Both the mysticetes and odontocetes are thought to be descendants of the archaeocetes. Cetaceans, along with bats, are considered some of the most derived mammals on the planet. They evolved from terrestrial animals to an entirely aquatic life form that is completely separated from the land in all aspects of biology.
Cetaceans live, breed, rest, and carry out all of their life functions in the water.
The phylogenetic relationships discussed above are in the currently accepted form but many biologists still classify the Mysticetes and Odontocetes as separate orders. Many of these characteristics are adaptations to reduce drag for fast swimming in an aquatic environment. Protuberances such as external ears or genitals would create turbulence and would be very inefficient for an animal in the water. Many cetaceans can dive for extended periods of time to great depths. Some cetaceans are thought to be the most intelligent non-primates and many have proportionately large brains.