Pandas, koalas and some dinosaurs found their own ways to grip like thumbs
The human thumb gets its name from an ancient root meaning swelling, since it is the stoutest digit. What makes it special is a mobile base that lets its pad meet each fingertip squarely. Nature has invented rival grips many times, from the panda's wrist bone to koala paws and clawed dinosaur hands.
Anatomists call the thumb the pollex, the hand's counterpart of the big toe. It shares hair, a nail and fingerprint ridges with the other digits, yet stands apart: it has been counted as having two phalanges instead of three, although reports since the early 2020s suggest it may instead have three and lack a metacarpal. Its broad tip and highly mobile metacarpus supply most of its ability to oppose. Even the word finger descends from an ancient Proto-Indo-European root tied to the idea of five.
Defining opposition is surprisingly contentious; researchers use hundreds of definitions. Primatologists John and Prudence Napier set a demanding one: the pad of the thumb pressed squarely against the pads of the other fingers, which requires the thumb to rotate about its long axis at its base. Apes and Old World monkeys can turn the thumb this way, but the broad pad-to-pad contact between thumb and index finger is distinctively human. Tarsiers and marmosets have no opposable thumb at all, and lemurs manage only a partial version.
Other animals improvise. The nearly thumbless spider monkey grabs with the bare underside of its tail. Giant pandas use an elongated sesamoid bone beside the first finger as a stand-in thumb. Koalas oppose their first two front toes to the other three to grip thin branches, and opossums carry grasping thumbs on their hind feet. Some polydactyl cats end up with opposable toes on both sides of the paw.
The idea is old. The Eocene fossil primate Darwinius masillae already had opposable thumbs and big toes. Among dinosaurs, the bird-like troodontids had a partially opposable finger, and the small hunter Bambiraptor may have been able to bring its first and third fingers together and reach its hand to its mouth. Even a pterosaur, Kunpengopterus, bore an opposable first digit on each wing, probably for climbing trees.
Source: Thumb
