Evolution insight: Fish reared on land replay the transition to four legs

It was one of the key events in the evolution of animals – and now it has been replayed in the lab. Evolutionary biologists reared air-breathing fish on land for eight months and found that the experience encouraged the fish to develop skeletons better adapted for walking.

Fish first adapted to life on land about 350 to 400 million years ago, when they evolved four legs to form the tetrapods, a group that includes amphibians, reptiles, birds and mammals. This crucial evolutionary transition is preserved in the fossilised remains of animals called stem tetrapods, which have some features of fish and some of four-legged animals.

Bichir fish (Polypterus), which are native to Africa's Nile basin, are the closest living counterparts of those ancient stem tetrapods. They have true lungs as well as gills, so they can survive on land as well as in their preferred underwater habitat. For these reasons, Emily Standen at the University of Ottowa in Canada and her colleagues decided to try rearing them on land to see whether this made them more adept at walking than bichir fish that were raised exclusively in water.

The short answer is that this is exactly what happened. "The anatomy of the fish raised on land changed, and those changes reflected what we see in the fossil record in the transition from fish to four-legged terrestrial vertebrates," says Standen.