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phenomenal rate of ten cubic inches every hundred thousand years. No other organ in the history of life is
known to have grown as fast as this
2)
.
10 2.What pressures generated the explosive growth of the human brain? A change of climate that set in
about two million years ago may supply part of the answer. At that time the world began its descent into a
great Ice Age, the first to afflict the planet in hundreds of millions of years. The trend toward colder
15
weather set in slowly at first, but after a million years patches of ice began to form in the north. The ice
patches thickened into glaciers as more snow fell, and then the glaciers merged into great sheets of ice, as
20
much as two miles thick. When the ice streets reached their maximum extent, they covered two-thirds of
the North American continent, all of Britain and a large part of Europe. Many mountain ranges were buried
entirely. So much water was locked up on the land in the form of ice that the level of the earth's oceans
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dropped by three hundred feet. These events coincided precisely with the period of most rapid expansion of
the human brain. Is the coincidence significant, or is it happenstance?
3.The story of human migrations in the last million years provides a clue to the answer. At the beginning
30 of the Ice Age Homo 3) lived near the equator, where the climate was mild and pleasant. Later he moved
northward. From his birthplace in Africa 4) he migrated up across the Arabian peninsula and then turned to
the north and west into Europe, as well as eastward into Asia.
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4.When these early migrations took place, the ice was still confined to the lands in the far north; but eight
hundred thousand years ago, when man was already established in the temperate latitudes, the ice moved
southward until it covered large parts of Europe and Asia. Now, for the first time, men encountered the
40
bone-chilling blasts of freezing winds that blew off the cakes of ice to the north. The climate in southern
Europe had a Siberian harshness then, and summers were nearly as cold as European winters are today.
5.In those difficult times, the traits of resourcefulness and ingenuity must have been of premium value.
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Which individual first thought of stripping the pelt from the slaughtered beast to wrap around his shivering
limbs? Only by such inventive flights of the imagination could the naked animal survive a harsh climate. In
every generation, the individuals endowed with the attributes of strength, courage, and improvisation were
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the ones more likely to survive the rigors of the Ice Age; those who were less resourceful, and lacked the
vision of their fellows, fell victims to the climate and their numbers were reduced.
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6.The Ice Age winter was the most devastating challenge that Homo had ever faced. He was naked and
defenseless against the cold, as the little mammals had been defenseless against the dinosaurs one hundred
million years ago. Vulnerable to the pressures of a hostile world, both animals were forced to live by their
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wits; and both became, in their time, the brainiest animals of the day.
7.The tool-making industry of early man also stimulated the growth of the brain. The possession of a
good brain had been one of the factors that enabled Homo to make tools at the start. But the use of tools
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became, in turn, a driving force toward the evolution of an even better brain. The characteristics of good
memory, foresight, and innovativeness that were needed for tool-making varied in strength from one
individual to another. Those who possessed them in the greatest degree were the practical heroes of their
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day; they were likely to survive and prosper, while the individuals who lacked them were more likely to
succumb to the pressures of the environment. Again these circumstances pruned the human stock, expand-
ing the centers of the brain in which past ex periences were recorded, future actions were contemplated,
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and new ideas were conceived. As a result, from generation to generation the brain grew larger.
8.The evolution of speech may have been the most important factor of all. When early man mastered the
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loom of language, his progress accelerated dramatically. Through the spoken word a new invention in tool-
making, for example, could be communicated to everyone; this way the innovativeness of the individual
enhanced the survival prospects of his fellows, and the creative strength of one became the strength of all.
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More important, through language the ideas of one generation could be passed on to the next, so that each
generation inherited not only the genes of its ancestors but also their collective wisdom, transmitted
through the magic of speech.
9.A million years ago, when this magic was not yet perfected, and language was a cruder art, those
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bands of men who possessed the new gift in the highest degree were strongly favored in the struggle for
existence. But the fabric of speech is woven out of many threads. The physical attributes of a voice box,
lips, and tongue were among the necessary traits; but a good brain was also essential, to frame an abstract
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thought or represent an object by a word.
10. Now the law of the survival of the fittest began to work on the population of early men. Steadily, the
physical apparatus for speech improved. At the same time, the centers of the brain devoted to speech grew
100 in size and complexity, and in the course of many generations the whole brain grew with them. Once more,