Cross-breeding blind cave fish with those from separate populations of blind cave fish can partially restore their vision, overriding half a million years of evolutionary change, say U.S. scientists.

"These fish are descended from ancestors that have been isolated in the dark for nearly one million years and most likely haven't had the capacity for vision for at least half that time," New York University biology professor Richard Borowsky, the study's lead author, said in a release.

"But by recombining the right genes through hybridization, you can partially restore vision. Not only are the structures of the eye restored to the point where they regain function, but all the connections to the brain for proper processing of information not used for that enormous length of time are restored."

The finding, which may have implications for understanding human eyes, was published in Monday's issue of the journal Current Biology.

The study suggests that genetic engineering can override, at least in part, evolutionary change in just one generation. That's because mutations in different genes are responsible for the loss of sight in separate cavefish lineages. 

"Restoration of the ability to see comes in a single generation because the populations residing in different caves are blind for different reasons — i.e., different sets of genes are non-functional in the different populations," said Borowsky.

"[In the hybrids], the deficiencies in one lineage are compensated for by the good gene copies in the other lineage, and vice versa."

Researchers crossed four populations of blind fish from caves in northeast Mexico. Sightless for at least half a million years, the fish evolved from sighted surface fish.

By creating hybrids of the different cave fish populations, researchers found that nearly 40 per cent of some hybrid crosses could see.

The farther apart the caves of the hybrids' parents were, the more likely it was that their offspring could see.

That backs up the theory that populations separated by greater distances are more distantly related and therefore have less overlap in the genes responsible for their blindness, said Borowsky.