"Mmm. Basically? If you're looking at it as a story. Random mutations that can pass on advantages, so they're kept around. Or are neutral."

She leans her head against his, brief but affectionate.

"Red hair is neutral, so it's kept around just because. But our ability to run for long distances is an advantage, so it shaped how humans evolved.

Or so I've been informed by documentaries," she adds with a quick smile.