The Maloti-Drakensberg Mountains are southern Africa's highest and give rise to South Africa's largest river, the Orange-Senqu. At Melikane Rockshelter in highland Lesotho (~1800 m a.s.l.), project AMEMSA (Adaptations to Marginal Environments in the Middle Stone Age) has documented a pulsed human presence since at least MIS 5. Melikane can be interrogated to understand when and why early modern humans chose to increase their altitudinal range. This paper presents the results of a multi-proxy paleoenvironmental analysis of this sequence. Vegetation shifts are registered against a background signal of C3-dominated grasslands, suggesting fluctuations in temperature, humidity and atmospheric CO2 within a generally cool highland environment with high moisture availability. Discussing Melikane in relation to other paleoenvironmental and archeological archives in the region, a model is developed linking highland population flux to prevailing climate. It is proposed that short-lived but acute episodes of rapid onset aridity saw interior groups disperse into the highlands to be nearer to the Orange-Senqu headwaters, perhaps via the river corridor itself.