How Climate Change and Plate Tectonics Shaped Human Evolution

A new study links the emergence of new hominin species, expanding brain capacity and early human migration with the appearance of deep freshwater lakes

By Mark Maslin and The Conversation

Editor's note: The following essay is reprinted with permission from The Conversation, an online publication covering the latest research.
It should not be a surprise that East Africa was a hotbed of evolution, because over the last five million years everything about the landscape has changed.
The extraordinary forces of plate tectonics and a changing climate have transformed East Africa from a relatively flat, forested region to a mountainous fragmented landscape dominated by the rapid appearance and disappearance of huge, deep-water lakes. And from this highly variable landscape emerged an ape smart enough to question its own existence.
A cradle rocked by tectonics
Twenty million years ago the Indian and Asian continental plates clashed and pushed up the massive Tibetan plateau. In summer this plateau acts as a huge heat engine, absorbing solar energy which it transfers to the atmosphere, causing immense convection currents. With all this hot air rising, air is sucked in from all round, including moist air from the Indian Ocean that produces intense South East Asian monsoons.
This has a knock on affect of drawing moisture away from the African continent, and it was this that began the progressive drying out of East Africa. In terms of human evolution, this distinct split between the climate of Asia and Africa coincides with the split between Asian and African apes, the latter eventually evolving into us.
At the same time as the peaks of Tibet were thrusting upwards, the rifting process began in Ethiopia and gradually moved south finishing in Mozambique about one million years ago. This rifting was caused by a hotspot of magma under northern East Africa heating the crust causing it to split down the middle like an overdone apple pie.
The rifting process produced a deep, wide, hanging valley half a mile above sea level with uplifted shoulders or mountain ranges on either side rising up to two miles high. The affects of the Rift Valley formation on the local climate was dramatic.
The East Rift’s mountains prevented moist air from the Indian Ocean from passing over East Africa, causing the region to dry even further. The topography of East Africa completely changed: from a homogeneous flat region covered in moist forest, to a mountainous landscape with plateaus and deep rift valleys, where vegetation varied from cloud forest to desert scrub.
Evolution, our coping strategy
Presented with fragmented vegetation and greater distances between sources of food may have led to the evolution of human bipedalism – walking upright on two legs – around six million years ago. These highly successful early bipedal hominins such as Ardipithecus ramidus or Australopithecus afarensis, were nevertheless relatively small-brained, with a cranial capacity of about 450cm³ compared with modern humans with over 1,500cm³.
The development of the East African Rift valley fragmented the landscape and formed a large number of separate lake basins. The mountainous landscape makes these basins very sensitive to small changes in rainfall. Martin Trauth of Potsdam University and colleagues found geological evidence that deep, freshwater lakes existed around 2.6 million, 1.8 million and 1 million years ago – key dates in human evolutionary history.