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Prehistoric push-ups

Reconstructing the posture of an extinct animal from isolated bones. The humerus of a Devonian tetrapod, found in Pennsylvania, shows that it could support the front end of its body in a low, wide stance. This posture reflects an intermediate stage in the evolution of amphibian limbs from fish fins. Image courtesy of Kalliopi Monoyios.
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Scientists reporting the discovery of the world's oldest known arm bone say that the first arms and legs developed for use in the water. In the past, scientists thought that arms and legs evolved specifically to outfit animals for life on land.

But, this scenario doesn't fit into the picture painted by recent fossil discoveries, including the 370-million-year-old arm bone described in the 02 April 2004 issue of the journal Science.

Walking the Devonian walk. Tetrapods exist in a range of different forms (tetrapodomorphs). Here they are arranged in a cladistic sequence, with left forelimbs on forefins, in dorsal view from above. Image © Science/Emese Kazár.
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If you've ever tried to swim in a wading pool packed with splashing ankles and plastic toys, you can start to imagine how limbs could have first developed for life in the water.

In a wading pool, floating on your stomach and pushing off the bottom with your arms is a good way to avoid getting kicked by a foot or rammed by a floating toy.

Now imagine that the wading pool is a shallow stream, that you are one of the first animals with arms and legs instead of fins, and that those inflatable toys are a bunch of vicious meat-eating fish.

The arm bone in the new creature, and its equivalent in a fish and human. By 365 million years ago, this bone evolved to support a limb instead of a fish fin. [Image courtesy of Kalliopi Monoyios]
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The site is known as “Red Hill” because it is a roadcut through the bright red rocks of the Devonian Age. The different layers of rock at this site represent 365-million-year-old streams. Fossils are found within these ancient stream deposits. Image courtesy of Kalliopi Monoyios.
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The first limbs developed from fins to help their owners get around in this plant-choked, predator-filled, shallow river environment, according study author Neil Shubin of the University of Chicago.

After studying the fossil arm that they dug out of rocks along a highway, Shubin and his coauthors say that many of the features we associate with life on land –- including arms and legs –- have their origins in fish.

They found the fossil in rocks that hold many other plants and animals that made their homes in the same tropical stream in present-day Pennsylvania.


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