A study of abnormal mice with extra digits has revealed why we have five fingers, rather than six or seven, says a British researcher.
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鈥檚 ability to catch prey or flee from predators 鈥� and explains why evolution drew the line at five digits, says Chris Hayes of Merck, Sharp and Dohme鈥檚 Neuroscience Research Centre in Harlow, Essex.
鈥淚t seems that five is the maximum,鈥� says Hayes, who studied the abnormal mice at the UK Medical Research Council鈥檚 Mouse Genome Centre in Harwell, Oxfordshire.
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Hayes found that the legs of mice were forced to grow in an arc to accommodate any extra digits. 鈥淭his affects the mobility and gait of animals, and that dramatically affects their ability to survive,鈥� he says.
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. One mouse studied by Hayes had 10 toes on each foot, rather than five. This was down to a natural mutation. The mouse鈥檚 mobility was severely affected. 鈥淭hey end up walking on their ankles,鈥� Hayes says.
However, animals can easily cope with fewer than five digits. 鈥淗orses essentially walk on the tips of one finger which has fused together from all the others through evolution,鈥� says Hayes. 鈥淪o you can decrease digit number and still increase evolutionary advantage.鈥�
Limb deformities are the second most common body malformation after heart defects, affecting 1 in 500 live births.
More at: Journal of Theoretical Biology (Vol 209, p497)
