June 27, 2008

A Walk on Dry Land

Scientists unearth a skull of an ancient four-legged creature

By Lara Sokolowski with AP reporting

When did creatures first walk on Earth? For centuries, scientists have asked that question. The discovery of a skull of the most primitive four-legged creature ever found is bringing researchers a step closer to understanding the transition from fish to animals that walk on land. This week, scientists explained their findings in a report in the science journal, Nature.

PHILIP RENNE An artist's idea of what Ventastega curonica may have looked like

The 365 million-year-old fossil skull, shoulders and part of the pelvis of Ventastega (venta-ste-ga) curonica were unearthed in Latvia, a country in northern Europe. Researchers believe that the sharp-toothed water-dweller probably measured about three or four feet long, swam through shallow waters, and ate other fish.

"If you saw it from a distance, it would look like a small alligator," explains the report's lead author Per Ahlberg. He is a professor of evolutionary biology at Uppsala University in Sweden. "But if you look closer, you would find a fin in the back."

A Fishy Tale

Scientists believe that life began in the sea. Gradually more complex animals began to evolve and appear on land. Tetrapods are animals with backbones and four limbs. Their descendants include amphibians, reptiles, birds and mammals.

Although the new find is helpful and sheds light on the origin of tetrapods, scientists don't think four-legged creatures are directly descended from Ventastega. Instead, it's more likely that in the family tree of tetrapods, Ventastega is an offshoot branch that eventually died off, not leading to animals we now know, Ahlberg says.

Ahlberg didn't find the legs or toes of Ventastega. But, he was able to determine that it was four-limbed because key parts of its pelvis and shoulders were found. From the shape of these parts, scientists could conclude that limbs, not fins, were attached.

From Sea to Land

All of this occurred more than 100 million years before the first dinosaurs roamed Earth. And while Ventastega is the most primitive of the transition animals, even older ones have been discovered. But oddly, they are more advanced.

Neil Shubin is a professor of biology and anatomy at the University of Chicago, in Illinois. He helped find Tiktaalik (tik-taa-lik), a fish that was one step earlier in evolution. "It's sort of out of sequence in timing," Shubin says of Ventastega.

Now, one puzzle that scientists are trying to solve is why fish started to develop what would later become legs.

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