A study of abnormal mice reveals why we have five fingers on each hand. If animals grow more than five digits other bones in the limb become distorted to the point where the animal cannot walk or run properly. This dramatically affects an animal's ability to catch prey or flee from predators and explains why evolution drew the line at five digits. Animals with more than five digits cannot place their feet flat on the ground. Instead, they are forced to angle their feet to cope with bowed legbones. However, animals can easily cope with fewer than five digits reports www.newscientist.com