October 29, 2008

Professor discovers missing evolutionary link--fish with feet

Ten high school biology students got in touch with their inner fishes last Thursday during an encounter with a prehistoric fish with legs.

University paleontologist and biology and anatomy professor Neil Shubin presented his talk “Finding Your Inner Fish” at I-House for the students of St. Charles North High School.

Students viewed a plaster cast of Shubin's latest discovery: a fossil skull from a carnivorous creature called Tiktaalik, also known by its moniker “fishapod”--or fish with feet. Tiktaalik lived approximately 370 million years ago and appears to be a transition species between lobe-finned fish that lived about 380 million years ago--which are ancestors to modern lungfish--and vertebrate land animals that appeared roughly 365 million years ago.

The fossil discovery is the is the centerpiece of Shubin's newest book Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body, which was published in January this year.

For Janice Young's AP biology class, Shubin’s book was the students’ summer reading assignment.

"Everything we’ve done has come back to this,” she said.

Shubin discovered the fossil in 2004 on Ellesmere Island in Nunavut, Canada after spending nearly 20 years searching for just such a transitionary creature. In the four short years since its initial discovery, 30 individual specimens of Tiktaalik have been discovered on the same site, ranging from four feet to nine feet in length.

In his talk, Shubin presented three main reasons for why he thinks Tiktaalik was a transitionary species between fish and land animals. First, much like early amphibians, Tiktaalik's head was flat and had both eyes on top--as opposed to a fish’s round, conical head with eyes on either side. Second, while fish have bones fusing their heads to their bodies, Tiktaalik had developed a neck, allowing it to move its head independently from its body as land animals do. Third, while Tiktaalik has fins like a fish, its bone structure is similar to those of later land animals. While fish skeletons have primitive bone structure in their fins, Tiktaalik’s skeleton had well developed upper and lower arm bones, wrist bones, and individual digits like that of land animals.

Tiktaalik lived in a shallow Upper Devonian era river delta that was surrounded by carnivorous fish of every length, from less than an inch long to some that were 16 feet long. In this “fish eat fish world,” Shubin said that the options were basically “get big, get armor, or get out.” He added that the Tiktaalik opted for the latter strategy by developing fins that enabled it go on short forays on land to hunt invertebrates that had already made the shift from water to land--and perhaps to escape the jaws of larger fish.

Shubin began writing his book before Tiktaalik had been discovered. Although the fossil fish takes the book's headlining role, Shubin's central thesis is that “inside every organ in our body, resides over 3.5 billion years of history. The basic structures, basic road maps in our body are found in other animals.”

Alongside a discussion of Tiktaalik, Shubin discussed some aspects of human anatomy that exemplify that 3.5 billion year history. He noted that the arches on the heads of shark embryos that develop into gills in mature sharks are analogous to the arches on human embryos that develop into the ears, jaw, throat and those structures’ corresponding blood vessels and nerves.

Shubin characterized his book as "pro-evolutionary."

"I wanted to go into the beauty of science,” he said.