Thumb

In humans, the thumb is the first of five digits on the hand. It is shorter and thicker than the other four fingers and has a much wider range of motion. The human thumb is opposable, meaning it is on the opposite side of the hand from the four fingers. This allows the thumb to move around and touch the tips of the other fingers. As a result, an opposable thumb gives humans the ability to better grasp and hold onto objects. Most primates also have opposable thumbs, but humans have a wider range of motion and a more precise grip. These aspects likely served as an evolutionary advantage for early humans, allowing them to develop and use tools to shape their environment. The thumb’s internal structure is different than the other four fingers, leading to debate over whether it can be classified as a finger or a separate appendage. While there is no definitive answer to that question, typically, the thumb is considered to be one of five digits on the hand but is not classified as a finger.

Background

For much of the first 3.7 billion years that life existed on Earth, it was confined exclusively to the planet’s oceans. The early forms of life evolved to fit their environment, developing fins and other appendages to help them survive in the seas. Then, during the Devonian period about 420 to 360 million years ago, an evolutionary change occurred that saw the development of four-legged vertebrates called tetrapods. The first tetrapods were the ancestors of all modern mammals as well as amphibians, reptiles, and birds. Early in the Devonian period, many of the tetrapods resembled fish and had anywhere from five to thirteen digits on their four limbs. However, by the end of the period, most tetrapods had evolved pentadactyl limbs—limbs with five digits on the end of each hand or foot.

This evolutionary advancement allowed for more mobility in limbs and digits. Over the course of several hundred million years, tetrapods further evolved to move onto land. Eventually, some species lost additional digits, while in others, the ends of their claw-like limbs became less pronounced. Without claws, some species developed more flexible digits that allowed them to better grasp objects and hold onto trees while climbing. About 65 million years ago, some mammals evolved a limb structure in which the phalanges—small bones that make up the digits on the hands and feet—developed a 2–3–3–3–3 pattern. This meant that one smaller digit had two phalanges while the other four digits had three.

Overview

The 2–3–3–3–3 pattern was common in most early primates, some of which eventually developed a form of opposable thumb. However, in contrast to human thumbs, primates such as chimpanzees and apes had longer fingers and shorter thumbs. This type of evolutionary arrangement was better suited for swinging on branches and moving about in trees. The hands of the earliest humans had larger opposable thumbs with shorter fingers. This allowed the thumb to become more flexible with a greater range of movement and more precise grip.

Some anthropologists suggest that human ancestors developed an opposable thumb capable of a more precise grip as far back as 3.4 million years ago. One study, published in 2015, examined fossils from the period and speculated that the bone structure of the hands would have been capable of a more forceful grip. However, anthropologists have not uncovered any definitive evidence that these human ancestors used this ability to create tools. As a result, the first known evidence for the modern human thumb dates back about 2.6 million years. It was about that time that early humans began making crude stone tools that were used for hammering, chopping, and scraping. In addition, some scientists believe this thumb-finger alignment was also useful in gripping weapons such as clubs or stone projectiles. The use of tools was the main development that separated the evolutionary line of humans from that of other primates. Tools allowed them to manipulate their environment and gain access to more nutrient-rich sources of food. In turn, these factors led to the development of a larger brain and the eventual rise of modern humans.

Some scientists believe that the human hand alignment—larger thumb and smaller fingers—developed first and primate hands evolved later to acclimate them to a life in the trees. Humans are one of several modern animals that have thumbs, although they are unique in that they can move their thumbs further across their hands than any other animal. Apes, such as chimpanzees, gorillas, and orangutans, and several species of monkeys can also rotate their opposable thumbs, although not to the extent of humans. Spider monkeys and colobus monkeys are among the few primates that do not have thumbs while tarsiers and marmosets are among those with non-opposable thumbs. South America’s waxy monkey tree frog has an opposable thumb that allows it to grasp tree branches, while giant pandas have evolved a bony structure in their wrists that acts like a thumb.

Anatomy of the Thumb

The human hand is composed of twenty-seven small bones. The eight carpal bones comprise the wrist and are connected to the ulna and radius bones of the arm. Five metacarpal bones make up the palm of the hand. The fingers and thumb have fourteen phalanx bones, or phalanges. Each of the four fingers has three phalanges—the distal, middle, and proximal. The distal phalanx bone is at the tip of each finger, the middle phalanx connects the distal and proximal bones, while the proximal phalanx connects to a metacarpal bone. Unlike the fingers, the thumb has only two phalanges: the distal and the proximal. These bones are connected by the interphalangeal joint (IP), which is the only joint in the thumb. The IP joint is found near the top of the thumb, just below the nail.

The thumb’s proximal phalanx is joined to a metacarpal bone at the metacarpophalangeal joint (MP). It is this MP joint that allows the thumb to bend and extend outward and is responsible for the thumb’s grip. The thumb’s metacarpal bone is connected to a carpal bone called the trapezium at the carpometacarpal joint (CMC). The CMC joint is what gives the thumb its wide range of movement and ability to rotate.

In medical terminology, the thumb is referred to as the pollex, from the Latin word pollere, or “to be strong.” As a result, the arteries, tendons, andmuscles of the thumb are referred to with the term pollicis. The princeps pollicis artery is the main blood vessel that carried oxygen-rich blood to the thumb. This artery branches off from the radial artery, a major artery in the forearm. The blood flowing through the princeps pollicis artery gives the thumb its own pulse.

The thumb also has four tendons that attach to muscles in the hand and forearm. The abductor pollicis longus is the tendon that moves the thumb toward and away from the palm of the hand. The flexor pollicis longus is the tendon that bends the thumb. The extensor pollicis brevis runs from the proximal phalanx to the back of the forearm and helps straighten the thumb. The extensor pollicis longus also helps straighten the thumb and runs from the distal phalanx to the forearm.

Is It a Finger?

The anatomical differences between the thumb and the other digits have led to some debate as to whether the thumb can be classified as a fifth finger. Scientific research on the force patterns and function of the thumb suggests that the body’s central nervous system considers the thumb to be a fifth finger. However, in many languages and cultures, the thumb is thought of as a separate, and even special, digit. English dictionaries follow this way of thinking, classifying the thumb as the first digit of the hand, but set it apart from the other four fingers. The modern English word for thumb comes from the Old English þuma (thuma), meaning “stout or thick.” This, in turn, originated from the older root word tum-, or “swell.” In contrast, languages such as Russian and Icelandic view it as the “big finger.” In some Native American dialects, the thumb is known as the “chief finger.” The ancient Greeks had several words for the thumb, one of which was antikheir, or “what is opposite the fingers.”

Bibliography

Chung, Andrew. “Anatomy of the Hand and Wrist: Bones, Muscles, Tendons, Nerves.” HealthPages.org, 23 Sept. 2019, www.healthpages.org/anatomy-function/anatomy-hand-wrist/. Accessed 9 Mar. 2020.

Cooperrider, Kensy. “Where Do Finger Names Come From?” JSTOR Daily, 1 Jan. 2020, daily.jstor.org/where-do-finger-names-come-from/. Accessed 9 Mar. 2020.

Gibbons, Ann. “How Human Ancestors Got a Grip.” Science, 22 Jan. 2015, www.sciencemag.org/news/2015/01/how-human-ancestors-got-grip. Accessed 9 Mar. 2020.

“Joints.” American Society for Surgery of the Hand, 2020, handcare.assh.org/Anatomy/Joints. Accessed 9 Mar. 2020.

Jones, Lynette A., and Susan J. Lederman. Human Hand Function. Oxford UP, 2006.

Nanayakkara, Visakha K., et al. “The Role of Morphology of the Thumb in Anthropomorphic Grasping: A Review.” Frontiers in Mechanical Engineering, 30 June 2017, www.frontiersin.org/articles/10.3389/fmech.2017.00005/full. Accessed 9 Mar. 2020.

Olafsdottir, Halla, Vladimir M. Zatsiorsky, and Mark L. Latash. “Is the Thumb a Fifth Finger? A Study of Digit Interaction During Force Production Tasks.” Experimental Brain Research, vol. 160, no. 2, January 2005, pp. 203–213, www.ncbi.nlm.nih.gov/pmc/articles/PMC2827037/. Accessed 9 Mar. 2020.

Solly, Meilan. “Did the Human Hand Evolve as a Lean Mean Bone-Smashing Machine?” Smithsonian, 13 July 2018, www.smithsonianmag.com/smart-news/human-hand-evolve-bone-smashing-180969632/. Accessed 9 Mar. 2020.

“Thumb.” Healthline, 14 Apr. 2015, www.healthline.com/human-body-maps/thumb#1. Accessed 9 Mar. 2020.