for National Geographic News
Today researchers announced their discovery of a 365-million-year-old fossil limb bone of an ancient tetrapod. Tetrapods, including humans, are four-limbed animals with backbones. The fossil was found during road construction that revealed an ancient streambed.
See an illustration of the animal and a fin-to-limb evolution chart.
Scientists say the find will help shed light on how early animals evolved limbs from fins. This crucial adaptation enabled Earth's animal life to crawl from water to land.
The bone—a humerus, or upper arm or forelimb—is one of the earliest tetrapod limb bones ever found. (Tetrapods today include amphibians, mammals, reptiles, and birds, among others.)
The ancient bone shares features with primitive fish fins, but also has characteristics of a true limb bone. It bridges the gap between fish and amphibian.
"The transition wasn't all or nothing," said Ted Daeschler, a vertebrate zoologist with the Academy of Natural Sciences in Philadelphia "It's not that some animals were thrown on land. There were certainly other functions intermediate."
Daeschler and colleagues Neil Shubin and Michael Coates, paleontologists at the University of Chicago, say the fossil bone offers a window onto this intermediate stage.
The National Geographic Society Committee for Research and Exploration and the National Science Foundation supported the scientists' research. The trio describe the fossil in tomorrow's issue of the journal Science.
From Stream to Shore
"This new humerus shows some primitive features that are lost in later tetrapods but in this specimen have already begun to change their orientation into tetrapod-like configuration," said Jennifer Clack, a paleontologist at the University Museum of Zoology Cambridge, United Kingdom.
Clack, an expert on the fish-tetrapod transition, also wrote an essay on the discovery for Science.
To understand how tetrapods evolved limbs from fins, Shubin said it helps to imagine the environment that these creatures lived in. "Think of a shallow stream choked with plants, not of an open sea," he said. "At some level, these shallow streams approach a more terrestrial environment in the ways that animals would move around."
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