By Randy Boswell
A celebrated missing-link fossil found in the Canadian Arctic seven years ago has led scientists to a significant new discovery that the genetic blueprint for arms, legs, fingers and toes existed in prehistoric fish long before some of them evolved into land animals exhibiting such features.
The finding by a team of U.S. researchers, published in the latest Proceedings of the National Academy of Sciences, reinforces the reputation of Tiktaalik roseae — a 375-million-year-old fossilized “fishapod” discovered on Ellesmere Island in 2004 — as an “evolutionary icon” because it symbolizes the crucial transition from sea to land for some of the Earth’s most primitive creatures.
The researchers said the evident similarities between fish fins and mammal limbs — “particularly the wrist and hand-like compartments present in the fins of Tiktaalik” and other ancient transitional species — “inspired a laboratory experiment to look at the homology, or shared physical and genetic traits, of fish and limbed animals.”
The experiment showed that a genetic “switch” needed to activate the development of limbs in present-day mice was also present in extinct species of fish some 400 million years ago. Not only that, the same patches of DNA traced to primitive fish could be transplanted from present-day fish into modern mice embryos and successfully trigger limb development.
Likewise, the limb-development genes from mice could be substituted in aquatic species such as zebrafish and skate and trigger the growth of fins.
“Because the last common ancestor of all these species pre-dates Tiktaalik-like ‘fishapods,’ the comparison offered a glimpse at biology before animals made their first steps on land,” the researchers stated in a summary of their findings.
“The genetic switches that drive the expression of genes in the digits of mice are not only present in fish, but the fish sequence can actually activate the expression in mice,” said Igor Schneider, a University of Chicago biologist who led the study. “This tells us how the antecedents of the limb go back in time at every level, from fossils to genes.”
Schneider’s colleague Neil Shubin — the University of Chicago scientist who discovered Tiktaalik and later authored the controversial book Your Inner Fish to highlight the fossil’s importance in understanding human evolution — said the latest study strengthens the links between pre- and post-Tiktaalik organisms.
“This is really a case where knowing something about the fossils and the morphology led us to think about genetic experiments,” Shubin, who co-authored the PNAS paper, said in a summary of the study.
“Tiktaalik and its cousins showed us that this limb compartment is not an utter novelty in tetrapods (all four-limbed animals, including humans), as was thought for a long time. So an antecedent of that program must exist.”
Tiktaalik was a predatory species that hunted in shallow waters at a time in Earth history when Ellesmere Island — now Canada’s northernmost land mass — was a subtropical swamp situated near the equator.
The fossilized bones of the three-metre-long Tiktaalik (its Inuktitut name means “big, shallow-water fish”) showed that it had the scales and fins of a fish but the ribs, neck, head and limb-like bones of a land animal.
In 2008, the U.S. National Academy of Sciences highlighted the discovery of Tiktaalik in a special report aimed at promoting the study of evolution in U.S. schools and countering calls for more teaching of “intelligent design” and other forms of creationism.
“The major bones in our own arms and legs are similar in overall configuration to those of Tiktaalik,” noted the NAS report. “The discovery of Tiktaalik, while critically important for confirming predictions of evolutionary theory, is just one example of many findings made every year that add depth and breadth to the scientific understanding of biological evolution.”
In December 2008, Shubin and other researchers published a paper on Tiktaalik braincase anatomy showing how features of the animal’s head bones — not only its limb-like fins — also presaged the evolutionary shift between sea and land for such species.