Monday, September 8, 2008

Fish fingers: Acanthostega and tetrapod evolution

This handsome devil is Acanthostega. One of my lecturers refers to him as 'friend Acanthostega', and, for reasons I will explain, I have come to think of Acanthostega as a friend.
There are no photos to show you, because Acanthostega has been dead for approximately 365 million years.

A cultural theory enthusiast might say that Acanthostega inhabited the liminal zone. A normal person might say that he was a half-fish, half reptile who lived in a swamp. When you hear about creatures crawling out of the primeval soup, it's Acanthostega who did the crawling.

As you can see, Acanthostega's limbs sit a little awkwardly on his body. This is because limbs have only just been invented. Acanthostega's predecessors were fish with fins. He was probably not mobile on land, but could use his legs to brace himself against aquatic plants, and push up out of the mud.

Early limb-bearing creatures like Acanthostega usually had six to eight digits on each hand. It was only later in evolutionary history that most animals settled on five digits as the optimal number.

Of all the species that have ever existed, it is estimated that 99% are extinct. It's sad to think of the last Acanthostega sinking lifelessly into the Upper Devonian mud. However, there is a happy ending. Limbs proved to be a very successful evolutionary strategy. Acanthostega is not just a friend, but a relative. It seems (my textbook is not explicit on this) that his descendants include ourselves!

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