Melbourne: Humans are just modified fish, say scientists who claim to have discovered how the muscles controlling the pelvic fins of the marine species have paved the way for the evolution of back legs in higher animals.

A team, led by Prof Peter Currie at Monash University and Dr Nicholas Cole at University of Sydney, says this innovation gave rise to the tetrapods, or four-legged creatures, along with human's distant ancestors who made the first steps onto land some 400 million years ago.

The scientists studied primitive cartilaginous fish: Australia's bamboo shark and its cousin, the elephant shark; and three bony fish: The Australian lungfish, the zebrafish and the American paddlefish.

Prof Currie said the genetics of a fish are not vastly different to our own.   

"We have shown that the mechanism of pelvic muscle formation in bony fish is transitional between that in sharks and in our tetrapod ancestors.   

"By examining the way the different fish species generated the muscles of their pelvic fins we were able to uncover the evolutionary forerunners of the hind limbs. Humans are just modified fish," he said.

In fact, the scientists used the fish species living today to trace the evolution of pelvic fin muscles to find out how the load bearing hind limbs of the tetrapods evolved.

To find differences in pelvic fin muscle formation, they compared embryos of the descendants of species representing key turning points in vertebrate evolution.   

The scientists genetically engineered fish to trace the migration of precursor muscle cells in early developmental stages as the animal’s body took shape. These cells in the engineered fish emitted red or green light.

The team found that the bony fish had a different mechanism of pelvic fin muscle formation from that of the cartilaginous fish, a mechanism that was a stepping stone to the evolution of tetrapod physiology.

The findings have been published in the 'PLoS Biology' journal.