Between 135,000 and 70,000 years ago, the east African climate was highly unstable and subject to megadroughts. In the worst droughts, Lake Malawi had less than 15% of the water it has today. Crucially, around 70,000 years ago, the climate changed and became wetter and more stable. Surely it's no coincidence that this is when the human species underwent a rapid expansion and began to leave the continent to eventually colonize the rest of the world. PNAS.