Millions of years ago, ancient land-dwelling mammals returned to the sea. Their bodies became streamlined for swimming, articulated fingers turned into flippers and fins, hypnotic songs slowly filled the oceans—and, somewhere during the evolutionary process, the newly evolving marine mammals lost a particular gene called Paraoxonase 1, or PON1.

Perhaps that gene was no longer necessary for organisms adapted to life in the water; as evolution streamlined aquatic bodies, it may have similarly streamlined genomes. But whatever the reason, PON1 ceased to function in each of the three marine mammal lineages that are today’s whales and dolphins, manatees and dugongs, seals and sea lions.

“The fact that it’s so absolutely conserved in terrestrial species, and is completely lost from most,