Hundreds of millions of years ago life climbed out of the oceans and started the process of evolution that created the world as we know it. About 50 million years ago the cetaceans returned to the oceans never to walk on the land again. Descending from a small, four-legged, dog-sized mammal with a tail, cetaceans have evolved into a group of almost 80 species that includes whales, dolphins and porpoises.
The modern cetaceans are broken down into two main types based on the method by which they feed. Mystyceti, or baleen whales, are filter feeders who have large membranous plates in their mouths that hang down. These plates are covered in small hairs that trap microscopic lifeforms as hundreds of gallons of sea water are inhaled by the whale. Only 10 species within cetacea belong to the mystyceti group and all of them are whales. Odontoceti, or toothed whales, have teeth in both their upper and lower jaws. These toothed whales instead of filter feeding use their teeth to catch large prey. All dolphins and porpoises are members of the odontoceti group as are most whale species.
Cetaceans are all fully aquatic and have adapted perfectly to life in the water. Their rear limbs have fused together to create flattened tails that are used by all cetaceans as their main propulsion power. The front limbs have developed into flattened, paddle-like limbs called flippers that are held at the side of the body. In some species these also help with propulsion but are mainly for steering purposes. Unlike land mammals, whales have essentially developed to be fully hairless though some whales do have patches of hair and young whales do often have hair at some point of their development.
The range of size in cetaceans is quite vast. The smallest is the finless porpoise at 6 feet in length and 99 pounds, while the largest is the blue whale at over 100 feet and 200 tons. The whales tend to be the largest of the cetaceans with many species being very long and weighing several tons. Dolphins are usually smaller although the killer whale, which is actually a type of dolphin, is as large as some whales. The smallest of all cetaceans are the porpoises, which consist of just six species.
Members of cetacea are found throughout the world's oceans, from the frigid Arctic and Antarctic waters to the tropics. Some species are widespread such as the blue whale, which is found throughout the planet's oceans, while others may only be found in isolated areas. Most cetaceans live entirely in the oceans although many cases of whales and dolphins swimming up river have been recorded. There are also a few dolphin species that have adapted to live entirely in freshwater rivers.
All cetaceans are carnivores and need to feed on other lifeforms. The largest of all whales, the blue whale, is a baleen whale and ironically feeds on the smallest of all oceanic life. Fish, crustaceans and other ocean living life make up the diets of various cetaceans. The killer whale is one of the few which will feed on animals from the land and have actually been known to rush up onto beaches to snatch seals from dry land.
The order cetacea is very diverse and a representation of what evolution is capable of. Varying from leviathans of the deep to small, coastal-dwelling mammals, cetaceans draw our eyes to the oceans to watch them and marvel at their beauty. Many of them have faced over hunting throughout the centuries and some have come close to extinction. It is important to protect and treasure them for future generations.