Wilderness Underfoot: Steppin' Out Part 1

StoryImage( ‘/Images/Story//Auto-img-11605902657974.jpg’, ‘Image provided’, ‘A living fossil- While some lobe-finned fish were busy evolving, others retained primitive features right up to modern times, including the Coelacanth (above) and the lungfish.’);
StoryImage( ‘/Images/Story//Auto-img-116059014515687.jpg’, ‘Image provided’, ‘Acanthostega's incredible evolutionary feet- Gaining legs and toes allowed this tetrapod to push through mucky swamps, or hold on to plants and debris so it could stay still while waiting to ambush prey.’);

How did ancient fish first find their way onto dry land? By evolving, of course!

One of the most remarkable explosions of diversity among the earth’s animals began with a humble lobe-finned fish. It was the ancestor of all the bony, four-limbed animals that ever shimmied, crawled, walked or flew over the earth’s continents. Every amphibian, reptile, dinosaur, mammal and bird that ever lived descended from that wonderful fish, whose lobed fins finally carried the vertebrates out of the water about 360 million years ago, in the Devonian Period.

Exactly how it all happened has been one of the most intriguing mysteries of science. Scientists originally speculated ancient fish might have wriggled away from shallow, dried-out ponds to reach new bodies of water. Or, perhaps, fish were searching for food. Either way, the prevailing scientific opinion held that legs evolved in fish that had already been adapting to longer and longer excursions onto land.

But a 1987 discovery by University of Cambridge paleontologist Jenny Clack radically changed scientists’ views of the matter. Clack was interested in the evolution of tetrapods (animals that either possess four limbs or were derived from four-limbed ancestors).

On an expedition to Greenland, Clack uncovered a wealth of fossils from the extinct creature known as Acanthostega. This animal had a deep, finlike tail, and gills to breathe underwater. But it also had four limbs, each with eight fingers. And it had lungs, so it could raise its head from the water and gulp air. Its legs, however, weren’t sturdy enough to carry its chunky body over land—lacking suitable ankles and weight-bearing pads. Instead, the feet were more like paddles.

Acanthostega was built for aquatic life. For scientists, the fossils were convincing proof that limbs first evolved to give certain fishes an advantage under water.

Acanthostega lived along freshwater shorelines and in swamps on the edges of emerging tropical forests. Legs would have allowed the 2-foot long animal to push itself along in water dense with plants, roots and debris. Legs would have also helped it hold perfectly still while waiting in the weeds to ambush its prey.

Over time, amphibious tetrapods adapted more functionality in their limbs. They gained flexibility and strength in the joints, improved dexterity of their fingers and toes, and lost the webbing. It was one small step for a fish, and one giant leap for vertebrates!

Next week: Steppin’ out, Part 2

From the Oct. 11-17, 2006, issue

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