Dogs are friends of man,But a long time before(改為ago),dogs were wild all over the world.It(改為They) can go back to the Stone Age.All dogs had(改為have) the same ancestor(祖先),It is believe(改為believed) that this ancestor was much like a wolf .Thousands of years ago,man began (加上to)tame (使馴服) the wild dogs.Then the strong dogs became work animalshad是過去式，不是對的嗎all dogs have the same ancestor狗有相同的補(bǔ)祖先。（我句是說一直到現(xiàn)在的情況，所以要用一般現(xiàn)在時(shí)）