Missing link shows how fish grew feet

Missing link shows how fish grew feet
By Roger Highfield, Science Editor
Last Updated: 9:04am BST 24/05/2007
Telegraph UK

A long-held theory of the development of limbs is overturned by a study that shows the molecular wherewithal to make hands and feet was present long before the first fish took the momentous step on to land.

A model of the 375-million-year-old Tiktaalik roseae
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Animals with limbs - tetrapods - first came onto the scene about 365 million years ago. However, fish already possessed the pattern of gene activity associated with helping to grow hands and feet which was present in them at least 400 million years ago, according to research at the University of Chicago.

Genes are the DNA instructions to make the proteins that build and operate a body and it had been thought that newly-evolved patterns of activity of particular genes may have enabled the descendants of lobed-fin fish to dramatically alter their bodies genetically to adapt to living in streams and swamps.

But the study shows that the genetic and developmental toolkit that builds limbs with fingers and toes was around long before the acquisition of limbs and that a primitive version of this toolkit exists in a living primitive bony fish, the North American paddlefish (Polyodon spathula).

"We found that the genetic capability seen in tetrapods to build limbs is present in even more primitive fish," said lead author Marcus Davis, a postdoctoral fellow in Prof Neil Shubin's lab at the University of Chicago.

The scientists studied the development of paddlefish fins to test whether the genes activated to make hands and feet in tetrapods were different from the genes activated to make fish fins.

The team focused on so-called Hox genes - which play a vital role in limb development - in the pectoral fins of paddlefish, a sturgeon-like fish.

To track where the Hox genes are active in the fin, the team inserted molecular markers and showed the activity pattern has similarities to patterns of these same genes in the limbs of tetrapods, whether mice or man, which trigger the development of feet and hands.

Intriguingly, their results suggest that another fish that is the focus on genetic studies across the world - the zebrafish - appears to have lost this aspect of limb development and this had misled earlier researchers into thinking that a novel genetic toolkits paved the way from the sea to the land.

They conclude that fish originally used these genes to build their fins but it took environmental triggers to make use of that capability.

According to Prof Shubin, the ancestors of tetrapods evolved limbs with fingers using this genetic toolkit largely because their ecosystem - the small streams that they lived in - was new.

"It had the tools," he said, "but it needed the opportunity as well."

This is the first molecular support for the theory that the genes to help make fingers and toes have been around for a long time - well before the 375-million-year-old Tiktaalik roseae, the newly found species discovered three years ago by Prof Shubin and colleagues in Nunavut Territory, 600 miles from the North Pole.

Tiktaalik, with its crocodile like head, is considered a missing evolutionary link between fish and tetrapods and was among the first creatures that walked out of water onto land.

Now we know that animals like Tiktaalik already had the genes needed to make this historic step right at hand.