April 5, 2006
Scientists Call Fish Fossil the 'Missing Link'
By JOHN NOBLE WILFORD
Scientists have discovered fossils of a 375 million-year-old fish, a large scaly creature not seen before, that they say is a long-sought "missing link" in the evolution of some fishes from water to a life walking on four limbs on land.
In addition to confirming elements of a major transition in evolution, the fossils are widely seen by scientists as a powerful rebuttal to religious creationists, who hold a literal biblical view on the origins and development of life.
Several well-preserved skeletons of the fossil fish were uncovered in sediments of former stream beds in the Canadian Arctic, 600 miles from the North Pole, it is being reported on Thursday in the journal Nature. The skeletons have the fins and scales and other attributes of a giant fish, four to nine feet long.
But on closer examination, scientists found telling anatomical traits of a transitional creature, a fish that is still a fish but exhibiting changes that anticipate the emergence of land animals — a predecessor thus of amphibians, reptiles and dinosaurs, mammals and eventually humans.
The scientists described evidence in the forward fins of limbs in the making. There are the beginnings of digits, proto-wrists, elbows and shoulders. The fish also had a flat skull resembling a crocodile's, a neck, ribs and other parts that were similar to four-legged land animals known as tetrapods.
The discovering scientists called the fossils the most compelling examples yet of an animal that was at the cusp of the fish-tetrapod transition. The fish has been named Tiktaalik roseae, at the suggestion of elders of Canada's Nunavut Territory. Tiktaalik (pronounced tic-TAH-lick) means "large shallow water fish."
In two reports in Thursday's issue of the journal Nature, the science team led by Neil H. Shubin of the University of Chicago wrote, "The origin of limbs probably involved the elaboration and proliferation of features already present in the fins of fish such as Tiktaalik."
Dr. Shubin, an evolutionary biologist, let himself go in an interview. "It's a really amazing remarkable intermediate fossil — it's like, holy cow," he enthused.
Two other paleontologists, commenting on the find in a separate article in the journal, said that a few other transitional fish had been previously discovered from approximately the same Late Devonian time period, 385 million to 359 million years ago. But Tiktaalik is so clearly an intermediate "link between fishes and land vertebrates," they said, that it "might in time become as much an evolutionary icon as the proto-bird Archaeopteryx," which bridged the gap between reptiles, probably dinosaurs, and today's birds.
The writers, Erik Ahlberg of Uppsala University in Sweden and Jennifer A. Clack of the University of Cambridge in England, are often viewed as rivals to Dr. Shubin's team in the search for intermediate species in the evolution from fish to the first animals to colonize land.
In a statement by the Science Museum of London, where casts of the fossils will be on view, Dr. Clack said the fish "confirms everything we thought and also tells us about the order in which certain changes were made."
H. Richard Lane, director of paleobiology at the National Science Foundation, said in a statement, "These exciting discoveries are providing fossil 'Rosetta Stones' for a deeper understanding of this evolutionary milestone — fish to land-roaming tetrapods."
The science foundation and the National Geographic Society were among the financial supporters of the research. Besides Dr. Shubin, the principal discoverers were Edward B. Daeschler of the Academy of Natural Sciences in Philadelphia and Farish A. Jenkins Jr., a Harvard evolutionary biologist.
Michael J. Novacek, a paleontologist at the American Museum of Natural History in Manhattan, who was not involved in the research, said: "Based on what we already know, we have a very strong reason to think tetrapods evolved from lineages of fishes. This may be a critical phase in that transition that we haven't had before. A good fossil cuts through a lot of scientific argument."
While Dr. Shubin's team played down the fossil's significance in the raging debate over Darwinian theory, which is opposed mainly by some conservative Christians in the United States, other scientists were not so reticent. They said this should undercut the creationists' argument that there is no evidence in the fossil record of one kind of creature becoming another kind.
One creationist Web site (emporium.turnpike.net/C/cs/evid1.htm) declares that "there are no transitional forms," adding: "For example, not a single fossil with part fins part feet has been found. And this is true between every major plant and animal kind."
Dr. Novacek responded in an interview: "We've got Archaeopteryx, an early whale that lived on land and now this animal showing the transition from fish to tetrapod. What more do we need from the fossil record to show that the creationists are flatly wrong?"
Dr. Shubin and Dr. Daeschler began their search on Ellsmere Island in 1999. They were attracted by a map in a geology textbook showing the region had an abundance of Devonian rocks exposed and relatively easy to explore. At that time, the land was part of a supercontinent straddling the equator and had a warm climate.
It was not until July 2004, Dr. Shubin said, that "we hit the jackpot." They found several of the fishes in a quarry, their skeletons largely intact and in three dimensions. The large skull had the sharp teeth of a predator. It was attached to a neck, which allowed the fish the unfishlike ability to swivel its head.
"Fish feeding in water readily orient the mouth toward food by maneuvering the entire body," said Dr. Jenkins, who assisted in the interpretation of the fossils. "The head is rigidly attacked to the trunk by bones linking the skull and shoulder girdle, and thus fish have no neck."
If the animal spent any time out of water, he said, it needed a true neck that allowed the head to move independently on the body.
Embedded in the pectoral fins were bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals. The scientists said the joints of the fins appeared to be capable of functioning for movement on land, a case of a fish improvising with its evolved anatomy. In all likelihood, they said, Tiktaalik flexed its proto-limbs primarily on the floor of streams and may have pulled itself up on the shore for brief stretches.
In their journal report, the scientists concluded that Tiktaalik is an intermediate between the fish Panderichthys, which lived 385 million years ago, and early tetrapods. The known early tetrapods are Acanthostega and Ichthyostega, about 365 million years ago.
Tiktaalik, Dr. Shubin said, is "both fish and tetrapod, which we sometimes call a fishapod."