In humans, polydactyly is considered a common occurrence.
Dr Alan Greene (MD FAAP) author, pediatrician, lecturer and keynote speaker says;
“Polydactylism, or having one or more extra fingers or toes, is probably the most common abnormality of development found at birth. Polydactylism is reported in about 2 per 1000 children. However, many of the simpler cases are taken care of in the nursery by the obstetrician or pediatrician and don't show up in these statistics.” (Ref 1)

Anne Boleyn, Winston Churchill and King Charles VIII of France (ref World Knowledge Library) are some famous polydactyls.

“Polydactyly is an ancient trait and but for a quirk of evolution, all modern animals would have 7 or 8 digits instead of just 5. The oldest known four-legged animals, Ichthyostega and Acanthostega, had 7 or 8 digits per limb. The "extra" digits were next to the thumb. The extra digits disappeared 350 million years ago, leaving modern animals with just 5 per limb. 100 million years after evolution opted for five digits, throwbacks to ancestral polydactyly occurred, as a fossil of a seven-toed reptile demonstrates. The fossil, an aquatic marine reptile called Nanchangosaurus, was an mutant or evolutionary throwback which lived 100 million years after other seven-toed amphibians had died out.” (Ref 2)

There has been concern in the Cat Fancies that polydactyly may be associated with other abnormalities that we would not want to encourage in cat breeding. It is important that we separate true polydactyl from other syndromes that happen to include extra digits such as Ellis-van Creveld syndrome which is a disorder of the skeletal dysplasia type and includes cardiac malformation, dwarfism, cleft palate as well as extra digits. This is an autosomal recessive gene found on the chromosome 4 short arm and quite distinct to the simple dominant polydactyl gene found in the Maine Coon cat.