The largest group of marine mammals, the cetaceans, is also the group that has made the most complete transition to aquatic life. While most other marine mammals return to land at least part of the time, cetaceans spend their entire lives in the water. Their bodies are streamlined and look remarkably fish-like. Interestingly, even though all marine mammals have evolved from very different evolutionary groups, there are certain similarities in lifestyle and morphology, and they are considered good examples of the principle of convergence. Convergent evolution is the process by which creatures unrelated by evolution develop similar or even identical solutions to a particular problem; in this case, life in water.

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