The Social Leap

William von Hippel


Expelled from Eden

You and I are descendants of chimplike creatures* who left the rainforest and moved to the savannah six or seven million years ago. On first glance it would seem like an odd decision for our ancestors to leave the trees, as there were virtually no predators that could hunt them successfully when they were in the forest canopy. Even superb tree climbers such as leopards don’t attack chimps in trees, as chimps are simply too fast and too dangerous when they are in their element. On the ground, however, chimps are easy prey. They are ungainly on two legs, comparatively slow on all four, and their small size makes them an easy meal for large cats such as lions, leopards, or the sabre-toothed tigers that once roamed East Africa.

So why leave the trees? What compelled our ancestors to trade the safety and sheer exuberance of life in the canopy for a slow and clumsy existence on the ground? There is vigorous scientific debate on this question, but one widely endorsed theory is an updated version of the ‘savannah hypothesis’. This hypothesis was proposed by Ray Dart in 1925, when he published the discovery of Australopithecus africanus, or ‘the man-ape of South Africa’. After noting that humans were unlikely to have evolved in tropical forests because life there was too easy, Dart wrote, ‘For the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of the intellect — a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species.’


Dart was right that we evolved in the savannah, but in 1925 he had no idea what forces put us there. We now believe that tectonic activity along the East African Rift Valley is what split us from our chimpish ancestors. All the earth’s surfaces, including the landmasses that make up the continents and the bottoms of the oceans, sit on tectonic plates. These plates float around on an underlying mantle, which emerges as a viscous liquid when it flows from a volcano but is under so much pressure below the earth’s crust that it is more like pliable road tar. The heat emanating from the earth’s core creates incredibly slow but strong currents in the mantle, and these currents carry the plates around with them. Sometimes these plates ram into each other in super slo-mo, as is the case with India smashing into Asia, a by-product of which is the Himalayas (which continue to rise a few centimetres each year). Sometimes these plates tear apart and move away from one another. In Africa, the east side of the continent is slowly unzipping from the rest, starting at the Red Sea, in the north, and ending at the coast of Mozambique, in the south.

The tectonic activity along this geographic zipper created the East African Rift Valley and slowly and sporadically raised vast portions of Ethiopia, Kenya, and Tanzania to an elevated plateau. These changes in topography led to localised changes in climate, with the rainforests on the east side of the Rift Valley drying out one by one, to be replaced by savannah. So it turns out that we didn’t leave the trees after all —the trees left us.

Because our chimpish ancestors were so impressive in the trees and so unimpressive on the ground, the gradual replacement of the rainforest with savannah meant that they had to find a new way to make a living. The fruits, berries, and leaf buds they were accustomed to eating receded along with the trees, their opportunities to hunt for meat were greatly diminished by their slow speed on the ground, and, to top it off, enormous predators prowled the grasslands. So how did our ancestors respond to this double whammy of disappearing food and newly dangerous predators? No doubt many of our would-be ancestors perished, but some of them survived and eventually began to thrive, and their story is our own.

* Humans and chimps evolved from a common ancestor, but we don’t know exactly what that ancestor looked like. The fossil record strongly suggests that our shared grandparents looked far more like today’s chimps than like us. For this reason, I refer to our common ancestors as chimplike or chimpish.

The Social Leap William von Hippel