When extreme climate change made the going tough in prehistoric East Africa, one branch of humanity’s closest relatives opted for bigger jaws to eat tough tubers and nuts, rather than bigger brains to figure out better ways to adapt, now feted as Homo habilis -the direct ancestor of the humans who would eventually migrate out of Africa.
It seems that when our ancestors evolved, the climate of Africa rapidly switched from wet and humid periods when the landscape was dominated by huge ephermeral lakes over 300 meters deep to arid barren stretches when rainfall was highly seasonal. Over 80% of the new hominid species in Africa emerge during these periods of extreme environmental stress. As a result of the formation of the Rift Valley, shifts in global climate caused brief periods during which these huge lakes appeared and disappeared in East Africa. These rapid shifts from very wet to very dry may have been the environmental stress necessary to push our ancestors down a human path.
In 1985, Elizabeth Vrba of Yale University, showed a significant shift from woodland/forest species to those found in a more open environment — and made a compelling argument that increased aridity led to widespread grasslands in southern Africa around 2.7 million years ago and a shift from hominins such as australopithecines who were still at home in the trees to the more fully erect species of Homo better adapted to life on the plains.
This “savanna hypothesis,” the idea that the incredible change in the landscape of East Africa sometime after 6 million years ago caused the earliest hominins to evolve bipedalism. Before this time, food was plentiful in the rainforest. After this time, though, the hypothesis goes that the spread of grasslands broke up the forest and forced adaptation. For an ape, walking upright is an extremely efficient way of covering vast distances between food sources, which may have been scarce after the vegetation shift.
However, the story is probably more complex, as early bipedal hominins such as Ardipithecus ramidus or Australopithecus afarensis have been found as far afield as Ethiopia. The savanna hypothesis implies that hominin evolution occurred first in East Africa with subsequent migration. But the fossil record is so sparse and difficult to date that it is next to impossible to confirm whether fragmenting of the East African landscape actually led to our ancestors getting up and starting to walk.
Because grasses did not fully dominate these landscapes until much later, Loic Ségalen of the University of Pierre and Marie Curie in Paris concluded that the early presence of these grasses allowed for experimentation and adaptations that may have been employed later, when grasslands began to dominate the ecosystem. So it seems that as grasslands appeared, hominins were taking advantage of new opportunities, but the appearance of grasslands alone was probably not enough to cause bipedalism (nor a major turnover of new hominin species).
The fossil record supports Mark Maslin’s theory revealing two starkly differentiated methods adopted by different hominin species: brain expansion to think and manage environmental stresses, and massive jaws to eat anything, whatever the conditions.