Crudely summarized, the main lines of this evolution were probably determined for millions of years mainly by astronomical cycles and a few sudden events, such as eruption of massive volcanoes or the impact of asteroids. Climate was the all-important factor, changed by the earth’s position in relation to the sun or by short-term circumstances. A huge pattern emerges, of recurrent swings of temperature. The extremes which resulted, of climatic cooling on the one hand and aridity on the other, choked off some possible lines of development. Conversely, in other times, and in certain places, the onset of appropriately benign conditions allowed certain species to flourish and encouraged their spread into new habitats. The only major sub-division of this immensely long process which concerns us comes very recently (in prehistoric terms), slightly less than 4 million years ago. There then began a period of climatic changes which we believe to have been more rapid and violent than most observed in earlier times. ‘Rapid’, we must again remind ourselves, is a comparative term; these changes took tens of thousands of years. Such a pace of change, though, looks very different from the millions of years of much steadier conditions which lay in the past.
Scholars have long talked about ‘Ice Ages’, each lasting between 50,000 and 100,000 years, which covered big areas of the northern hemisphere (including much of Europe, and America as far south as modern New York) with great ice sheets, sometimes a mile or more thick. They have now distinguished some seventeen to nineteen (there is argument about the exact number) such ‘glaciations’ since the onset of the first, over 3 million years ago. We live in a warm period following the most recent of them, which came to an end some 10,000 years ago. Evidence of these glaciations and their effects is now available from all oceans and continents and they provide the backbone for prehistoric chronology. To the external scale which the Ice Ages provide we can relate such clues as we have to the evolution of humanity.
The Ice Ages make it easy to see how climate determined life and its evolution in prehistoric times, but to emphasize their dramatic direct effects is misleading. No doubt the slow onset of the ice was decisive and often disastrous for what lay in its path. Many of us still live in landscapes shaped by its scouring and gouging thousands of centuries ago. The huge inundations which followed the retreat of the ice as it melted must also have been locally catastrophic, destroying the habitats of creatures which had adapted to the challenge of arctic conditions. Yet they also created new opportunities. After each glaciation, new species spread into the areas uncovered by the thaw. Beyond regions directly affected, though, the effects of the glaciations may have been even more important for the global story of evolution. Changes in environment followed cooling and warming thousands of miles from the ice itself; and the outcome had its own determining force. Both aridification and the spread of grassland, for instance, changed the possibilities of species spreading themselves into new areas, especially if they could stand upright and move on two feet. Some of those species form part of the human evolutionary story, and all the most important stages in that evolution – so far observed – have been located in Africa, far from the ice-fields.
Climate can still be very important today, as contemplation of the disasters caused by drought show. But such effects, even when they affect millions of people, are not so fundamental as the slow transformation of the basic geography of the world and its supplies of food which climate wrought in prehistoric times. Until very recently climate determined where and how humans lived. It made technique very important (and still does): the possession in early times of a skill such as fishing or fire-making could make new environments available to branches of the human family fortunate enough to possess such skills, or able to discover and learn them. Different food-gathering possibilities in different habitats meant different chances of a varied diet and, eventually, of progressing from gathering to hunting, and then to growing. Long before the Ice Ages, though, and even before the appearance of the creatures from which humanity was to evolve, climate was setting the stage for and thus shaping, by selection, the eventual genetic inheritance of humanity itself.
One more backward glance is useful before plunging into the still shallow (though gradually deepening) pool of evidence. One hundred million or so years ago, primitive mammals were of two main sorts. Some, rodent-like, remained on the ground; others took or had taken to the trees. In this way the competition for resources was lessened and strains of each survived to people the world with the creatures we know today. In the second group were the prosimians. We are among their descendants, for they were the ancestors of the first primates.
It is best not to be too impressed by talk about ‘ancestors’ in any but the most general sense. Between the prosimians and ourselves lie millions of generations and many evolutionary blind alleys. It is important none the less that our remotest identifiable ancestors lived in trees because what survived in the next phase of evolution were genetic strains best suited to the special uncertainties and accidental challenges of the forest. That environment put a premium on the capacity to learn. Those survived whose genetic inheritance could respond and adapt to the surprising, sudden danger of deep shade, confused visual patterns and treacherous handholds. Strains prone to accident in such conditions were wiped out. Among those that prospered (genetically speaking) were some species with long digits which were to develop into fingers and, eventually, the oppositional thumb, and other forerunners of the apes already embarked upon an evolution towards three-dimensional vision and the diminution of the importance of the sense of smell.
The prosimians were little creatures. Tree-shrews still exist which give us some idea of what they were like; they were far from being monkeys, let alone men. Yet for millions of years they carried the traits which made humanity possible. During this time geography counted for much in their evolution, by imposing limits on contact between different strains, sometimes effectively isolating them, and thus increasing differentiation.
Changes would not happen quickly but it is likely that fragmentations of the environment caused by geographical disturbance led to the isolation of zones in which, little by little, the recognizable ancestors of many modern mammals appeared. Among them are the first monkeys and apes. They do not seem to go back more than 60 million years or so.
These monkeys and apes represent a great evolutionary stride. Both families had much greater manipulative dexterity than any predecessor. Within them, species distinct in size or acrobatic quality began to evolve. Physiological and psychological evolution blur in such matters. Like the development of better and stereoscopic vision, the growth of manipulative power seems to imply a growth of consciousness. Perhaps some of these creatures could distinguish different colours. The brains of the first primates were already much more complex than those of any of their predecessors; they were bigger, too. Somewhere the brain of one or more of these strains became complex enough and its physical powers sufficiently developed for the animal to cross the line at which the world as a mass of undifferentiated sensations becomes at least in part a world of objects. Whenever this happened it was a decisive step towards mastering the world by using it, instead of reacting automatically to it.