A fish tale is circulating through the scientific community and popular news outlets, thanks to a recent discovery by Dr. Neil Shubin, Professor and Chair, Organismal Biology and Anatomy, Dr. Michael Coates, Associate Professor, Organismal Biology and Anatomy and Dr. Ted Daeschler from the Academy of Natural Sciences in Philadelphia. Only their story isn't about the one that got away. Their story is about the one that took the first step towards walking on land 365 million years ago.

Drs. Shubin, Coates and Daeschler recently published their discovery in the April 2, 2004 issue of Science. The article describes a bone, a small humerus, first discovered along a highway in north central Pennsylvania more than ten years ago, that sheds light on the question of how land-dwelling creatures may have evolved from aquatic life forms.

"Think of a shallow stream choked with plants, not of an open sea," Shubin says. "At some level, these shallow streams approach a more terrestrial environment in the ways that animals would move around."

Such an environment would necessitate the ability to push along in very shallow waters or lift up to breathe air at the surface. This adaptation is evidenced in the humerus, which shows evidence of a large area for the attachment of muscles at the shoulder.

This configuration would allow the creature to perform a kind of modified push-up. The discovery of this specimen helps scientists bridge the evolutionary? gap from fish to amphibian.

This bone tells us how the function of the limb evolved in the earliest stages of creatures that learned to walk on land, which is what makes it so important to my colleagues in the field," Shubin continues. "This fossil allows us to make comparisons among land-walking tetrapods and fish that we couldn't make before. Once we do those comparisons, we find that it fits along the notion proposed by Michael Coates that many of the features that were necessary to allow creatures to walk on land actually evolved in fish or other aquatic creatures."

"This animal lived in the hinterland between truly aquatic and terrestrial environments, and the development of the humerus in this manner is a response to that," Coates says. "Survival in that type of habitat encouraged the development of load-bearing forelimbs, even though the rear of the creature would probably have maintained paddle structures."

The animal was approximately two feet long and lived during a time when there were no land-dwelling vertebrates. It existed for a "brief moment in time," according to Shubin, and perhaps had both gills and lungs, as did other similar tetrapods from the period, providing it with one more mechanism by which to survive in it's hostile river delta environment.

"Having the ability to lift the chest off of the ground would allow more efficient lung breathing, although it is likely that this animal retained a substantial set of gills," says Coates. "The discovery of this humerus hints at a previously unknown amount of diversity in animals approaching tetrapod conditions, and keys right into the gap that existed in the fin to limb transition."

"If you were to find one bone from the earliest known creatures to walk on land, you would want this particular bone," Shubin claims. "The humerus has a lot of joint surfaces and muscle attachments that tell you how the animal moved. Other bones, such as vertebrae or skull fragments, wouldn't provide the same amount of information. The structures on the humerus represent a mosaic of features, some of which are seen in fish, some of which are seen later on very advanced land-dwelling creatures, and some of which are very unique. A lot of this paper was dedicated to trying to make sense of those primitive features, those unique features, those very advanced features."

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