阅读下列短文，从每题所给的四个选项（A、B、C和D）中选出最佳选项。
When did the first man appear? Although early humans didn't leave any written records, they left many paintings on the walls of the caves (洞穴) where they lived. From these paintings, we know that early humans walked on four limbs. Also, they did not live at one place but moved from place to place in search of food and water.
Without any tools, early humans used stones and wood to dig and cut their food. They also used these to protect themselves from animals. Because stones were used to make tools, this age was called Stone Age.
With time going, tools made of stones and wood were replaced (代替) with metal ones. Metals(金属) were also used for farming and making knives.
The first fire that early humans noticed was natural fire caused by lightning. With fire, early humans could now light up dark caves. They could keep themselves warm during cold weather and cook meat. Also the fire could drive animals away.
Early humans noticed new trees growing from the seeds (种子) that fell to the ground from the fruits. This gave them the idea that they could make their own food by planting seeds, which led to farming. With the beginning of farming, early humans began to live in one place. They moved out of the caves and lived on farms and near the fields together, then the first villages appeared.
Raising animals changed the life of early humans. The wild dogs were probably the first animal to be raised. As time went, goats, sheep, cows, and horses were kept, too. This led to herding (畜牧业).