Whale Evolution
"Whale
evolution is
different from the evolution of most other animals."
Evolution of Whales
The first animals evolved in the water.
They evolved from bacteria to jellyfish, corals, shellfish, fish, and finally reptiles.
Once reptiles had evolved, they were the first animals to get out of the water and start invading land.
Reptiles lived on the land for a long time before mammals evolved, so mammals were the first group of animals to evolve entirely independent from the life in the water - they all evolved directly to land animals.
While there are mammals that spend some time in the water (to various degrees from having a swim to depending on the water in parts of their life cycle), whales (including dolphins) are the only group of mammals that have returned to the water for their entire life - they die when they are too long out of water.
Evolution of Dolphins
Dolphins are also whales and returned to the water after having been land animals.
They are toothed whales and belong to a different family (Odontoceti) from baleen whales (Mysticeti).
Both baleen whales and dolphins got the evolutionary adaptations for a life in water - their legs developed to flippers and their bodies got a streamlined shape, which helps moving around in the water.
They got large lungs which enables them to spend more time under the water. The closest related animals to whales and dolphins are hippopotamuses.
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