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Planet Earth

Why there are no Six-toed Sloths

By Kathy A SvitilSeptember 1, 2001 5:00 AM

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Ten fingers and toes is one of life's immutable limits. No modern land-lubbing vertebrate has more than that, a consistent pattern that has long puzzled biologists. Christopher Hayes thinks the explanation is simple: It was just a matter of evolution weeding out the clumsy.

Congenital malformations regularly lead to animals with extra digits, "so clearly there must be some evolutionary disadvantage to having more than five," says Hayes, a molecular biologist at the Merck Sharp & Dohme Neuroscience Research Center in Essex, England. To find that factor, he studied mice with a genetic defect called polydactyly (multiple digits) and noted that this condition always seemed linked with unnaturally shortened bones in the lower arms and legs.

Stunted limbs make polydacty mice and men trip over their own feet. "We can keep our feet flat when we walk because of the mechanical strut of the tibia, the big bone of the lower leg. So if you lose the far end of the tibia, your foot will rotate and you'll have the equivalent of a congenital clubfoot," Hayes says. Shortening in the arm causes similarly awkward twisting, especially for tree-swinging animals. Creatures with extra digits probably would not have survived, which explains why we give high fives and count in base 10.

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