Sometime between 360 and 390 million years ago, a group of fishes made the move to life on land. Along the way, their fins gradually transformed into weight-supporting limbs with distinct elbow and wrist joints. Fins became legs. Swimmers became walkers.

Some aspects of this evolutionary transition are now clear. We know how the bones of those pioneering fish evolved into those that I’m now using to type, thanks to a number of beautiful fossils. We’re even starting to uncover the genetic changes that were involved in this makeover.

But there are still mysteries. How, for example, did the owners of the first legs control their limbs?

Hold your arm out and bend it at the elbow. Several things are