Read more: Cosmic accidents: 10 lucky breaks for humanity
When the going got tough in prehistoric East Africa, some of humanity鈥檚 closest relatives went for bigger jaws, rather than bigger brains. Big mistake
By some 30 million years ago, the primate upstarts had come to dominate the canopies of the once more lush tropical rainforests. For one particular group, this was a mere staging post.
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Before about 20 million years ago, east Africa boasted Amazon-like jungles that were a stable and plentiful home to our forebears, still swinging from the trees. Then the Earth moved, quite literally. A plume of magma started pushing up from beneath what is now northern Ethiopia.
During the following 15 million years, two massive mountain ranges running north to south, each about 2 kilometres high, rose up out of the east African plateau. Saddled in the middle was the Great Rift Valley, a depression a kilometre above sea level.
The mountains to the east deflected moisture-laden winds arriving from the Indian Ocean, and those in the west stopped similar winds from the Congo. Deprived of rain, the valley gradually began to change from lush rainforest to sparser savannah. For our African ancestors, living in the trees was no longer such a viable survival strategy.
The newly mountainous terrain also became host to ephemeral deep-water lakes that formed and disappeared within hundreds of years (). This environmental variability was a source of tremendous evolutionary pressure. 鈥淵ou needed an ability to migrate and move from food source to food source,鈥� says of University College London. One way or another, that led to a seminal moment in primate development around 6 million years ago: one species learned to stand and walk on two feet.
The rapidly changing environment meant primate evolution couldn鈥檛 stop there. 鈥淓ither you had to think your way out, or eat your way out,鈥� says Maslin. About 2.5 million years ago, evolution took two turns: one towards bigger brains to figure out better ways to adapt, the other towards bigger jaws to eat tough tubers and nuts. The first strategy had the greatest staying power. The lantern-jawed hominins eventually died out, while the brainier Homo habilis is feted as the direct ancestor of the humans who would eventually walk out of Africa.
