Read the text below and answer the question that follows.
Prehistoric Man lived at first chiefly on wild berries, nuts, roots, and herbs. As his implements improved and his skill increased, he became hunter, trapper, and fisher. A tribe of hunters, however, requires an extensive territory and a constant supply of game. When the wild animals are all killed or seriously reduced in number, privation and hardship result. It was a forward step, therefore, when man began to tame animals as well as to kill them.
Why prehistoric man became hunter, trapper and fisher?