From Fins to Limbs 



Recent fossil discoveries show how four-legged land animals 

 evolved from fishes whose filllike paddles had already adapted 

 to functions such as pushing through shallows and swamps. 



By Jennifer A. Clack 



When I became a paleontologist about 

 twenty-five years ago, the evolution of 

 four-legged animals from their fish an- 

 cestors was embodied in khthyostcga, a partly terres- 

 trial creature that lived 36< I million years ago in what 

 is now East Greenland. Ichthyostega was the oldest 

 known tetrapod — an animal having four legs with 

 toes. On the other side of the evolutionary divide 

 was Eusthenopteron, a fish that was about ten million 

 years older than Ichthyostega. In Eusthenopteron pale- 

 ontologists saw the model ancestor to the tetrapods: 

 the skeleton of its fin seemed the archetype from 

 which all limbs evolved, including our own. 



Those two iconic animals stimulated speculation 

 about how creatures crawled out of the water to 

 "conquer the land." According to the simplest sce- 



nario of the day. a fish such as Eusthenopteron left one 

 drying-up pool to find another and thus grew legs. 

 But Eusthenopteron and Ichthyostega were only two 

 widely separated points of reference; the interme- 

 diate steps were missing. And little could be said 

 about what encouraged animals onto land, how the 

 transition happened, or when it took place. 



Ichthyostega had first come to light in the early 1 930s. 

 In 1897 a party of explorers known as the Andree ex- 

 pedition, traveling by balloon, had been lost attempt- 

 ing to find the Northwest Passage — the fabled navi- 

 gational shortcut across the top of the world. Decades 

 later. Scandinavian scientists were searching for the 

 remnants of that expedition in East Greenland, when 

 they found a cache of Ichthyostega fossils, among oth- 

 er eeoloeical discoveries. After that. Greenland be- 



36 NATURAL HISTORY July. 'August 2006 



