Human hand-walkers: five siblings who never stood up

Human beings begin life as quadrupeds, crawling on all fours, but none has ever been known to retain this gait and develop it into a proficient replacement for adult bipedality. We report the case of a family in which five siblings, who suffer from a rare form of cerebellar ataxia, are still quadrupeds as adults - walking and running on their feet and wrists. We describe the remarkable features of this gait, discuss how it has developed in the members of this family, and consider whether a similar gait may have been used by human ancestors.