"Walking Fish" provides insight into Early Land-Based Animal Development

http://www.livescience.com/47582-unusual-fish-bichir-animal-evolution.html

This article outlines the evolutionary implications of the behavior of the Bichir, a fish with legs.
It begins by outlining the challenges faced by fish in an early environment, with many
species of fish competing for different resources. The stiff competition faced by the different fish species pushed the fish towards occupying a land-based niche.

The Bichir could reflect a modern-day intermediate step between a water and land based animal.
Scientists conducted an experiment in which they separated two groups of young Bichir, and had one group live on land, and one in water. The two Bichir groups developed differently, with the group raised on land adapting its behavior to make better use of its lungs and also its repurposed fins, and the group in the water relying on its gills and regular fins more.

These results show an approximation of how ocean-based life could have adapted to live on land. By slowly adapting its existing structures such as fins into legs, and also by relying less on its gills as a source of oxygen, an ocean-based animal can eventually adapt itself for life on land. This entire process is an example of adaptive radiation.