chimpanzees (Pan
troglodytes verus). This large-bodied Miocene ape consumed seasonally
variable food and water sources enriched in 18O compared to
contemporaneous terrestrial fauna (n = 66 fossil specimens). Reliance on fallback foods during documented dry seasons potentially contributed to
novel dental features long considered adaptations to hard-object feeding. Developmentally informed microsampling recovers greater ecological
complexity than conventional isotope sampling; the two Miocene apes
(n = 248 near weekly measurements) evince as great a range of seasonal
δ18O variation as more time-averaged bulk measurements from 101
eastern African Plio-Pleistocene hominins and 42 papionins spanning
4 million y. These results reveal unprecedented environmental histories
in primate teeth and suggest a framework for evaluating climate change
and primate paleoecology throughout the Cenozoic.

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