All cells in the human body—and the motions they enable, from our beating hearts to our wiggling toes—use energy generated by a molecule called ATP. Its energy is thought to be released to fuel cellular processes in only one way: when ATP's decomposition is initiated by an enzyme, a specific type of protein.

When scientists want to create a protein that can potentially treat disease, one approach is to begin with a different protein and evolve it in the lab, selecting over many generations for versions with a new, desired function.