WHY DO WHALES HAVE HIPS?

Have you ever mused on the question of why whales have hips, when they have no legs joined onto them?  Zoologists infer from this that whales must have at some stage quitted the sea and developed legs.  Then at some later date they returned to the sea and undeveloped them as they didn't need them in the water.  All of this, I surmise, must have taken some time.