Whales: An aquatic existence
The problems associated with a return to an aquatic existence include locomotion, respiration and reproduction. Yet such adaptations were undertaken in an extremely short period, although it is difficult to comprehend how such an immense animal as the 130 ton blue whale (Balaenoptera musculus) really descended from a tiny creature like the tree shrew. Their ancestors must have entered the sea at a time when the only mammals in existence were the little insectivores. But their anatomy is now so extreme in their adaptation to swimming that it gives no clue as to how the transition back to the seas was made. It may be that the two main groups of whales; the carnivorous forms possessing teeth (suborder Odontoceti) and the filter feeding forms using a baleen (suborder Mysticeti) have different ancestries, those with teeth having come from insectivores by way of primitive carnivores and the rest, the baleen whales, being descended more directly.