But for the Frankenfish that the FDA approved yesterday—the very first genetically modified organism declared safe to eat—the journey took more than 20 years.

It didn't take that long because the science was hard. Researchers had already nailed down the genetic tweaks to bulk up the fish—technically called the AquAdvantage salmon—by the early 1990s. Starting with the genome of the Atlantic salmon, a heavily farmed species that's nearly extinct in the wild, scientists made two changes. They took the gene for a growth hormone from the Chinook (or king) salmon, the largest of the Pacific salmon species, and kicked that hormone into overdrive with a promoter gene taken from ocean pout, an eel-like fish that can survive and grow in near-freezing waters. "Usually the salmon's growth hormone gets turned off during colder months," says Eric Hallerman, fish conservation scientist at Virginia Tech University. The pout's promoter gene basically makes sure the Chinook growth gene never gets shut off. Voila: a mega-fish.