How land-living animals evolved from fish has long been a scientific puzzle. An important part of the mystery is the transformation of the fins of fish into the arms and legs of our ancestors. Paleontologists Neil Shubin and Michael Coates from the University of Chicago, and Ted Daeschler from the Academy of Natural Sciences in Philadelphia found a 365-million-year-old fossil that sheds light on this transformation.

In the April 2, 2004, issue of Science, the scientists describe this bone, a humerus from the Late Devonian Period found in Pennsylvania. It's the earliest of its kind from any limbed animal. This specimen bridges the gap between the fins of fish and the limbs of amphibians.

The newly discovered arm bone is a keystone to allow new comparisons of fish and limbed animals because it represents a part of the anatomy where many functional changes were taking place. "It's a mosaic of primitive fish and derived amphibian," said Shubin, lead author of the paper. This integral piece of the evolutionary puzzle not only shares features with primitive fish fins, but also has characteristics of a true limb bone.

"This bone is a lot more robust than a humerus from any of the ancient species," said Coates, who specializes in fish-tetrapod transition and the divergence of modern lineages. "Relative to other tetrapods, this is almost over-engineered. There's a massive space for the attachment of substantial muscle going across to the chest." Action of these muscles would have produced a motion similar to a pushup or a benchpress.

Which is why the researchers believe that the arm bone served to prop up the body. They argue that "this function represents the intermediate condition between primitive steering and braking functions in fins and the derived aquatic or terrestrial walking gait."

"The size and extent of these muscles means that the humerus played a significant role in the support and movement of the anim
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Contact: Catherine Gianaro
cgianaro@uchospitals.edu
773-702-6241
University of Chicago Medical Center
1-Apr-2004

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