Scientists have decoded the DNA of a "living fossil" fish which was found off the coast of Africa in 1938, and is now offering new insights into how today's mammals, amphibians, reptiles and birds evolved from a fish ancestor.

The coelacanth, which lives in deep-sea caves off the coast of Africa, was once known only from its fossils and so was thought to have gone extinct at least 70 million years ago until a recently-dead specimen was discovered by South African fishermen in 1938.

The coelacanth can grow to two meters (six feet) and live as many as 60 years in the wild, according to National Geographic. Their front fins are fleshy, resembling the limbs of four-legged land animals.

The gene sequencing enabled researchers to see changes between water and air, to the sense of smell, the immune system, and the body's ability to eliminate waste.

The study, published in the journal Nature, suggested that another fish called the lungfish, which also has four limbs, had more genes in common with land-based animals.Even so, the coelacanth is a remarkable source, for it will help show which genes were squeezed out, and which emerged, in the touted sea-to-land transition.

"What we can see is that while the genome as whole changes, the protein-coding genes - that make the living fish - are much more stable and much more unchanging," said Professor Kerstin Lindblad-Toh, from the University of Uppsala in Sweden and the Broad Institute of MIT and Harvard.

"And if you think about it, this might be correlated to the fact that the coelacanth lives in a rather extreme and stable environment.

"It lives several hundred meters down in the ocean, and it may also be in an environment where it doesn't have a lot of competitors. So maybe it adapted to that environment a long time ago and it doesn't have a huge need for change."