A new study suggests that the American paddlefish duplicated their own genome some 42 million years ago, which might question the theory of fin-to-limb evolution.

To find out how limbs develop in humans, researchers from the San Francisco State University compared the limb-building genes that are found in mice with fin-building genes that are found in fish.

For their study, they tested paddlefish which are known to have a genetic toolkit that helped in the evolution of limbs from fins much before the evolution of four-footed creatures known as tetrapods. Amphibians, sauropsids and mammals are said to be tetrapods.

Earlier studies have used paddlefish as a representative in the evolutionary tree making them a proxy for the ancestor of the bony fishes that evolved into tetrapods.

Researchers sequenced the chromosomal regions that have the 19 Hox genes in the American paddlefish. Hox genes basically determine the basic structure of an organism. They are responsible for body shape and proper placement of the limbs. Hox genes also play a significant role in determining whole genome duplications.

Genome duplications provide an opportunity for the genes to mutate and play a role in the formation of a new species or create new features within a species. If a twin copy gene is evolved, one will function the way it is and the other may be lost or take on a new purpose, the researchers said.

They believe that genome duplication observed in the paddlefish may change the way they have been interpreted in studies for the fin-to-limb transition. "This creates extra genetic material that adds complexity to comparative studies. It may change the way we interpret studies on limb development," Karen Crow, assistant professor of biology at San Francisco State University, said in a statement.

Researchers also noted that whole genome duplications have taken place in plants and animals. Two genome duplications are said to have occurred before the jawed vertebrates evolved. "Our findings on the paddlefish suggest that whole duplication is not as uncommon in animals as previously thought," Crow said.

The findings of the study have been published in the journal Genome Biology and Evolution.