China Shandong Evolution Model Chapter 6: The LSED Framework and Reverse Migration – Physical Barriers and the One-Way Valve

The Southern Dispersal Hypothesis posits that the southern route of South Asia functioned as a “coastal highway” facilitating the eastward expansion of modern humans from Africa to East Asia during the Late Pleistocene. This chapter subjects that foundational presupposition – directional symmetry – to systematic interdisciplinary scrutiny. I develop the LSED (Lock-South-and-Exit-Doors) framework, a physical–geographical model that integrates oceanographic, geomorphological, palaeoclimatic, palaeoecological, and palaeodemographic lines of evidence to evaluate the passability of the South Asian southern route across glacial–interglacial cycles. The analysis identifies six independent barriers that collectively generate a systematic directional bias: (1) seasonally asymmetric monsoon-driven ocean currents; (2) glacial river-mouth incision forming impassable submarine canyons; (3) intergenerational knowledge zeroing from sea-level fluctuations; (4) the Sunda Shelf peat-swamp niche barrier; (5) multi-decadal monsoon resource unpredictability; and (6) the post-Toba African demographic vacuum. I formalise these barriers through an ordinal resistance ranking (eastward ΣR ≈ 42; westward ΣR ≈ 26; ratio ≈ 1.62) and validate the ranking’s robustness via one-at-a-time sensitivity analysis. The framework demonstrates that the South Asian southern route operated as a demographic one-way valve: sustained eastward population expansion from Africa toward East Asia was largely inhibited, while westward movement originating from East Asia remained viable. I derive three testable predictions (Prediction 13 〈P13〉– Prediction 15 〈P15〉) for empirical verification and outline the ecological and temporal windows – centred on MIS 3 (~55–40 ka) – during which westward dispersal along the “monsoon corridor” was most feasible. The LSED framework reframes the narrative of Late Pleistocene hominin dispersal from a unidirectional exodus out of Africa to an asymmetrical system in which eastward and westward flows were governed by fundamentally different physical and ecological constraints.