Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [59]
It was climate change, said Stanley—‘a flip of the climatic switch’—that had closed the door on its existence, but that had also opened the opportunity for a spurt of evolution towards Homo. As woodland areas shrank, australopithecines were forced to abandon the habit of tree-climbing and to make longer forays across more open terrain in search of food, constantly at risk from predators. By chance, one of its populations survived to become an earth-bound biped. ‘The accidental nature of our evolutionary birth is astounding’, wrote Stanley.
Few scientists were willing to support the idea that the driving force of climate change was enough to explain the emergence of humankind. In a statistical analysis examining the relationship between evolutionary and climatic patterns published in 1993, Robert Foley, a Cambridge biologist, concluded that climate was an important element but not the only one. ‘Another such element is competition, both within and between species’.
It is probably the case that where climate change is an important factor, it operates through competition. Climatic change will alter the nature, abundance and distribution of environments and resources within those environments. This will lead to changes in competitive relationships between and within species, and it is these altered competitive relationships that are likely to lead to evolutionary consequences. The consequences might be extinction of populations as the most direct effect, or speciation as a less direct one arising either out of reduced intra-community competition or the opening up of new ecological opportunities. Competition, therefore, is always likely to be the immediate cause of evolutionary change, played out within a framework determined by, among other factors, the climate.
Climate change alone, Foley concluded, was not sufficient to explain the patterns of speciation and extinction among hominids and other African primates. ‘Clearly competitive relationships can change independent of climate, or alternatively, the impact of climate change will vary markedly with geographical factors. In other words, it is probable that species appear and disappear as a result of local competitive conditions rather than broad global patterns of climatic change’.
Further evidence emerged, however, reinforcing the view that key stages of hominid evolution had been directly affected by the impact of environmental change. Researchers conducting geological surveys of ancient lakes in eastern Africa discovered that although the climate record there showed a long-term drying trend, it had been punctuated by short episodes of extreme variability, fluctuating from wet to dry conditions. Over the past 3 million years, giant lakes up to 1,000 feet deep and stretching for hundreds of square miles had been formed and had then vanished as the climate changed; the disappearances of the lakes had been followed by periods of severe drought. ‘At one extreme’, observed Mark Maslin, a leading palaeoclimatologist, ‘the landscape would have been a true Garden of Eden, with beautiful freshwater lakes, beautiful shorelines and forests along the rivers. There would have been open spaces allowing early humans to exist easily, with water and lots of resources. But occasionally these quickly flipped into bone-dry periods, where it’s 45 degrees centigrade in the middle of the day and no natural water resources’.
Three of these periods of extreme variability, said the researchers, had occurred around 2.5 million, 1.5 million and 1 million years ago, when global climate changes coincided with tectonic disruption in the Great Rift Valley. They had acted as a catalyst for evolutionary change, putting enormous pressure on hominids to adapt to the new environment. Some species were driven to the brink of extinction; others survived, acquiring higher brain power and new skills.
CHAPTER 13
NEW FRONTIERS
THE BOUNDARIES OF human origins were pushed back even further