The movements of humans in the last million years provide a clue to the answer. At the beginning of the Ice Age, humans lived near the equator, where the climate was mild and pleasant. Later they moved northward. From their birthplace in Africa they moved up across Arabia and then turned to the north and west into Europe, as well as eastward into Asia.
When these early movements took place, the ice still only covered the lands in the far north; but eight hundred thousand years ago, the ice moved southward until it covered large parts of Europe and Asia. Then, for the first time, humans encountered the bone-chilling, freezing winds from the cakes of ice in the north. The climate in southern Europe had a Siberian coldness then, and summers were nearly as cold as European winters are today.
In those difficult times, resourcefulness and inventiveness must have been of great value. Which individual first thought of stripping the fur from dead animals to wrap around hi body? Only by (human beings/ imaginative acts/ could/ such inventive/ survive/ and) a cold climate. In every generation, the individuals with strength, courage, and creativity were the ones more likely to survive the Ice Age; those who were less resourceful fell victim to the climate and their numbers were reduced.
The Ice Age winter was the greatest challenge that humans had ever faced. They were naked and defenseless against the cold, as some little mammals had been defenseless against the dinosaurs one hundred million years before. Facing the pressure of a hostile world, both those mammals and humans were forced to live by their wits; and both became, in their time, the most intelligent animals of the day.