How the fish got his legs

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The history of life on earth has involved many spectacular metamorphoses. From the primeval soup containing the first life to the first single celled organisms, and then to the first multicellular organisms which made possible the evolution of the human body. One of the most spectacular of these metamorphoses was the evolution of terrestrial four-legged animals from fish. An important step in this transition from aquatic to terrestrial life was the transformation of fins into limbs.

Around 375 million years ago comes the first evidence of a courageous fish species taking its first steps towards terrestrial living. This species is called Tiktaalik rosaea. Its first fossil was discovered in 2004 (which you can read about in this book and this paper). It has a unique physiology, and is thought to represent an example of an evolutionary transitional species between fish and amphibians. It is therefore the direct ancestor of both you and me, as well as every other amphibian, reptile, mammal and bird on earth. Tiktaalik has a mixture of physiological features similar to tetrapods (four-legged animals) and to fish. It has fins and scales like all fish, but unlike fish it has a flat head and a neck. It also has holes on the top of its skull called spiracles. These indicate that the fish had primitive lungs, as well as gills, allowing it to breathe both on land and underwater.

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Skull of Tiktaalik showing spiracles (holes above eyes)

Perhaps most importantly the skeleton inside Tiktaalik’s fins has a similar bone structure to that of the tetrapod limb. Neil Shubin, author of Your Inner Fish, describes how the tetrapod limb structure follows the same pattern in all tetrapods, from horses to bats to humans. This structure you can see in the diagram of the human arm below, simply put there is one big bone (humerus) followed by two bones (the radius and the ulna) followed by a bunch of little bones.  You can see in the picture below that Tiktaalik has a bone structure that shows this pattern.

 

It is also evident that it used these specialized fins to move around on land, as it had a robust enlarged shoulder which would have been able to support the weight of the animal.This shows that fish ventured onto land before they got legs. You can watch Neil Shubin, one of the discoverers of Tiktaalik talk about the discovery below.

So why did this species spend some of its time on land? Scientists believe that the environmental conditions that this species was living in encouraged the species to venture onto land. Tiktaalik lived in shallow waters which were oxygen poor. The ability to use its fins to move on the land allowed it to access more areas of food, and to move from one aquatic area to another. Its ability to breathe oxygen also gave it an edge against other fish species, as it did not rely on the oxygen dissolved in the water to breathe.

I have focused this blog post on Tiktaalik because it represents a crucial step in the evolution of tetrapods. They are the oldest fossils discovered which have the novel traits associated with tetropods and therefore represent the earliest examples of at least partial terrestrial living. The cocktail of fish-like and tetrapod-like characteristics of Tiktaalik have paved the way for a better understanding of how fish ventured onto land, and ultimately got their legs.

As ever thanks for reading, and if you would like to read more about tetropod fossils from later in the fossil record, this paper has some great information.

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Photo credit Tiktaalik 1:

<a href=”http://www.flickr.com/photos/62281335@N07/6662865611″>Field Museum: Tiktaalik</a> via <a href=”http://photopin.com”>photopin</a&gt; <a href=”https://creativecommons.org/licenses/by-nc-nd/2.0/”>(license)</a&gt;

Photo credit Tiktaalik skull: By photographed by Richard G. Clegg, tweaked by dave souza [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)%5D, via Wikimedia Commons

Photo credit Tiktaalik’s limb: photographed by Richard G. Clegg, tweaked by dave souza (http://en.wikipedia.org/wiki/Image:Tik_limb_1a.jpg) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)%5D, via Wikimedia Commons

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