(SCIENCE) Scientists and evolutionary theorists have always surmised that man’s fishy four-limbed ancestors initially grew limbs, followed by digits; left the water, and then developed the ability to walk. However, research performed on the African lungfish at the University of Chicago is now disproving that chain of events. Read on for more on the research team’s evolutionary discoveries. — Global Animal
African lungfish are being studied as their primitive attempts to walk may be rewriting evolution. Photo credit: Yen-Chyi Liu/University of Chicago

New York Times, Ritchie S. King

Roughly 400 million years ago, an ancient lobe-finned fish left its watery habitat to become the first four-limbed terrestrial creature. Its descendants — which are called tetrapods and include tree frogs, blue jays and human beings — typically get around by stepping, flying or jumping.

So the first tetrapods had not only to leave the water, but to learn how to push themselves off a solid surface by alternating the movement of their back legs. And they had to grow limbs with digits for balance.

While biologists have long thought that the digitized limbs came first, followed by the move to a terrestrial habitat, new research suggests a different sequence.

“It’s possible that walking evolved before feet or hands or digits or toes — or even being on land,” said Heather King, a biologist at the University of Chicago and a member of the research team, whose work appeared in Proceedings of the National Academy of Sciences.

Ms. King and her team studied the movement patterns of an African lungfish, a modern-day lobe-finned fish that shares many features with the ancient precursors of the tetrapods: It has four nondigitized fins, lungs and no sacrum — the triangular bone that joins the hips to the spine and, in tetrapods, conveys energy from a stepping leg to the rest of the body.

The team observed a few fish taking alternating steps with their rear fins along the bottom of a test aquarium or pushing off with both rear fins at the same time in a hopping motion. Even though the fish lack a sacrum, Ms. King suspects they are able to push themselves forward because their lungs make them buoyant.

More New York Times: http://www.nytimes.com/2011/12/20/science/african-lungfish-study-indicates-that-walking-evolved-in-the-water.html?_r=1&ref=animals