The great climatic changes of the past 2 million years created stress among living things. Forest, grassland and animal populations were repeatedly forced to change and migrate as glaciers waxed and waned. The most significant changes occurred in the sub-tropical deserts of the northern hemisphere. The Sahara was wetter (rock paintings there depict hunters of big game in extensive Savannah grasslands). The Indus Valley civilization of Mohenjo-Daro and Harappa arose during this wetter period, but subsequently faded because of climatic change. In the past 4000 years, this desiccation has predominated in much of the sub-tropical belt.