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One of the most important milestones in the evolution of life began some 400 million years ago, when the first animals made their way from water onto land. At that time, known as the Devonian Period, the world was changing dramatically: complex plant ecosystems formed on land, the first woody plants appeared, and the water's edge was becoming a new kind of environment.

The move to land was a very gradual process, and the evolution of limbs wasn't a simple adaptation resulting from animals crawling onto the shore and never looking back. In fact, the new picture of this transition shows that most of the changes needed for life on dry land happened in creatures that were still living in the water. Some fishlike vertebrates had already begun to evolve limbs by around 400 million years ago: They were called "lobe-fins," with fins that looked like fleshy paddles, and they had lungs as well as gills.

The transition from these lobe-fins to the earliest tetrapods -- four-legged animals that walked on land -- has long been of intense interest to biologists. Many of the most telling fossils have been dug up in Greenland in the latter part of the 20th century, particularly in the past 15 years.

One of the first vertebrates that may have ventured onto land, whose remains date from about 364 million years ago, is called Ichthyostega. Although fishlike in many ways, it had robust bony legs, arms, and digits. Ichthyostega clearly spent some time out of water.

In 1987, fossils of another related form, named Acanthostega, were discovered in Greenland. This creature had stumpy legs and a long tail, which were probably used for propulsion in water. In fact, Acanthostega, more so than Ichthyostega, was basically an inhabitant of the water; its limbs were too floppy and its backbone too weak to support itself on land. Not only that, but despite the presence of lungs, Acanthostega had very fishlike gills.

From these finds, it now appears that the four legs common to land animals today really evolved for another purpose: navigating swampy wetlands, not as a means of moving to land. But once on land, the animals found their limbs a survival advantage there, too. Evolution frequently produces adaptations that come to be useful in the future for a different purpose.

April 16, 2006

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