Cetaceans were the only group of mammals to adapt to a totally aquatic lifestyle. Recently, several fossils have been discovered that document this evolutionary process, making it a classic example of ongoing evolution.
Throughout the geological ages, evolution has been promoting drastic changes that challenged human understanding and sometimes generated great controversies regarding its possibility. Two examples are: the development of articulated limbs from muscular fins in the tonic peaks of the Devónico; and more recently, the development of bipedal locomotion in a lineage of quadrupedal primates at the end of the Miocene. These changes allowed the biological groups where they occurred to occupy ecological niches hitherto unexplored, and thus guarantee their current place in the tree of life. However, one of the most drastic transitions in the history of evolution, seems to have occurred in response to the (re) occupation of an “ancestral” environment … the sea.
History thinkers and naturalists quickly realized that Cetaceans had a greater affinity for land animals such as mammals than for fish. After all, they breathed atmospheric oxygen through structures that were clearly lungs, in addition to drinking milk in childhood and being “warm-blooded”. However, the notion that these animals had their origin in entirely terrestrial ancestors remained controversial for quite a while. When Darwin (1895) argued (for purely argumentative purposes) that bears that were in the habit of feeding on aquatic insects (as was actually observed) could in time originate an animal like a whale, he was widely criticized on several fronts, from such a way that he reformulated his argument in later editions of his book. Within today, cetaceans are used by the American creaceonist movement as proof that evolution is not possible. However, in recent years, several transition fossils have been discovered that confirm the terrestrial origin of cetaceans, making this group a classic example of the changes that large groups of organisms may undergo during their evolution.