Two-stage mid-Brunhes climate transition and mid-Pleistocene human diversification

Global climate shifted to markedly warmer interglacial conditions across the “mid-Brunhes transition” (MBT, ~400 ka). However, a global MBT synthesis that spans marine and terrestrial evidence remains elusive, which limits our understanding of the role of the MBT in mid-Pleistocene human evolution. We synthesize Asian precipitation reconstructions within a context of global palaeoclimatic records and find that the MBT occurred in two stages. First, stronger warming of northern hemisphere continents, weaker southern hemisphere warming, and related more extensive northward displacement of the intertropical convergence zone (ITCZ) during interglacial marine isotope stage (MIS) 13 intensified and expanded precipitation in Asian monsoon regions and in other widespread northern hemisphere regions, with accompanying carbon reservoir changes featuring globally high marine benthic δ¹³C values because of vegetation expansion at ~500 ka. Subdued southern hemisphere warming and northward ITCZ displacement decreased southern hemisphere precipitation simultaneously during MIS 13. Second, a shift to globally warmer interglacials at ~400 ka, with elevated atmospheric CO₂ concentrations, smaller ice volume, and higher sea level resulted in sustained high interglacial precipitation in East Asia from MIS 11 onward and sustained high marine benthic δ¹³C values during MIS 11. We also synthesize palaeoanthropological data and find that the climate and ecosystem changes across the MBT coincided with the timing of human lineage diversification, including the emergence of Neanderthals and Denisovans in Eurasia and Homo sapiens in Africa, and their potential coexistence with H. heidelbergensis, H. erectus, H. floresiensis, H. naledi, and other Homo archaics. The timing of the MBT also coincided with novel hominin behavioural developments, including fire control and the transition from handaxe industries to more versatile Levallois techniques. Combined with environmental theories of human evolution, this chronological coincidence suggests a potential link between mid-Pleistocene environmental changes and human evolution.