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The Turkana skeleton gave a far more complete picture. Aside from hand and foot bones, nearly an entire skeleton was recovered. The teeth revealed it to be a youth, about nine years old; the pelvis suggested it was a male; the leg bones confirmed it walked upright. It possessed a full forehead and a round, smooth cranium, indicating a brain size significantly larger than an australopithecine; the brain size was later calculated to have been about 830 cubic centimetres, or 880 cubic centimetres for a fully grown adult, about two-thirds the size of a modern man’s brain. Its height was estimated to be about five feet three inches—as an adult it would perhaps have reached six feet. In general, its body proportions matched those of modern people.

Of crucial importance was the date when it lived. Most of its bones were found lying right on top of a layer of volcanic tuff which the geologist Frank Brown assessed to be about 1.65 million years old. The age assumed for Turkana Boy, therefore, was 1.6 million years.

After an exhaustive study of the skeleton, Alan Walker concluded that Turkana Boy and his species Homo erectus represented an impressive advance from their predecessors. Long-legged and immensely strong, they used techniques to make stone tools that were more developed than before. They were also successful in obtaining higherquality foods, almost certainly by hunting. Although they were not capable of speech, they lived in groups with strong social ties. Yet for all these physical attributes, Homo erectus was still essentially an animal—‘a clever one, a large one, a successful one—but an animal nevertheless’. Turkana Boy, wrote Walker, may have looked much like a human, but he almost certainly did not act like a human. ‘There was no human consciousness within that human body’.

One year later, during a second season at West Turkana, Alan Walker alighted upon another remarkable fossil. While searching a site on the Lomekwi River, twenty miles south of Nariokotome, Walker came across a small pile of stones left by a member of the Hominid Gang a few weeks before to mark the location of a piece of dark-coloured fossil. Walker picked up the fossil, examined it, then put it down. Then he picked it up again. It was part of an upper jaw with enormous tooth roots. For a moment he thought it was from some kind of extinct antelope. Then he saw another piece of bone that looked like the front of the skull of a large monkey. When he turned it over, he realised from its frontal sinus that it was hominid. With increasing excitement, excavations began.

The ‘Black Skull’, as it came to be known because of its dark patina, caused the family tree to be redrawn once more. It belonged to a robust australopithecine, a hominid similar to Olduvai’s Zinj—Australopithecus boisei—but with more pronounced features: an apelike jaw, massive brow ridges and a huge sagittal crest running the length of the brain case. The brain was extremely small for a hominid, no more than 410 cubic centimetres. Walker and Leakey decided to describe it as a ‘hyper-robust’ example of Australopithecus boisei. But whereas Zinj had been dated as just under 2 million years old, the Black Skull, according to the age of deposits where it was found, was estimated to be 2.5 million years old, predating almost everything else that had been thought of as a robust type of australopithecine. It was acclaimed in Time magazine as ‘the most exciting find since Lucy’.

The ramifications of discovering a 2.5-million-year-old robust australopithecine were considerable. The family tree hitherto accepted by most palaeoanthropologists showed the doomed australopithecine line running from the smaller-toothed africanus to the big-toothed robustus to boisei before reaching a dead end about 1 million years ago. When drawing up their family tree in 1979, Johanson and White had followed the same pattern, while placing afarensis as the common ancestor of the australopithecine line as well as the Homo line.