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have a mixture of ‘primitive’ as well as distinctive Homo traits. But Mary insisted that they belonged to the genus Homo and not to Australopithecus. In her view, australopithecines were an offshoot of the hominid line, a contemporary of early man that had died out. But she was unsure where on the Homo line the Laetoli fossils should be placed. Hoping to find more and better specimens, she delayed putting a name to them. It was a decision that later she would come to regret bitterly.

In August 1975, Richard Leakey’s team at Koobi Fora scored another success. At a location not far from the main camp, Bernard Ngeneo, the Hominid Gang member who had found the 1470 skull, spotted fragments of bone poking above the surface. After days of careful excavation, Ngeneo’s find turned out to be a skull, with the face almost complete; but it resembled nothing else that had been found at East Turkana. While examining it, Leakey noticed strong similarities to specimens of Homo erectus from Zhoukoudian—Peking Man—thought to date back to between 500,000 and 700,000 years. Hitherto, few fragments of Homo erectus had been discovered in Africa. But Leakey was soon convinced of the skull’s identity. ‘There was no doubt’, he wrote in his autobiography, ‘that this was not Australopithecus nor even Homo habilis but rather H. erectus, a more immediate ancestor of ourselves’. Its brain size just reached the lower limits of Homo erectus. But what was particularly striking about it was its age. The initial estimate was between 1.3 million and 1.6 million years. But subsequent evaluation showed it to be 1.8 million years.

The discovery of skull 3733, as it was numbered, had ramifications that went far beyond the matter of its age. For it put an end to a theory about human evolution that had gained increasing attention in the 1960s and 1970s—the single species hypothesis. For several decades, most scientists had held fast to the notion that only one type of hominid could have been alive at any one point in time. When Robert Broom and John Robinson in 1949 announced the discovery of a novel species of hominid they called Telanthropus, suggesting that two extinct species had once coexisted, they set off a prolonged controversy. Louis Leakey faced the same furore in the 1960s by arguing that Zinj, a robust australopithecine, and Homo habilis had lived ‘side by side’.

At the forefront of the single species theory in the 1960s were two influential American anthropologists, Loring Brace and Milford Wolpoff, of the University of Michigan. Like the architects of the modern synthesis of the late 1940s, they viewed human history as progressing by gradual change up a single evolutionary ladder that started with australopithecines and ended in modern humans. They conceded that there were measurable differences between individual fossils but saw no need to create new species to accommodate those differences. Their case relied on a piece of ecological theory—the principle of competitive exclusion—that maintained that two species with similar adaptations could not occupy the same ecological niche in the same place at the same time; sooner or later, one of the two species would out-compete the other. Applying the principle of competitive exclusion to hominid evolution, they argued that because of cultural adaptation—such as toolmaking—only one species could have existed at any given period.

The discovery of 3733—a clear example of Homo erectus—blew this theory apart because it had been found at the same stratigraphic level as a skull numbered 406 that Richard Leakey had discovered in 1969 and that had proved to be a clear example of Australopithecus boisei. The differences between them were easily recognised. The British anatomist Alan Walker described 406 as ‘an eating machine’—it possessed enormous teeth, jaws and chewing muscles that were anchored to striking bony crests at the top and back of the skull and to its strong cheekbones. By contrast, 3733 was ‘a thinking machine’—its cranium was dominated by a large, globular braincase housing a brain that was almost