National Geographic : 1997 Feb
Liise V Li Savanna and scrub Transitio woodland Rain forest . ANCIENT GLOBAL CLIMATE Before 3.5 million years ago Atlantic and Pacific waters mix inthe open passage between the Amer icas, balancing salinity levels. Buoyant Atlantic cur rents temper the chill of the Arctic Ocean. MODERN GLOBAL CLIMATE The Pacific no longer mixes with west-moving Atlantic waters in the tropics. The North Atlantic Cur rent, heavy with salt, sinks before it reaches the polar sea. Deprived of warmth, the Arctic freezes, ultimately causing acooling, drying effect in Africa. Challenged to survive in a shifting landscape Wind-borne dust and pollen deposited in coastal ocean-floor sediments provide clues to rainfall, temperature, and vegetation patterns in Africa over nearly five million years (graph, above). Many scientists find significant correlations between periods of climate change and impor tant junctures in hominid development. Early hominids like the australopithecines illustrated above possessed features that helped them survive in avariety of landscapes. A biped's free hands are able to gather a broad range of foods. Erect posture offers less surface to absorb the sun's heat and exposes more skin to moving air, helping bipeds keep cool on the open plains. According to geologist Steven Stanley, Africa grew cooler and drier after the Isthmus of Pan ama rose (globes, left). Rain forests shrank; woods were broken by broad stretches of grass land. Stanley believes this change drove the split between Australopithecus and Homo. Paleoanthropologist Robert Blumenschine calls Tanzania's Ngorongoro Crater (right) "a modern analog" of an eastern African landscape that likely challenged hominids between one and two mil lion years ago. Dense forest darkens the crater's southern rim, foreground. Downslope, open acacia woodland thins into savanna. Grasses and sedges ring a dry lake bed. Inthe distance a stream winds across the crater floor, and on the far rim trees thicken again.