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million or so years. But we do not know for certain whether this was through the spread of one stock, or because similar creatures evolved in different places. It is generally held, though, that tool-making was carried to Asia and India (and perhaps to Europe) by migrants originally from East Africa. The establishment and survival in so many different places of these hominids must show a superior capacity to grapple with changing conditions, but in the end we do not know what was the behavioural secret which suddenly (to continue to talk in terms of prehistoric time) released that capacity and enabled them to spread over the landmass of Africa andAsia. No other mammal settled so widely and successfully before our own branch of the human family, which was eventually to occupy every continent except Antarctica, a unique biological achievement.
    The next clear stage in human evolution is nothing less than a revolution in physique. After a divergence between hominids and more ape-like creatures, which may have occurred more than four million years ago, it took rather less than two million years for one successful family of hominids to increase its brain size to about twice that of Australopithecus . One of the most important stages of this process and some of the most crucial in the evolution of humanity were already reached in a species called Homo erectus . It was already widespread and successful a million years ago, and had by then spread into Europe and Asia. The oldest specimen of the species so far found may be about half a million years older than that, while the last evidence for its survival (from Java) suggests members of it were still living between thirty and fifty thousand years ago. Homo erectus therefore successfully exploited a much bigger environment than Homo habilis and did so for much longer than has Homo sapiens , the branch of hominids to which we belong. Many signs once more point to an African origin and thence to a spread through Europe and Asia (where Homo erectus was first found). Apart from fossils, a special tool helps to plot the distribution of the new species by defining areas into which Homo erectus did not spread as well as those into which he did. This is the so-called ‘hand-axe’ of stone, whose main use seems to have been for skinning and cutting up large animals (its use as an axe in the usual, hafted sense seems unlikely, but the name is established). There can be no doubt of the success of Homo erectus as a genetic product.
    When we finish with Homo erectus there is no precise dividing line (there never is in human prehistory, a fact it is only too easy to overlook or forget), but we are already dealing with a creature who has added to the upright stance of his predecessors a brain approaching that of modern man in magnitude. Though we still know only a little of the way in which the brain is organized, there is, allowing for body size, a rough correlation between size and intelligence. It is reasonable, therefore, to attribute great importance to the selection of strains with bigger brains and to reckon this a huge advance in the story of the slow accumulation of human characteristics.
    Bigger brains meant bigger skulls and other changes, too. An increase in antenatal size requires changes in the female pelvis to permit the birth of offspring with larger heads, and another consequence was a longer period of growth after birth; physiological evolution in the female was not sufficient to provide antenatal accommodation to any point approachingphysical maturity. Human children need maternal care long after birth. Prolonged infancy and immaturity in their turn imply prolonged dependency: it is a long time before such infants can gather their own food. It may be with the offspring of Homo erectus that there began that long extension of tolerated immaturity, whose latest manifestation is the maintenance of young people by society during long periods of higher education.
    Biological change