Your Inner Fish, is the first installment of a three part PBS series by Neil Shubin. After a quick review of our shared anatomy with the fish and a whirlwind tour along the evolutionary timeline, the program plunges us into the fascinating saga of discovery of the tetrapods that explains how limbs of the terrestrial vertebrates came to be. Watch how the legendary 375 million year old (Devonian) tetrapod fossil Tiktaalik was discovered after a series of adventurous Arctic expeditions.
The anatomy of Tiktaalik was a perfect encapsulation of a body plan of all land animals with a backbone (vertebrates) from humans to frogs to lizards.
First we have to project ourselves and imagine this ancestral tetrapod was shaped. It has a flat head with eyes on top, much like a crocodile. It has nostrils on the side. It has an appendage or a fin that’s able to bend its elbow and its wrist. It has a ribcage which suggests that it was able to support itself in gravity. It has a neck—no fish has a neck. What you have is an animal that’s clearly specializing for a life on the water bottom, in the shallows or even out in the air for periods of time. The geology of the site it was found in was a very small stream in a large delta or a swampy environment. These Devonian shallow water habitats was the cradle for the evolution of vertebrates to move onto land.
First publications that came out focused on the head (cranium) and fore limbs (fin structure) and left out the rear half of the fossil. Simultaneously with the Your Inner Fish documentary the analysis of the rear half of the fossil was published. The pelvic bone was a real suprise because it was quite large. The hind appendages were ready to move on land and unlike the “front wheel drive” hypothesis that held that the closest relatives of tetrapods emphasized chest support and locomotion. Tiktaalik showed us that pelvic support and locomotion was a tetrapod innovation.
There are quite a few interesting side points in this documentary. Retention of embryonic gill arches is most certainly a unique phenomena in few people. Preaurucular pit is the scientific term for this ancestral structure.
We also delve into the genetics of the limb development and get introduced into Sonic hedgehog gene. Sonic hedgehog gene produces a protein that determines limb development. Tiktaalik fossil showed us when fins became limbs. Genetics is now showing how fins became limbs and hands and fingers.
Tiktaalik means “little fish in water” in Netsilik Inuit language. The Shubin interview starts 14 minutes into the program: