We know that the earliest hominids begin showing up around five to six million years ago (Orrorin tugenensis, Ardipithecus ramidus and Australopithecus anamensis) and that this coincides with increased aridity and expansion of grasslands. This piece of information dovetails nicely into that theory.
They argue that the accelerated uplift of mountains and highlands stretching from Ethiopia to South Africa blocked much ocean moisture, converting lush tropical forests into an arid patchwork of woodlands and savannah grasslands that gradually favored human ancestors who came down from the trees and started walking on two feet - an energy-efficient way to search larger areas for food in an arid environment.
In their Geotimes article, the Ganis - a husband-and-wife research team who met in college in their native Bangladesh - describe this 3,700-mile-long stretch of highlands and mountains as "the Wall of Africa." It parallels the famed East African Rift valley, where many fossils of human ancestors were found.
"Because of the crustal movement or tectonism in East Africa, the landscape drastically changed over the last 7 million years," says Royhan Gani (pronounced rye-hawn Go-knee), a research assistant professor of civil and environmental engineering. "That landscape controlled climate on a local to regional scale. That climate change spurred human ancestors to evolve from apes."
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