How did two groups of fish separately evolve genes for making antifreeze?

These animals survive at temperatures that would kill other fish because they produce their own antifreeze—a protein that courses through their blood and prevents ice from forming. In 1997, Cheng and DeVries, both at the University of Illinois Urbana-Champaign, found that the gene that makes this antifreeze protein had an unexpected origin—it arose from an ancestral gene that makes a digestive enzyme. Over millions of years, that snippet must have duplicated itself, again and again, turning an old digestive gene into a new ice-binding one, and allowing the notothens to survive in Antarctic waters.

Source: www.theatlantic.com