Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [57]
The conclusion that Vrba reached was that environmental change was more likely to promote the formation of new species in groups of dietary specialists than in those groups that were generalists. What had caused such a marked change 2.5 million years ago, she maintained, was an abrupt shift in global climate. ‘Specialist species replace each other as the climate changes, like partners in a gavotte—a dance through time’.
Vrba went on to develop a radical theory about the impact of climate change on evolution. Climate change had long been regarded as an important factor. Charles Darwin acknowledged that evolution speeded up when conditions changed, but he viewed climate change as a subsidiary mechanism of natural selection that served to tighten the screws on competition. ‘As climate chiefly acts in reducing food, it brings about the most severe struggle between the individuals’, he wrote in On the Origin of Species.
Modern research shows that over the last 50 million years, the earth has experienced a progressive shift towards cooler conditions. In the early stages of the Miocene, about 20 million years ago, an immense band of forest stretched across Africa, from the Atlantic coast to the Indian Ocean, giving shelter and sustenance to a wide variety of apes, among them the ancestors of all existing apes. But by the end of the Miocene, about 5 million years ago, the forests had broken up into a mosaic of landscapes including scattered woodlands and grassland savannahs. In eastern Africa, the formation of the great highland domes of modern Kenya and Ethiopia, beginning about 10 million years ago, had added to the change, preventing moist air from the Indian Ocean from passing over the wall of mountains, throwing up a rain shadow that led the once-continuous forest to shrink and fragment.
Other changes in the global climate also had an impact. Using new techniques, palaeoclimatologists concurred with Vrba that as well as the general trend towards cooler conditions, there had been abrupt falls in temperatures. A steplike drop about 14 million years ago brought about a significant extension of the Antarctic ice sheet. A further episode occurred about 6 million years ago. About 2.8 million years ago, another sharp fall in temperatures marked the onset of continental glaciation in the northern hemisphere and the formation of the Arctic ice cap. The Pleistocene epoch—between 1.8 million and 10,000 years ago—was a time of continuing climatic instability, shifting from long cold glacial episodes to warm interglacial conditions and back again. Ice sheets and other glaciers advanced and retreated periodically from the north. At the peak of the Ice Ages, nearly onethird of the earth’s surface was covered by glaciers. In Europe, northern Germany and most of England were buried under ice hundreds of feet thick; in North America, the ice sheet advanced as far south as what is now New York City.
The prevailing view until the 1960s was that the tropics had been more stable than temperate zones, providing a more consistent ecological arena for human evolution. The theory holding sway—the ‘Pluvial Theory’—maintained that periods of glaciation at the earth’s poles had been accompanied by increased rainfall and vegetation in the tropics. But during the 1960s, new research showed that the opposite was the case: that global cooling had led to more arid conditions around the equator; and that far from being part of a stable environment, Africa’s landscape had constantly been reworked by tectonic movements, volcanic eruptions and lava flows.
Vrba’s theory was that abrupt changes in global climate had led to evolutionary