Fish Fossil Shows Why Humans Have Two Arms, Legs

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An unusual prehistoric fish with fins near its butt has helped to solve the mystery over why most animals, including humans, have paired limbs.

The fish, Euphanerops, is possibly the first creature on the planet to have evolved paired appendages, which in this case were fins. The 370-million-year-old species is described in the latest issue of Biology Letters.

"Fins are the world's first limb-like appendages," lead author Robert Sansom told Discovery News. "Paired limbs would subsequently develop from paired fins in the transition from sea to land, but the first evolution of paired appendages was a big, important step in the evolution and development of vertebrates," which are animals with a backbone or spinal column.

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Sansom, a researcher at both the University of Leicester and the University of Manchester, and colleagues Sarah Gabbott and M.A. Purnell analyzed 36 Euphanerops specimens unearthed in Quebec, Canada. This was a jawless fish that lived long before dinosaurs first emerged.

Many living fish have a single anal fin, located at the center back of the fish’s underside near its rear end. The fin is thought to help maintain control of body position.

Euphanerops, however, evolved two such fins. Some subsequent fish did not evolve the paired appendages, so fish with all sorts of fin combinations existed for a while.

"What this research leads us to believe is that, at this early stage (in evolutionary history), vertebrates were trying out lots of different body plans, some familiar, some less familiar, and only some that survived," Sansom explained.

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The change happened at a radical point in fish history when some of them were starting to evolve jaws and teeth. (There is currently a big chicken and egg-type debate among fish experts as to which evolved first: teeth or jaws.) These attributes likely emerged for reasons similar to fish gaining fins -- improved hunting and escape skills.

As Sansom shared, "The evolution of paired appendages and more sophisticated fins will probably be for improved locomotion, potentially related to an arms race between tracking down prey and avoiding predators."

He continued, "Paired fins allow for more sophisticated control of movement."

This movement, which at first just happened underwater, later helped some species make the transition from water to land.

Heather King of the University of Chicago and colleagues studied living lungfish to see how that transition might have happened.

"Lungfish are very closely related to the animals that were able to evolve and come out of the water and onto land, but that was so long ago that almost everything except the lungfish has gone extinct," she explained.

King and her team found that lungfish could, as their name suggests, blow up with air like a balloon, giving their body buoyancy. Their scrawny back paired appendages can then either sort of hop or actually walk by alternating the movement of these limbs.

Co-author Neil Shubin said, "This shows us -- pardon the pun -- the steps that are involved in the origin of walking."

Since those first steps from water to land were taken, some animals evolved four limbs for walking. Even for these animals, like dogs and cats, the limbs come in pairs. For that innovation, we can probably thank the unusual, long-extinct jaw-less fish Euphanerops.

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