Polypterus senegalus, a type of fish that walks on land and breathes air, may reveal how our distant ancestors may have first left the sea.

All tetrapods - those animals with four limbs, like reptiles, birds and humans, evolved from these ancient fish.

Roughly 400 million years ago, biodiversity of fish was extremely high, which could have driven fish onto land for the first time in history. The species which first ventured onto terra firma are long extinct, requiring researchers to study Polypterus.

McGill University biologists raised juvenile members of the species "on land" for nearly a year, to see how their development would differ from their cousins brought up in water. To keep the animals alive in their aquarium, a thin layer of water was provided in the bottom of the tank, and misters kept the environment moist.

Polypterus raised in drier conditions developed in a different manner than the control group kept in water. Their legs were held closer together than normal, and slipped less frequently in sand. The land-raised animals also held their heads higher than the others when they walked. Not only were behavioral changes seen, but changes in the bone structure of the animals also took place.

"Anatomically, their pectoral skeleton changed to became more elongate with stronger attachments across their chest, possibly to increase support during walking, and a reduced contact with the skull to potentially allow greater head/neck motion," Trina Du, a doctoral student at McGill and collaborator on the study, said.

This study was the first major investigation of what anatomical and behavioral changes may have taken place in the first fish that climbed up on land. It also gave biologists a chance to see how these early land dwellers utilized their fins as a means of locomotion.

Research revealed that fish raised on drier surfaces were able to move more quickly across sand than those raised in water. Investigators believe this advantage may have allowed early tetrapods to survive on land.

Polypterus, also called Bichir fish, are native to the Nile basin in Africa. Biologists believe they are the closest living relative to fish which first left the oceans. These animals possess both lungs and gills, allowing them to survive in either aquatic environments or on land. The study examined 100 bichirs raised in a drier environment and 50 allowed to develop in water.

"Because many of the anatomical changes mirror the fossil record, we can hypothesize that the behavioral changes we see also reflect what may have occurred when fossil fish first walked with their fins on land," Hans Larsson, McGill researcher and an associate professor at the Redpath Museum, said.