Cetaceans

We were watching a nature movie about whales on TV and I asked my grandson, "how can you tell the difference between a fish and a whale?" and he didn't know, so I told him "by the tail." The tail of a fish is vertical and that of a whale is horizontal. Remember seeing the fluke of the whale dipping down as it begins to dive. I told him that whales like dolphins and porpoises, a group known as Cetaceans, are mammals, they need to breathe air because they have lungs, while fish have gills and hence obtain their oxygen directly from the water. That's why whales have to periodically go to the surface to spout (exhale carbon dioxide) and breathe, but fish never need to do this. I pointed out that this was because whales and other Cetaceans evolved in the sea from land mammals that returned to the sea, and were not connected to other fish.

He expressed skepticism as did his father, who apparently does not believe in evolution. But, the terrestrial origins of whales and other Cetaceans are clearly indicated by several characteristics:

Their need to breathe air from the surface
They have lungs and not gills
They are mammals and feed their young with milk (unusual in the sea)
The bones of their fins resemble the limbs of land mammals while those of fish do not
The vertical movement of their spines are characteristic more of a running mammal than of the horizontal movement of fish

They have solved the problem of swimming in the water with horizontal tails and vertical body movement instead of vertical tails and horizontal body movement as in fish, a process called "convergent evolution." They have adapted to the same environment by a parallel but different mechanism over millions of years. It was long disputed how such sea-going animals could have evolved from land animals instead of from fish, but fossil finds have revealed several stages in the transition of Cetaceans from land to sea (for more details see Wikipedia). Also, this is not unique since there are other species, such as walrus, seals and sealions which are amphibious mammals that have adapted back to a sea environment. Also, molecular genetics, i.e. comparative analysis of the DNA of animal species, have revealed an evolutionary tree that shows the phylogeny (group relationships) of a series of related Cetaceans.

From this it is clear that Cetaceans evolved from land animals that returned to the sea and gradually adapted to their environment, losing certain characteristics, but retaining their most basic ones of breathing air thru lungs and feeding their young with milk. This not only explains why fish and whales are so different, but also is a great example of evolution.