Hominoid dispersal during Neogene, from tectonics to Milankovich forcings as major driving factors to explain the spread of population

During the Neogene, major tectonics events occurred: uplift of mountain ranges (including the Tibetan Plateau and surrounding regions), opening and closing of seaways, and large variations of atmospheric CO₂ and sea level. What were the consequences of such changes on the dispersal of hominoid populations? We focus on the Miocene to Pliocene time interval (23 Ma-2.6 Ma). First, we analyze the spread of hominoids from their original geographic range, the tropical forests of Africa during the Early Miocene to the first expansion to Eurasia during the mid-Miocene Climate Optimum. Niche modelling combined with paleoclimatic simulations, provides means to circumvent the fragmentary nature of this record. We identify how the large climatic changes during Miocene Transition impact the potential habitats of hominoids and compare our findings to both the related fossil records and paleoenvironmental proxies.

Second, we analyze human distribution during the Mid to Late Pliocene (4-3 Ma), i.e. before the triggering of the large perennial Greenland ice sheet, and of huge amplitude northern hemisphere glacial interglacial cycles, while CO₂ evolved between 400 and 300 ppm. In Africa, tropical areas experienced drastic hydrological changes, mainly driven by precession cycles, which deeply modulated monsoon intensity and precipitation patterns, as illustrated by the paleoshore level record of Mega Lake Chad. To explore how orbital parameters strongly modify hydrological cycles over tropical Africa and, the associated dispersion of the genus Australopithecus, we simulated the response of climate, vegetation, and hydrological cycles of the mid to Late Pliocene conditions.

For both geological contexts, we provide and analyze a series of Earth System models (IPSL-CM5A2 LR-low resolution-) coupled simulations. Moreover, we associate these fully-coupled simulations with high resolution atmosphere-only model simulations, and a dynamic vegetation model (ORCHIDEE). Both models were used to estimate ecological niches with a spatial resolution of 50 km.

We describe the imprint of slow climate changes during the Miocene Climate Transition (MCT) as well as more rapid climate evolution during mid to Late Pliocene, associated with higher frequency oscillation of orbital parameters.

This study demonstrates how, for these periods, climate and especially hydrological variations were pivotal to the understanding of hominoid migrations. We compare our findings to fossil records and paleoenvironmental proxy reconstructions.

To conclude, we discuss the strengths and limitations of such approaches, which were made possible through a large trans-disciplinary effort between paleontologists, paleoanthropologists, paleoenvironmentalists, paleoclimatologists, and niche modelers.