Fish have different styles of fins, but they all share the same evolutionary pattern for positioning the limbs — a finding that could help explain the genetics of human birth defects.

The genetic template for building and positioning our arms and legs were laid out in paired fish fins.

"Given that paired fins made their evolutionary debut at a particular location on the sides of the body, intuitively one would think the genetic tools for fin development would be brought together in that place," said developmental biologist Martin Cohn of the University of Florida.

"We've discovered that the genetic circuitry for building limbs first appeared in an entirely different place — the midline of the animal."

The midline runs along the centre of a fish's back and belly.

The genetic pattern was important not only for the placement of symmetrical pairs of fins but also the median or middle fin, the researchers said in Thursday's online issue of the journal Nature.

Genes involved in medial fin development are also important for the healthy development of human limbs.

"Knowing that many of these genes are responsible for limb defects in humans is intriguing," Cohn said. "What we've done is identify where those developmental pathways originated during our evolutionary past and how they became involved in limb development."

Lampreys, a living relic from the time before fish evolved paired fins, also showed the same genetic cues, which suggests the template may predate fish.