Read: The wild experiment that showed evolution in real time

While discovering all this, Cheng and DeVries learned that at the other end of the world, Arctic cod also make antifreeze proteins, and their versions are built from exactly the same thralala units as the notothens’. The two groups had evolved almost identical antifreezes independently—a stunning example of convergent evolution, where two organisms turn up to life’s party in the same outfit. But there was a big difference between them: The cod antifreeze gene did not arise from a digestive one, and for the longest time Cheng couldn’t find its ancestor. “The gene had to have come from somewhere,” she says.

“These 1997 papers are classics,” says Aoife McLysaght from Trinity College Dublin, who studies the evolution of new genes. “The case for the Antarctic one was very clear, but all they could say was that the Arctic one wasn’t the same. It was also a new gene, doing the same thing. But it had a different origin, and what that origin was wasn’t clear.”

Now, after 22 years, Cheng has finally solved the mystery, and the answer is stranger and more convoluted than she imagined. Her team, which includes her colleagues Xuan Zhuang, Chun Yang, and Katherine Murphy, compared three species of cod that make antifreeze with four that do not. They compared pieces of antifreeze genes from the former against the DNA of the latter, in the hope of finding sequences that shared a vague resemblance. They found a hit—but in a functionless stretch of cod DNA that doesn’t include any genes at all. Somehow, this region of useless junk gave rise to a new and very useful gene. And Cheng’s team has deduced how this happened, step-by-step.

First, through random chance, a short stretch of junk DNA was duplicated twice, creating four identical segments in a row. The stretches between these segments were very close to the code for the thralala unit, and through a single mutation, one of them turned into exactly the right code. This snippet then duplicated, over and over, creating the core of a new antifreeze gene.

But for genes to be useful, they need more than the right sequence. They also need to be next to switches that allow them to be activated in the right time and place. In this case, the newborn antifreeze gene was serendipitously reshuffled to a different spot in the cod genome, where it landed next to one such switch.

At this point, the ancestral cods had a gene for antifreeze and could switch that gene on. But they needed one last tweak: a small signal sequence that acts like a shipping label, allowing the antifreeze proteins to be exported from the cells that make them and into the rest of the animal’s body. Fortunately, that signal was already almost in place: It took just one mutation—the loss of a single DNA letter—to create it.