Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [27]
Mayr reached a similar conclusion, though from a different perspective. After several years of fieldwork on the birds of New Guinea and the Solomon Islands, Mayr became convinced that geographic features played a crucial role in the production of species. Speciation, he observed, occurred only through an initial phase of geographical isolation, breaking a once homogenous population into subpopulations; these then began to evolve in different directions, according to the demands of their local environment. Mayr introduced the term ‘allopatric speciation’ to describe the process, meaning speciation ‘in another place’. Small isolated populations on the fringe of a main population, separated by barriers such as mountains or stretches of water, were the most likely source of new species, he said. Once they had become distinct, they were no longer able to interbreed with the main population. In other words, geographical forces rather than biological factors were responsible for dividing a population. Speciation could only occur when there was a vacant ecological niche that a subspecies could occupy.
Mayr went on to argue that because humans were so geographically widespread and so diversely adapted, there had been few opportunities for speciation except, perhaps, at the very beginning. There was no justification, he said, for palaeontologists and palaeoanthropologists to designate large numbers of human types, as Robert Broom had done with the australopithecines. Mayr was ready to include Australopithecus as part of the hominid family. But he insisted initially that only one species of Australopithecus could have existed, on the grounds that according to the principle of competitive exclusion, if there were two rival species occupying the same territory, one would have driven the other to extinction. He eventually accepted that because of the degree of morphological difference, there must have been two species, a robust form which had died out and the more lightly built Australopithecus africanus. It was Australopithecus africanus, he decided, that had evolved into Homo.
As a result of Mayr’s strictures, scientists agreed on a new dispensation: australopithecines were accepted as the first member of the hominid family; next came Homo erectus, a category said to have evolved from australopithecines that included Pithecanthropus from Java and similar fossils from China; then came Homo sapiens. The Neanderthals were no longer regarded as an extinct primitive species but as an early variant of Homo sapiens, perhaps adapted to the colder climate of the Ice Ages; how to fit Piltdown Man into this scheme remained unsolved. Thus, human evolution was envisaged as a straight line of continuous transformation of one species into the next.
Although the modern synthesis provided a more straightforward structure for understanding human evolution, it lost the diversity that palaeoanthropologists had hitherto enjoyed to accommodate the range of fossils they had discovered. A diverse assortment of fossils was now lumped together as variations on a theme because they originated from roughly the same period. Species became no more than segments of steadily evolving lineages. According to the modern synthesis, with time and anatomical change marching in tandem, the fossil record should have shown gradual intergradations from one species to the next. The difficulty was that fossil hunters had so far found too few hominid specimens to ascertain whether the theory fitted the facts on the ground.
In Africa, meanwhile, the Leakeys had found a benefactor who was to transform their fortunes. Responding to a letter that Louis Leakey wrote to the London Times in February 1948, explaining the difficulties and expense of organising field expeditions, Charles Boise, an American-born London businessman, sent him a cheque for £1,000. An art connoisseur, Boise had once worked in