In a South African fish market in 1908, and in a small trawler fishing off the coast of west Africa in 1938, a certain type of fish was caught—a sea cave-dwelling, 5-foot (16.5-meter) ferocious predatory fish with limblike fins and a three-lobe tail called an African coelacanth. This was what is now called a “living fossil” (a term coined by naturalist Charles Robert Darwin), based on the fact that the fish closely resemble their more than three-hundred-million-year-old fossil ancestors—animals thought to have gone extinct around seventy million years ago. It took until the early 1950s for another live specimen to be caught, and since then, several more have been found.

In 2013, an international team of researchers delved even deeper into the fish’s past—decoding the genome of a coelacanth—with its 2.8 billion units of DNA, about the same size as a human genome. They found that the genes in the fish are evolving more slowly than other organisms—possibly because of a lack of predators and/or that the fish do not have to change. This may be due to a characteristic of the fish’s habitat: They live off the Eastern African coast (and another species off the coast of Indonesia) at dimly lit ocean depths of about 500 feet (496.7 meters)—regions that have not changed for thousands, if not millions, of years.