Читать книгу The Times History of the World - Richard Overy - Страница 7

Global cooling between five and six million years ago saw savannahs replace the tropical forests of sub-Saharan Africa. The appearance of this new environment was in turn matched by an evolutionary pulse that gave rise to new carnivores and omnivores. Among them were the hominines, the ancestors of modern man.

The earliest hominine fossils, discovered in the Afar region of Ethiopia, are the fragmentary 4.5-million-year-old remains of Ardipithecus ramidus. Better evidence is available of the later and more widespread Australopithecenes, or “southern apes”. Skeletal and fossilized footprints of Australopithecus afarensis, dated to between three and four million years ago, indicate a serviceable if not fully bipedal gait, hands still partly adapted for specialized tree climbing and a brain approximately one-third the size of ours. This species is the probable ancestor both of the robustly built Australopithecines boisei, aethiopicus and robustus, all with large teeth and herbivorous diets, and of our genus, Homo, meaning “man”. A major discovery thrown up by fieldwork since the 1950s has revealed that these closely related but nonetheless distinctive species not only lived at the same time but side by side in the same habitats. Finds of more species are expected.

From between two and three million years ago, there is evidence of important evolutionary trends in Homo: brains became much bigger, a process known as encephalization; and full bipedalism was attained—as the 1974 discovery of the fossil skeleton known as ‘Lucy’ shows. As larger brains need better diets to sustain them, the increase in brain size could only have occurred as a result of significant evolutionary pressures. The problem was compounded because hominines stayed the same size, with the result that their bigger brains could be achieved only by reducing the size of another organ, the stomach, a trade-off which in turn reduced the efficiency of the digestive tract, which in turn demanded a still better diet.

EARLY TECHNOLOGIES

The most convincing explanation of this development—the expensive tissue hypothesis—holds that a move towards an energy-rich diet, particularly animal proteins, was responsible. And indeed the earliest-known stone tools, found in Gona, Ethiopia, suggest that 2.5 million years ago meat was a central part of hominines’ diet, with the sharpened stones used to cut flesh and pound marrowrich bones from carcasses either scavenged or brought down and then defended against carnivores. Burnt bones found in southern Africa indicate that by 1.5 million years ago hominines had learned to “cook” their food, a development which again would have compensated for smaller stomachs by breaking down animal proteins before digestion took place.

OUT OF AFRICA

This pattern of development was the basis for the first colonization, by Homo erectus, 1.8 million years ago of areas outside sub-Saharan Africa. Then, around 500,000 years ago, Homo heidelbergensis migrated into north Africa and the Near East, reaching northern Europe about 400,000 years ago (however there is also good evidence in northern Europe for hominines 700,000 years ago). Homo erectus and heidelbergensis are sometimes considered to have shared a common ancestor, a type designated as Homo ergaster, and best known from the skeleton found at Nariokotome in Kenya’s Rift valley. By perhaps 1.5 million years ago, all three had brains of about 1000cc (61cu in) and an adaptable stone technology: the weight and careful shaping of the edges of their distinctive handaxes, whether pointed or oval, made them effective butchery tools.

Stone technology was not the only factor in the evolutionary pressures that led to larger brains. It was also to do with allowing hominines to remember, to manipulate, to support and to organize others in more complex ways. Perhaps paradoxically, as hominines developed these more sophisticated social structures, so they simultaneously became less reliant on one another and better adapted to living in smaller groups. This in turn allowed them to colonize harsher barrier habitats such as the Sahara at the margins of their homelands from where they could colonize new, more temperate areas beyond.

MODERN HUMANS

From about 500,000 years ago, this early burst of colonization came to a halt. Instead, though there were undoubtedly many dispersals of populations and much intermingling of genes, regional groups of separate populations living side by side such as the Neanderthals developed. But from 100,000 years ago, another major dispersal began when anatomically modern people—Homo sapiens sapiens—emigrated from sub-Saharan Africa. By 50,000 years ago, Australia had been reached, by boat; 33,000 years ago, the western Pacific islands were colonized; 15,000 years ago, the Americas were reached. Major expansion into the Arctic began about 4500 years ago as the continental ice sheets retreated. Finally, 2000 years ago, humans began to settle the deep Pacific islands from where they reached New Zealand around 1200 years ago, 1000 years before the island’s discovery by Captain Cook.