it's true that that is a common feature really from the time of  for the last five hundred million years, from the time of the earliest fish to ourselves and to the birds and everybody else, but it's like that not because there is some kind of profound law of form, which says that's the kind of organism which is in  permitted by the laws of development to arise, erm i mean the law form would be something like erm a law of physics which says that if objects move round the sun they're going to do so in ellipses with the sun at one focus.