Could birds evolve from fish?

That’s not what actually happened, but there is no rule against it. The evolution of animals is simply the result of contingency, not driven by any necessity.

Yet intuitively it seems obvious that it is easier to exploit traits already present, such as fins, to crawl on land rather than to fly over the surface of the water. A process called exaptation.

The surface of the earth was at that time a risk-free place full of possibilities, with little if any evolutionary pressure to take flight, which is moreover energetically very costly.

However, it is possible to conceive of cases in which aquatic creatures might first evolve into birds and only then populate the land, if we look beyond the Earth.

On a habitable planet where the density of the atmosphere is greater than that of the earth, the lower density difference between water and air, coupled perhaps with a lower gravity, could make direct transitions from aquatic to bird-like animals possible.

If the planet also had little or no land, such a transition would almost be a necessity.

Similar reasoning to mine was probably also made by the makers of the film Avatar, whose creatures were designed to be as scientifically accurate as possible.

Reconstructions have even been made of the phylogenetic tree (how creatures are related to each other) of Pandora, and it seems that there, aquatic creatures actually became birds before they became quadrupeds.

The jaws of these animals are very fish-like, and curiously, Pandora has both a lower gravity and a denser air.

This article is referred to in: Why bioluminescence?