The coelacanth intrigued scientists because it was a kind of "living fossil": It had changed so little over the last 400 million years that it might reveal how fish first grew limbs and walked on land.
Deepening the mystery, other research showed that fish, mice and other animals carry many of the same genes. But in fish, those genes code for fins, whereas in land-based animals, they create limbs.
Because the fish were so endangered, it was difficult to study their body plan in detail. But Amemiya and his colleagues managed to get tissue samples from a coelacanth from the Comoros Islands.
Using that tissue, the team sequenced the primitive fish's genome and analyzed it. They found a small snippet of DNA called an enhancer that was present in both coelacanths and four-legged creatures, but missing in other fish.
The enhancer was part of the "dark matter" of the genome — the large fraction of the genome that doesn't code for proteins, but somehow turns genes on and off.
When they put the DNA snippet into mice, it seemed to turn on the genes to make the forelimbs and hind limbs in mice, Amemiya told LiveScience.