Dogs are friends of man,But a long time before(改为ago),dogs were wild all over the world.It(改为They) can go back to the Stone Age.All dogs had(改为have) the same ancestor(祖先),It is believe(改为believed) that this ancestor was much like a wolf .Thousands of years ago,man began (加上to)tame (使驯服) the wild dogs.Then the strong dogs became work animalshad是过去式，不是对的吗all dogs have the same ancestor狗有相同的补祖先。（我句是说一直到现在的情况，所以要用一般现在时）