Why do humans only have 5 digits on each hand and foot? How did having 5 or fewer digits per hand become "the norm" in most modern-day jawed vertebrates? (Hox genes probably came to mind, but I won't delve into that discussion here...)
Scientists believe that the condition of having no more than 5 fingers or toes most likely evolved prior to the divergence of amphibians and amniotes. Fossil evidence from 360 million years ago reveals that before this evolutionary split, there existed tetrapods that possessed limbs bearing arrays of 6 to 8 digits. Reduction from these polydactylous (polus= many, daktulos= finger) patterns to the common arrangement of 5 or fewer digits coincided with the evolution of sophisticated wrist and ankle joints -- sophisticated in the sense of greater number of bones present and complex articulations.
It is believed that animals bearing 6 or more digits had simple limb skeletons, comparable to that of modern-day cetaceans (e.g., dolphins and whales). The reasons for digit number reduction may be related to the functional demands of simple "walking limbs." For example, a whale fin does not provide a platform for an efficient "push-off," whereas walking limbs allow some rotation relative to the limb bones as the body moves forward.
All modern day tetrapods possess limbs characterized by 5 or fewer digits, with 5 being the more prevalent number. The interesting question here is: is there good reason that 5 digits (rather than 4) was functionally and biomechanically preferable to the common ancestor of modern tetrapods?
No, there was not.
But why not? Well, many tetrapod species have reduced their number of digits further. An interesting observation is that while digit numbers can be reduced, they rarely increase. This is a prime example of the wisdom in evolution that it is much easier to lose than to regain something. There are actually no truly six digit limb examples for us to study. Furthermore, while individuals of many species - mice, chickens, dogs, cats, and humans - carry mutations that result in polydactyly, having more than 5 digits has never been adopted as a norm throughout the evolutionary history of the common ancestor of tetrapods to modern tetrapods.
The panda's "extra" thumb (marked rs in the right figure) looks like a thumb and acts like a thumb, but is not homologous to the human digit that makes our hands so useful and contributes so much to our dexterity. The panda's thumb is just an enlarged carpal bone, not a true digit.
The panda may come to mind when you think of an animal with 6 digits. However, the extra finger is not truly a digit, but rather a modification of the wrist bones. The psuedo-digit is actually an enlarged carpal bone and not really an extra thumb.
Interesting side note: Hand-Foot-Fenital syndrome is a rare condition in which the urogenital tract and limbs are malformed. The mutation is an example of how a single gene can influence more than one phenotypic trait.
1. Coats, Michael. "Why do most species have five digits on their hands and feet?" Scientific American. 25 April 2005. Scientific American, Inc.
2. Tabin CJ. Why we have (only) five fingers per hand: Hox genes and the evolution of paired limbs. Development. 1992;116:289-296.