Humankind's common ancestor with other mammals may have been a roughly rat-size animal that weighed no more than a half a pound, had a long furry tail and lived on insects.from Rat-Size Ancestor Said to Link Man and Beast
So they must have found evidence of a single successful random mutation, instead of the usual unsuccessful ones, i.e. birth defects. Either that, or they found Nature's magic wand in some faraway jungle where all these successful mutations have been taking place all at once, converting rats into humans as if by magic, although some not as completely as others. Don't hold out on us, guys!