Evidence of a once-lush, green Sahara can be found in the presence of depressions across the region that point to the existence of dried lake beds. More importantly, alternating layers of sedimentary rock in nearby marine environments contain different amounts of dust, which suggests that environmental conditions swing between drier, dustier phases and wetter, less-dusty ones. The rock record of lake rocks from Lake Tanganyika and marine rocks near Africa’s northwestern coast also point to alternating periods in which plants shed higher and lower amounts of leaf wax. Plants have evolved waxy leaves to help control water loss, and sediments with higher concentrations of hydrogen isotopes associated with waxy leaves have been shown to correspond with periods in which northern Africa was drier.
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