Choosing Water over Land

It’s a known fact that life originated out of water. The first living organisms – micro organisms took birth in the sea. And slowly they evolved into higher forms of life. Fishes originated and then they took reptilian form and these reptiles came to land and they evolved into mammals. But some of these mammals, strangely enough went back to water.

Modern whales owe their existence to a small wolf like carnivorous mammal PAKICETUS – the name being so because its fossils were first found in Pakistan. Pakicetus has now been included within the order ‘CETACEA’.  Pakicetus used to spend some amount of time in water – sometimes for feeding and sometimes to escape from other land-predators. The cetaceans finally fully settled in water, their lungs and bones – all developing to help them lead an aquatic life.

Let me now introduce another class of animals which you are familiar to – The ARTIODACTYLs or the EVEN TOED UNGULATES. These are all those herbivores you almost usually see – the cattle, pig, camel, giraffe etc. and most importantly hippopotamus. They are distinguished by the presence of 4 toes/hooves (as against 3 in case of odd toed ungulates like horses, donkeys, tapirs, rhinos etc), the weight of the body being borne wholly by 2 of the 4 toes. The closest living relatives of cetaceans are artiodactyls, Hippopotamus being the closest among them all. In fact the two orders – those of cetaceans and artiodactyls are together given a Clade – Cetartiodactyla. So how do early Cetaceans differ from artiodactyls? The structure of the tympanic bone in the ear is what differentiates the cetaceans from artiodactyls.

Interestingly enough, there is another close relative of whales – INDOHYUS (Meaning, Indian Pig), which was a herbivore. It was almost a contemporary of Pakicetus and most probably more recent than Pakicetus. It is a relative of the cetaceans in the sense that it has the same ear structure and belongs to the family Raoellidae.

Another set of mammals which took the aquatic route are the SIRENIA – the sea cows, manatees and dugongs.  SIRENIA are completely herbivorous and feed on plants growing under the sea. Now guess who are the closest relatives of these creatures? Elephants. Yes, Elephants and the Sirenia both evolved from common ancestors PRORASTOMIDAE which were mostly terrestrial animals known to be amphibious too. While they finally went and settled to lead a fully aquatic life to have evolved into Sirenia, somewhere down the line some of them have even come out of water and evolved into Elephants. No marks for guessing why elephants are such good swimmers and why they like to wallow in water so much. The skin of pachyderms is also fit to lead a dual – aquatic plus terrestrial lifestyle. Thus, in a way elephants represent a set of animals who went back to water and then again came back to land.

Here, I would like to touch upon another set of mammals that have gone back to water, though not fully. They are the PINNIPEDs or the super family of mammals having seals, walrus, sea lions etc who feed in water but stay on land. Thus they are semi- aquatic. But it is true that they have evolved from bear-like animals which were fully terrestrial.

Thus, we can see that though evolution forces most animals into being aquatic or terrestrial or amphibious, some of them still like to go back to what suits them the most by evolving themselves.