The biological ‘five finger rule’ is strikingly consistent throughout living tetrapod vertebrates. Humans and other primates, most carnivorous mammals, crocodiles, lizards and tortoises all typically possess the five digits (fingers and toes) characteristic of tetrapod limbs. It wasn’t always so – the ancient ancestors of the first vertebrates to walk on land, such as Ichthyostega and Acanthostega, had up to…