Dogs are friends of man. They work for us. They hunt with us. They also play with us. But a long time ago, all over the world, dogs were wild. Dogs can go back to the Stone Age. All dogs had the same ancestor (祖先). It is believed that this ancestor was much like a wolf. Other animals, like fox, also came from this ancestor. Thousands of years ago, man began to tame (使驯服) the wild dogs. When the dogs were tamed, they were trained well. The strong dogs then became working animals. They were trained to pull heavy loads (担子). They learned to keep watching over sheep and other animals. Working dogs could do other things, too. Some dogs were not strong. But they could help man hunt. Other dogs were best at pets. At first, there were only a few kinds of dogs. Today, there are more than 100 kinds.
