Scientists have long sought the critical point at which fish evolved to the first land-walking creatures, when fins became limbs. Neil Shubin, PhD, associate dean for organismal and evolutionary biology, and provost for the Field Museum of Natural History, is one of these scientists. Recently his research has begun to provide the answers to one of evolutionary biology's most tantalizing questions: when did fins become limbs?

Features such as wrists, ankles and digits have long served as the line of demarcation between fish and tetrapods, with no direct evidence for how or when this transition took place. That has changed with the discovery of Tiktaalik roseae, a crocodile-like creature ranging from 4 to 9 feet in length. Shubin and his team, which included Ted Daeschler of the Academy of Natural Sciences and Farish Jenkins of Harvard University, discovered the Tiktaalik over four summers of exploration on Ellesmere Island in Artic Canada, a location chosen when Shubin and Daeschler found it listed in a textbook as being comprised of unexplored Devonian rock.

“It turned out that no one ever worked there before, except the geologists that mapped it,” says Shubin. “So after seeing that image in the textbook, we went out and tried to raise money, get the permits, and then ran a series of expeditions up there to find just the kind of creature we ended up discovering six years later.”

In the April 6, 2006 issue of Nature, Shubin and his colleagues detail how the pectoral fin of the Tiktaalik shows the beginnings of a tetrapod limb. The Tiktaalik looks like a fish, in that it has scales and fins, but the skeletal structure reveals those interesting tetrapod-like features, such as a wrist and finger-like appendages. The creature also has a back of the head, like a limbed-animal, an amphibian, and a neck, a first in the archaeological record.

"It really is a mosaic of half limbed-animal and half fish, and that really tells us how fish evolved to walk on land, how they evolved to inhabit terrestrial ecosystems, and so forth. It is really remarkable, and has lots of surprises for us," explains Shubin. "It also has features we didn't fully expect before, such as ribs -big ribs- that attach to one another of a kind that we see only in animals that need to support itself in gravity. What's a fish doing with this kind of thing? Obviously, that tells us a lot about how fish developed to walk on land."

One of the most interesting aspects of this discovery is the fact that Tiktaalik leaves a clear record of a fish that had evolved to develop the very apparatuses necessary to venture through the shallows and even onto land, a discovery that provides insight into the transition of life on earth from aquatic to terrestrial.

“What we are dealing with are primitive pieces of our own bodies. This is not some dead end branch of evolution,” Shubin says. “This is our branch of evolution. This is our wrist. This is the evolution of our neck. This is the evolution of our ear. This is very much our distant past.”

 

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