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efficient ways to get food. By 1.8 million years ago, at the beginning of the Pleistocene, more modern hominins were established in Africa, though the earliest Homo sapiens , our own species, did not appear until a mere 100,000 years ago.
    One of these early members of our genus, Homo ergaster , is noteworthy because it may have marked the appearance of the first long-distance runner in our lineage. Although earlier hominins were also bipedal at least some of the time, Homo ergaster was tall (one skeleton with the fetching name of KNM-WT 15000 was 5 feet 4 inches tall as a young boy, projected to stand at 6 feet when he was fully grown) and had the long legs, narrow hips, and more barrel-shaped chest of modern running humans, as opposed to the long arms and stubbier legs of earlier forms that probably still spent a considerable amount of time in the trees.
    The running is significant not just because it contributes to the controversy over “natural” forms of exercise that I will discuss in Chapter 6, but because some anthropologists believe it was linked to the evolution of larger brains. Running would allow ancestral humans to get to carcasses left by predators like lions before other scavengers such as hyenas, and hence gain access to high-quality protein. Such concentrated food can fuel the demands of large brains, which are physiologically expensive to maintain. To obtain and process game animals, Homo ergaster made more sophisticated tools than earlier humans had made, including hand axes with nifty double-faced blades and sharp, narrow points. Interestingly, Homo ergaster also shared a doubtless unwelcome advancement with modern humans: intestinal parasites. Recent studies of the human tapeworm, a gut inhabitant that is specialized on humans and the animals they eat, show that at least two kinds of tapeworms date back to 1.7 million years ago, long before animal domestication. 15
    Homo ergaster spread from Africa into Europe, with other species of Homo also appearing in various parts of Eurasia. Between 900,000 and 130,000 years ago, during the Pleistocene, the Earth became cooler, with long periods when glaciers covered large parts of the world. While the glaciers covered some areas, deserts spread over northern Africa, separating the populations of plants and animals, including humans. Anthropologists do not agree about exactly how to classify the fossil humans found from this period, but it is clear that brain size evolved substantially from that of Homo ergaster ; and one kind of early human, Homo heidelbergensis , hunted and butchered large animals such as mammoths. By 300,000 years ago, humans had produced even more finely crafted stone tools, including some that were suitable for attaching to handles—a marked improvement that allows much greater force to be applied to the tool.
    The Neandertals, reconstructions of whom are probably responsible for most people’s conceptions of what cavemen supposedly looked like, lived in Europe and eastern Asia from 127,000 until about 30,000 years ago. They were not human ancestors, meaning that their lineage did not lead directly to that of modern humans, but because they lived in Europe and many paleontologists found their own European homeland a more convenient place to dig for fossils than many parts of Africa and Asia, we know more about them than we do about any other early human.
    The recent discovery that we humans share a small proportion of our genes with the Neandertals, 16 suggesting that early Homo sapiens and Neandertals mated with each other, has renewed interest in the lives of these hominins. The fossils reveal skulls with big front teeth that show heavy wear, perhaps because the Neandertals used to pull their meat through their incisors. The Neandertals also had rather stocky frames; according to Boyd and Silk, “A comparison with data on Olympic athletes suggests that Neanderthals most closely resembled the hammer, javelin, and discus throwers and