A million years of contrasting climate system influences shaped potential hominin habitats across Africa – Novel perspectives from Lake Bosumtwi (Ghana, West Africa)

After the climatic and environmental context of hominin evolution in East Africa centred the spotlight for decades, West Africa gains increasing interest considering a pan-African early human history. However, long and continuous continental climate records from this area illuminating regional hydroclimatic impacts and differences across Africa are missing. This is a major shortcoming as fundamental offsets in W-E hydroclimates are expectable that may have influenced human dispersal. Here we present the million-year-old hydroclimate record from Lake Bosumtwi in tropical West Africa, suggesting that this area has been strongly impacted by hemispheric system interactions (N-S-Atlantic, 100 ka cycles) and local insolation including half-precession. Comparing our findings with records from tropical East Africa governed by an Indian Ocean signal (20 ka, 400 ka cycles), we can confirm strongly contrasting hydroclimatic trends. To understand the meaning of these, we compare hydroclimatic signals from Lake Bosumtwi and Chew Bahir (tropical East Africa) providing us with important relative (moister-dryer) information with climate model output data delivering mean annual precipitation (MAP) estimates. This reveals striking similarities between the considered geoscientific data and climate models raising confidence that the MAP estimates can be reliably used to infer supported biomes. Whilst modelled MAP at Lake Bosumtwi varies between 1050 mm (wet savannah) and 1550 mm (rainforest), Chew Bahir may have been characterised by thorn to dry savannah conditions (550-750 mm/a). In this context, cross-Africa climate modelling suggests that large W-E savannah corridors supporting migration of large mammals and humans spread during periods, when the differences between MAP at both end-members have been low. In contrast, these corridors are interrupted by conditions supporting rainforests, when δMAP is high. These phases coincide with major steps in the evolution of the mega-fauna and hominins providing us with a basis for discussing new perspectives on climate and human co-evolution scenarios.