We can thank our heads and shoulders — and not just our knees and toes — that we evolved to run as well as we do.

The study, which involved treadmills, motion capture, hand weights and an eon’s worth of back story, finds that an unusual coordination between certain muscles in runners’ shoulders and arms helps to keep heads stable and runners upright.

The new findings may answer lingering questions about the role of our upper bodies in running and why we, unthinkingly, bend and swing our arms with each stride. They also add to the mounting evidence that, long ago, distance running began shaping human bodies and lives in ways that still reverberate today.