Newly found species fills evolutionary gap between fish and land animals

Paleontologists have discovered fossils of a species that provides the missing evolutionary link between fish and the first animals that walked out of water onto land about 375 million years ago. The newly found species, Tiktaalik roseae, has a skull, a neck, ribs and parts of the limbs that are similar to four-legged animals known as tetrapods, as well as fish-like features such as a primitive jaw, fins and scales.

These fossils, found on Ellesmere Island in Arctic Canada, are the most compelling examples yet of an animal that was at the cusp of the fish-tetrapod transition. The new find is described in two related research articles highlighted on the cover of the April 6, 2006, issue of Nature.

"Tiktaalik blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," said Neil Shubin, professor and chairman of organismal biology at the University of Chicago and co-leader of the project.

Tiktaalik was a predator with sharp teeth, a crocodile-like head and a flattened body. The well-preserved skeletal material from several specimens, ranging from 4 to 9 feet long, enabled the researchers to study the mosaic pattern of evolutionary change in different parts of the skeleton as fish evolved into land animals.

The high quality of the fossils also allowed the team to examine the joint surfaces on many of the fin bones, concluding that the shoulder, elbow and wrist joints were capable of supporting the body-like limbed animals.

"Human comprehension of the history of life on Earth is taking a major leap forward," said H. Richard Lane, director of sedimentary geology and paleobiology at the National Science Foundation. "These exciting discoveries are providing fossil 'Rosetta Stones' for a deeper understanding of this evolutionary milestone--fish to land-roaming tetrapods."

One of the most important aspects of this discovery is the illumination of the fin-to-limb transition. In a second paper in the journal, the scientists describe in depth how the pectoral fin of the fish serves as the origin of the tetrapod limb.

Embedded in the fin of Tiktaalik are bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals.

"Most of the major joints of the fin are functional in this fish," Shubin said. "The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."

At the time that Tiktaalik lived, what is now the Canadian Arctic region was part of a landmass that straddled the equator. It had a subtropical climate, much like the Amazon basin today. The species lived in the small streams of this delta system. According to Shubin, the ecological setting in which these animals evolved provided an environment conducive to the transition to life on land.

"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and the right ancient environments to provide the potential for finding fossils documenting this important evolutionary transition," said Ted Daeschler of the Academy of Natural Sciences in Philadelphia, a co-leader of the project. "Finding the fossils within this remote, rugged terrain, however, required a lot of time and effort."

The nature of the deposits where the fossils were found and the skeletal structure of Tiktaalik suggests the animal lived in shallow water and perhaps even out of the water for short periods.

"The skeleton of Tiktaalik indicates that it could support its body under the force of gravity whether in very shallow water or on land," said Farish Jenkins, professor of organismic and evolutionary biology at Harvard University and co-author of the papers. "This represents a critical early phase in the evolution of all limbed animals, including humans--albeit a very ancient step."

The new fossils were collected during four summers of exploration in Canada's Nunavut Territory, 600 miles from the North Pole, by paleontologists from the Academy of Natural Sciences in Philadelphia, the University of Chicago and Harvard University. Although the team has amassed a diverse assemblage of fossil fish, Shubin said, the discovery of these transitional fossils in 2004 was a vindication of their persistence.

The scientists asked the Nunavut people to propose a formal scientific name for the new species. The Elders Council of Nunavut, the Inuit Qaujimajatuqangit, suggested "Tiktaalik" (tic-TAH-lick)--the word in the Inuktikuk language for "a large, shallow water fish."

The scientists worked through the Department of Culture, Language, Elders and Youth in Nunavut to collaborate with the local Inuit communities. All fossils are the property of the people of Nunavut and will be returned to Canada after they are studied.

The team depended on the maps of the Geological Survey of Canada. The researchers received permits from the Department of Culture, Language, Elders and Youth of the Government of Nunavut, and logistical support in the form of helicopters and bush planes from Polar Continental Shelf Project of Natural Resources Canada. The National Science Foundation and the National Geographic Society, along with an anonymous donor, also helped fund the project.

Why do all fossils have a specific place in the tree of life?

The Tree of Life is a model illustrating the relationships between species. The relationships show the common descent of life on earth. It's like a jigsaw puzzle with millions of pieces (fossils and living species). After diligent research by generations of scientists -- not only biologists, but scientists from separate disciplines like geology, paleontology, organic chemistry, astronomy, and nuclear physics (for radiometric dating techniques) -- the pieces all fit. They lock into place in several independently cross-confirming ways, including form, function, chronology, and molecular biochemistry.

When jigsaw puzzles are properly assembled, a picture emerges; with data, a scientific theory emerges. In this case, the "picture" is the theory of evolution. There are still some missing pieces, and there probably always will be, but the evidence now assembled is more than sufficient to reveal the picture. There is so much evidence that at this point, the picture itself can confidently be used as a guide for predicting the fit of new pieces that are found. When they are put in place, DNA evidence shows a close, pre-existing relationship of the pieces that we've fitted together, thus confirming the picture; any re-arranging the pieces would be inconsistent with such evidence.

Other well-established theories function similarly. For example, after decades of matching stellar distances (determined by Cepheid variables) with the redshift of their images, the redshift alone is now used as a reliable guide to distance -- subject to verification whenever a Cepheid variable is available, which always confirms the redshift's information.

Creationists are forever claiming that the evolutionary picture revealed by the evidentiary jigsaw puzzle is nothing more than the arbitrary result of our prejudices, and that by using different presuppositions the puzzle's pieces could be assembled some other way, and the pieces would match up just as well -- but presumably would show their own preferred picture. So why don't they ever do it?

Instead of actually doing research, they always ask: "Where are the transitional fossils?" When presented with a truckload of them, they claim that they're all fakes, or they sometimes retreat to their fallback position: "Why don't you have more transitional fossils?" We find more every year, but somehow they are never enough for the creationists. No matter how many more are found, they will never be enough. They are the OJ jury, for whom no amount of evidence will suffice. They never stop to consider that even one transitional fossil contradicts the concept of special creation.

But the questions should flow in the other direction. Creationist should be asked: "Where are the anachronistic fossils -- the preposterous fossils, the incongruous fossils that we should expect if the tree of life is incorrect?"

If they want to challenge the tree of life, let them produce something like a pegasus fossil, or any other evidence that is inconsistent, incompatible, and irreconcilable with the theory of evolution.

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