Researchers sequenced the genome of the coelacanth: a deep-sea fish that closely resembles its ancestors, which lived at least 300 million years ago.
The study found that some of the animal's genes evolved very slowly, giving it its primitive appearance.
The work also shed light on how the fish was related to the first land-based animals.
The coelacanth has four large, fleshy fins, which some scientists believe could have been the predecessors of limbs.
It had been suggested that this fish was closely related to early tetrapods - the first creatures to drag themselves out of the ocean, giving rise to life on land.
But 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.