paleontology 5: Evolution from fish to four legged animals. (v1.0)

 In the second paleontology essay I stopped with the rise of bony fishes around 400 million years ago. Then I took a diversion in the next two essays to talk about first the colonization of land and then to complete discussing the evolution of plants. Now in this essay I resume the narrative from bony fishes. How did bony fishes evolve into four legged animals on land.

A small class of bony fish are called lobe finned fish, and there is good evidence they are the ancestors of tetrapod. The oldest tetrapod fossil is 365 million years ago, in the Devonian period (from 419 to 358 million years). A lobe finned fish fossil of a species called Tiktaalik from 375 million years is one of the most important fossils found in the last two decades. It is a true transitional animal between a tetrapod and a lobe finned fish. Although tetrapod means 4 legs, this group also includes animals that don’t have 4 legs but evolved from other tetrapod ancestors – some or all their legs disappeared – like snakes. Birds and humans walk on two legs but are still a tetrapod. Tetrapod includes amphibians, birds, reptiles and mammals.

The earliest tetrapod likely walked on the bottom of shallow bodies of water. The tetrapod though gradually made their way onto land and evolved to live on land. The early tetrapod were amphibians. Some Tetrapod’s found their way back to water later. The earliest evidence of a return to water is from the carboniferous period while later returns were as late as the Cenozoic era (age of mammals - from 66 million years ago to present) like whales, dolphins, porpoises, seals and sea lions who are all mammals.

The change from a body plan for breathing and navigating in water in lobe finned fish to a body plan in tetrapod enabling the animal to move on land and breathe is one of the most profound evolutionary changes known and is a major evolutionary change. It is also one of the best understood, largely thanks to several significant transitional fossil finds in the late 20th century combined with improved analysis.