The biological ‘five finger rule’ is strikingly consistent throughout living tetrapod vertebrates. Humans and other primates, most carnivorous mammals, crocodiles, lizards and tortoises all typically possess the five digits (fingers and toes) characteristic of tetrapod limbs. It wasn’t always so – the ancient ancestors of the first vertebrates to walk on land, such as Ichthyostega and Acanthostega, had up to eight digits per hand or foot – but living tetrapods descended from a common ancestor that had five digits on each limb. There are many examples of animals having reduced this number by losing some digits during development, such as horses, birds and salamanders, but their basic blueprint lays out the pattern for five fingers and toes. What’s much rarer is an increase over five digits (even though it was once common). This can happen by chance as a result of duplication, known as polydactyly, which we see when individuals have unusual extra digits, but polydactyly represents a developmental anomaly rather than evolutionary change that tends to persist across many generations (with some rare, isolated exceptions). Instead, some animals have adapted existing (but smaller) bones in their hands and feet, known as sesamoids, so they are able to act like digits. Sesamoid bones are associated with tendons and muscles, so these enlarged sesamoids or ‘predigits‘ can often even be manipulated like a true digit.
Remarkably, this exaptation (co-option) of sesamoids into predigits has been found in at least three different groups in mammals alone! Perhaps most famous is the panda’s ‘thumb’, which is thought to help it grip the tough bamboo it feeds on. The radial sesamoid is found in many mammals as a tiny nubbin of bone found inside a tendon on the inner (thumb) side of the hand, but in the giant panda it has grown much larger, rivalling the first digit for length and appearing externally as a sixth digit. However, its evolutionary ancestry as a sesamoid means that it is technically a predigit, not a real finger.
As in the panda, evolving enlarged radial or tibial (on the hind legs) sesamoids is often associated with some biomechanical function that requires extra purchase or support. This is a pattern also seen in moles and elephants, despite their stark differences to pandas in many other aspects!
Elephants in high heels?
A sixth digit was first illustrated in elephant skeletons more than 300 years ago by Scottish doctor Patrick Blair, but only recently has it been shown to be a sesamoid like that of giant pandas. Elephants walk in a tip-toed posture, with the heel positioned above large fatty pads in the foot. In both the fore and hind feet respectively, elephants have a prepollex (next to the thumb) and a prehallux (next to the big toe) which project downwards like a stiletto heel, spearing these fatty pads and stabilising each step. These predigits, as in the panda, correspond to the radial and tibial sesamoids of many other mammals.
It is thought that in this case, the evolution of the predigits is associated with the appearance of gigantic size (and increased time spent on land, relative to aquatic ancestors) in elephants and related species. Although the sesamoids are almost certainly present in their smaller ancestors, their flat-footed posture suggests they wouldn’t have required the same stabilisation.
Moles: built to dig
Our final example comes in a much smaller package: talpid moles are found throughout the world. Their burrowing lifestyle has shaped their evolution in several ways, including the development of an enormous prepollex (right), or “falciform bone”, in many species. This enlarged predigit is particularly evident in some of the species most specialised to life underground, indicating that the structure might help brace the wrist and expand the available surface area of the hand for burrowing. Recent studies in developmental genetics showed that the same genes used in the shrew cousins of moles to form fingers are switched on in the region of the prepollex, thus turning what remains a small radial sesamoid bone in shrews into a giant finger-like bone in moles. Evolution: gotta dig it!
Exaptation: The evolution of a new function for an existing feature.
Polydactyly: The development of extra digits by the duplication of existing fingers or toes.
Predigit: A modified sesamoid structure which acts like a digit but does not share the same developmental and evolutionary origin, as opposed to polydactyly (above).
Prehallux: A predigit on the inside (big toe side) of the hindlimb.
Prepollex: A predigit on the inside (thumb side) of the forelimb.
Radial sesamoid: A small bone found medial to the base of digit I of the forelimb (the inside digit; thumb in humans).
Sesamoid: A small bone or other hard, nodular tissue that is naturally found within a tendon or ligament near a joint; e.g. the kneecap or patella.
Tetrapod: A vertebrate (bony) animal with, or descended from ancestors with, four legs bearing fingers and toes.
Want to know more?
D. Dwight Davis 1964. The giant panda: a morphological study of evolutionary mechanisms. Fieldiana 3, 1.
Hutchinson et al. 2011. From Flat Foot to Fat Foot: Structure, Ontogeny, Function, and Evolution of Elephant ”Sixth Toes”. Science 334, 1699.
Images courtesy of D. Dwight Davis, Peter Deane and the BBC.