Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [58]
The example that Vrba chose when launching her ‘turnover pulse hypothesis’ in 1985 was based on events culminating at about 2.5 million years ago. A global cooling event, beginning about 2.8 million years ago, appeared to cause a relatively abrupt reorganisation of African ecosystems after about 300,000 years. It was about 2.5 million years ago, Vrba noted, that robust australopithecines first made their appearance; that stone tools showed up in the geological record for the first time; and that the first evidence of Homo emerged—in the form of a skull fragment found near Kenya’s Lake Baringo by her Yale colleague Andrew Hill.
Other scientists identified the same period as marking a significant change in climate and vegetation. A French palaeobotanist, Raymonde Bonnefille, used fossil pollen evidence to show that the environment at Hadar, where Lucy had been found, had changed from wet woodland conditions to arid grasslands between 3 million and 2.5 million years ago. A member of the Omo Research Expedition, Hank Wesselman, reached much the same conclusion from his study of micromammal fauna, such as rodents and other small forms. Species that were adapted to moist conditions gave way to species, such as gerbils, known to occupy arid habitats.
Vrba’s ‘turnover pulse hypothesis’ also fitted in with the theory of punctuated equilibrium that Stephen Jay Gould and Niles Eldredge had formulated in the 1970s. But whereas Gould and Eldredge had kept to orthodox Darwinism, seeing change as coming from within the species, Vrba argued that a species’ evolution was driven by the changing world around it. ‘Speciation’, she said, ‘does not occur unless forced by changes in the physical environment’.
An American palaeobiologist, Steven Stanley, took up the theme with dramatic flourish in a book entitled Children of the Ice Age: How a Global Catastrophe Allowed Humans to Evolve. Stanley argued that a biological revolution had taken place as a result of climate upheaval 2.5 million years ago. It had led not only to the eventual demise of the australopithecines but had created the conditions that enabled Homo to emerge with a powerful brain that elevated humankind far above the rest of the animal world.
The change had occurred at momentous pace. For 1.5 million years, the apelike australopithecines had endured in Africa without evolving significantly; their brain size was only a little larger than that of apes. In the ensuing crisis, most had no chance of avoiding extinction, but one small group managed to survive. ‘During an interval of perhaps a hundred thousand years—but possibly much less—one of its populations evolved into Homo’. Unlike the australopithecines, early Homo had the wherewithal—the brain size—to make its way in the Ice Age. The label that Stanley gave for this evolutionary event was the ‘catastrophic birth’ of humankind.
To underpin his argument, Stanley made use of the growing body of evidence that suggested that Australopithecus, far from being a fully earth-bound biped, had spent a good deal of time in trees; as well as foraging for food on the ground, it possessed considerable climbing skills, retreating to trees to avoid predators such as lions. It was a highly successful mammal, but it had been caught in an ‘evolutionary straitjacket’, living between two worlds, one