Staring about one million years ago, there was an increase in the growth of the human brain. It expanded at first at the rate of one cubic inch every hundred thousand years; then the growth rate doubled; it doubled again; and finally it doubled once more. Five hundred thousand years ago the rate of growth hit its peak. At the same time the brain was expanding at a rate of ten cubic inches every hundred thousand years. No other organ in the history of life is known to have grown as fast.
What pressures generated the explosive growth of the human brain? ( 2 ) may supply part of the answer. At that time the world began to enter into a great Ice Age, the first on the planet in hundreds of million of years. The trend toward colder weather set in slowly at first, but after a million years areas of ice began to form in the north. They thickened into glaciers as more snow fell, and then the glaciers joined together into great sheets of ice, as much as two miles thick. When the ice sheets 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 dropped by three hundred feet.
These events occurred precisely at the same time as the period of most rapid expansion of the human brain. Is this significant, or is it accidental?
( 2 )には
ア, The custom of wearing clothes that took root in the first ice age
イ, The fact that human beings had to fight against dinosaurs
ウ, The invention of language and the use of tools
エ, A change of climate that set in about two million years ago