Ancient Fish Had Primitive Fingers, Toes

September 24, 2008

An ancient fish possessed primitive digits that may have been the precursors to our own fingers and toes, according to a new study.

The finding supports recent studies that suggest primitive fish and sharks had the genes necessary to develop digits, even if the animals didn't grow those appendages.

"The contrary position was that the hand and foot was a complete novelty that appeared [out of nowhere]" in the first four-legged land animals, called tetrapods, said study team member Per Ahlberg, a paleontologist at Uppsala University in Sweden.

That theory "can now pretty much be dismissed," Ahlberg added.

Arm-Like Fin

Using medical x-rays, Alhberg and his team created a three-dimensional image of a fin belonging to Panderichthys, a coastal fish that lived about 385 million years ago during the Devonian period.

The scans revealed the ancient fish had many of the same bones that make up a modern human arm, including the humerus, radius, and ulna, as well as four small "radial" bones that look remarkably like rudimentary fingers, the authors said.

Unlike human fingers, however, Panderichthys's digits were concealed in the fleshy arm-like base of its fin.

With the origins of tetrapods, the outermost part of the fin was lost, allowing the "fingers" that were always present at the base of the fin to emerge, Ahlberg said.

The research was detailed September 21 in the journal Nature.

Missing Link

Curiously, the radial bones of Panderichthys are more finger-like than those of Tiktaalik, a fish with stubby leg-like limbs that lived about five million years later.

Many scientists regard Tiktaalik as a "missing link": the crucial transitional animal between fish and the first tetrapods.

One possibility, Alhberg said, is that finger development took a step backward with Tiktaalik, and that Tiktaalik's fins represented an evolutionary return to a more primitive form.

Michael Coates, an evolutionary biologist at the University of Chicago, called the new findings "intriguing" but is not convinced that the digit-like structures in Panderichthys's fin are the equivalent of our fingers.

For one thing, they seem unusually flat for radial bones, Coates said.

"Radials are generally cylindrical. When you look at [a] cross-section [of the digit], they're dumbbell-shaped."

The structures are so peculiar, they might just be fragments of damaged bone, he added.

Coates agreed, however, that fingers and toes—or at least their precursors—were probably present in early fish.

"Nothing comes from nothing in evolution," he said.

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