Thumb
Overview
 
The thumb is the first digit
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....

 of the hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

. When a person is standing in the medical anatomical position (where the palm is facing anteriorly), the thumb is the lateral-most digit. The Medical Latin English adjective for thumb is pollical.
The English word "finger" has two senses, even in the context of appendages of a single typical human hand:
  1. Any of the five digits.
  2. Any of the five terminal members of the hand, especially those other than the thumb.


Linguistically, it appears that the original sense was the broader of these two: penkwe-ros (also rendered as penqrós) was, in the inferred Proto-Indo-European language
Proto-Indo-European language
The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

, a suffixed form of penkwe (or penqe), which has given rise to many Indo-European
Indo-European languages
The Indo-European languages are a family of several hundred related languages and dialects, including most major current languages of Europe, the Iranian plateau, and South Asia and also historically predominant in Anatolia...

-family words (tens of them defined in English dictionaries) that involve or flow from concepts of fiveness.

The thumb shares the following with each of the other four fingers:
  • Having a skeleton of phalanges, joined by hinge-like joints that provide flexion toward the palm of the hand
  • Having a "back" surface that features hair and a nail, and a hairless palm-of-the-hand side with fingerprint
    Fingerprint
    A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...

     ridges instead


The thumb contrasts with each of the other four by being the only digit that:
  • Is opposable to the other four fingers
  • Has two phalanges rather than three
  • Has greater breadth in the distal phalanx than in the proximal phalanx
  • Is attached to such a mobile metacarpus (which produces most of the opposability)

and hence the etymology of the word: "tum" is Proto-Indo-European for " swelling " (cf "tumour" and "thigh") since the thumb is the stoutest of the digits.
In humans, opposition and apposition are two movements unique to the thumb but these words are not synonyms:

Primatologists and hand research pioneers J. Napier
John Napier (primatologist)
John Russell Napier, MRCS, LRCP, D.Sc. was a British primatologist, paleoathropologist, and physician, who is notable for his work with Homo habilis and OH 7, as well as on human and primate hands/feet...

 and P. Napier
Prudence Hero Napier
Prudence Hero Napier was one of Britain's most eminent primatologists, and the world's leading expert on the taxonomy of primates. She was the widow of the primatologist, John Napier. -Biography:...

 defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with - or diametrically opposite to - the terminal pads of one or all of the remaining digits." For this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint
Carpometacarpal joint
The carpometacarpal joints are five joints in the wrist that articulates the distal row of carpal bones and the proximal bases of the five metacarpal bones....

).
Discussions
Encyclopedia
The thumb is the first digit
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....

 of the hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

. When a person is standing in the medical anatomical position (where the palm is facing anteriorly), the thumb is the lateral-most digit. The Medical Latin English adjective for thumb is pollical.

As one of five digits, and as companion to four other fingers

The English word "finger" has two senses, even in the context of appendages of a single typical human hand:
  1. Any of the five digits.
  2. Any of the five terminal members of the hand, especially those other than the thumb.


Linguistically, it appears that the original sense was the broader of these two: penkwe-ros (also rendered as penqrós) was, in the inferred Proto-Indo-European language
Proto-Indo-European language
The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

, a suffixed form of penkwe (or penqe), which has given rise to many Indo-European
Indo-European languages
The Indo-European languages are a family of several hundred related languages and dialects, including most major current languages of Europe, the Iranian plateau, and South Asia and also historically predominant in Anatolia...

-family words (tens of them defined in English dictionaries) that involve or flow from concepts of fiveness.

The thumb shares the following with each of the other four fingers:
  • Having a skeleton of phalanges, joined by hinge-like joints that provide flexion toward the palm of the hand
  • Having a "back" surface that features hair and a nail, and a hairless palm-of-the-hand side with fingerprint
    Fingerprint
    A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...

     ridges instead


The thumb contrasts with each of the other four by being the only digit that:
  • Is opposable to the other four fingers
  • Has two phalanges rather than three
  • Has greater breadth in the distal phalanx than in the proximal phalanx
  • Is attached to such a mobile metacarpus (which produces most of the opposability)

and hence the etymology of the word: "tum" is Proto-Indo-European for " swelling " (cf "tumour" and "thigh") since the thumb is the stoutest of the digits.

Opposition and apposition

In humans, opposition and apposition are two movements unique to the thumb but these words are not synonyms:

Primatologists and hand research pioneers J. Napier
John Napier (primatologist)
John Russell Napier, MRCS, LRCP, D.Sc. was a British primatologist, paleoathropologist, and physician, who is notable for his work with Homo habilis and OH 7, as well as on human and primate hands/feet...

 and P. Napier
Prudence Hero Napier
Prudence Hero Napier was one of Britain's most eminent primatologists, and the world's leading expert on the taxonomy of primates. She was the widow of the primatologist, John Napier. -Biography:...

 defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with - or diametrically opposite to - the terminal pads of one or all of the remaining digits." For this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint
Carpometacarpal joint
The carpometacarpal joints are five joints in the wrist that articulates the distal row of carpal bones and the proximal bases of the five metacarpal bones....

). Arguably, this definition was chosen to underline what is unique to the human thumb.

Anatomists and other researchers focused exclusively on human anatomy, on the other hand, tend to elaborate this definition in various ways and, consequently, there are hundreds of definitions. Some anatomists restrict opposition to when the thumb is approximated to the fifth digit (little finger) and refer to other approximations between the thumb and other digits as apposition. To anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (the opponens pollicis
Opponens pollicis muscle
The opponens pollicis is a small, triangular muscle in the hand, which functions to oppose the thumb. It is one of the three thenar muscles, lying deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis.-Structure:...

 and opponens digiti minimi
Opponens digiti minimi muscle
The opponens digiti minimi is a muscle in the hand. It is of a triangular form, and placed immediately beneath the palmaris brevis, abductor minimi digiti, and flexor brevis minimi digiti...

 respectively).

Other researchers use another definition, referring to opposition-apposition as the transition between flexion-abduction and extension-adduction; the side of the distal thumb phalanx thus approximated to the palm or the hand's radial side (side of index finger) during apposition and the pulp or "palmar" side of the distal thumb phalanx approximated to either the palm or other digits during opposition.

Moving a limb back to its neutral position is called reposition and a rotary movement is referred to as circumduction
Circumduction (anatomy)
In anatomy, circumduction means to move a limb in a circular manner.Circumduction is defined as the movement pattern which is a combination of flexion, extension, adduction and abduction. It occurs at synovial joints where a circle can be described by the body part...

.

Skeleton

The skeleton of the thumb consists of the first metacarpal bone
First metacarpal bone
The first metacarpal bone or the metacarpal bone of the thumb is the first bone of the thumb. It is connected to the trapezium of the carpus at the first carpometacarpal joint and to the proximal thumb phalanx at the first metacarpophalangeal joint....

 which articulates proximally with the carpus
Carpus
In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers , whereas those of the metacarpus do. The corresponding part of the foot is the tarsus...

 at the carpometacarpal joint and distally with the proximal phalanx
Proximal phalanges
Proximal phalanges are bones found in the limbs of most vertebrates. In humans, they are the bones at the base of a toe or finger, the prominent, knobby ends of which are often called the knuckles....

 at the metacarpophalangeal joint
Metacarpophalangeal joint
The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint...

. This latter bone articulates with the distal phalanx
Distal phalanges
The distal or terminal phalanges are the terminal limb bones located at the tip of the digits...

 at the interphalangeal joint. Additionally, there are two sesamoid bone
Sesamoid bone
In anatomy, a sesamoid bone is a bone embedded within a tendon.Sesamoids are found in locations where a tendon passes over a joint, such as the hand, knee, and foot. Functionally, they act to protect the tendon and to increase its mechanical effect. The presence of the sesamoid bone holds the...

s at the metacarpophalangeal joint.

Muscles

The muscles of the thumb can be compared to guy-wire
Guy-wire
A guy-wire or guy-rope, also known as simply a guy, is a tensioned cable designed to add stability to structures . One end of the cable is attached to the structure, and the other is anchored to the ground at a distance from the structure's base...

s supporting a flagpole; tension from these musclular guy-wires must be provided in all directions to maintain stability in the articulated column formed by the bones of the thumb. Because this stability is actively maintained by muscles rather than by articular constraints, most muscles attached to the thumb tend to be active during most thumb motions.
The muscles acting on the thumb can be divided into two groups: The extrinsic hand muscles, with their muscle bellies located in the forearm, and the intrinsic hand muscles, with their muscles bellies located in the hand proper.

Extrinsic

A ventral forearm muscle, the flexor pollicis longus
Flexor pollicis longus muscle
The flexor pollicis longus is a muscle in the forearm and hand that flexes the thumb...

 (FPL) originates on the anterior side of the radius
Radius (bone)
The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...

 distal to the radial tuberosity
Radial tuberosity
Beneath the neck of the radius, on the medial side, is an eminence, the radial tuberosity; its surface is divided into:* a posterior, rough portion, for the insertion of the tendon of the biceps brachii....

 and from the interosseous membrane. It passes through the carpal tunnel
Carpal tunnel
In the human body, the carpal tunnel or carpal canal is the passageway on the palmar side of the wrist that connects the forearm to the middle compartment of the deep plane of the palm. The tunnel consists of bones and connective tissue...

 in a separate tendon sheath
Tendon sheath
A tendon sheath is a layer of membrane around a tendon. It permits the tendon to move.It has two layers:* synovial sheath* fibrous tendon sheathFibroma of the tendon sheath has been described....

, after which it lies between the heads of the flexor pollicis brevis. It finally attaches onto the base of the distal phalanx of the thumb. It is innervated by the anterior interosseus branch
Anterior interosseous nerve
The anterior interosseous nerve is a branch of the median nerve that supplies the deep muscles on the front of the forearm, except the ulnar half of the flexor digitorum profundus....

 of the median nerve
Median nerve
The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

 (C7-C8)
Three dorsal forearm muscles act on the thumb:

The abductor pollicis longus
Abductor pollicis longus muscle
The abductor pollicis longus muscle is one of the extrinsic muscles of the hand. It lies immediately below the supinator muscle and is sometimes united with it.-Origin and insertion:...

 (APL) originates on the dorsal sides of both the ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...

 and the radius, and from the interosseous membrane. Passing through the first tendon compartment, it inserts to the base of the first metacarpal bone. A part of the tendon reaches the trapezium, while another fuses with the tendons of the extensor pollicis brevis and the abductor pollicis brevis. Except for abducting the hand, it flexes the hand towards the palm and abducts it radially. It is innervated by the deep branch of the radial nerve
Deep branch of the radial nerve
The deep branch of the radial nerve winds to the back of the forearm around the lateral side of the radius between the two planes of fibers of the Supinator, and is prolonged downward between the superficial and deep layers of muscles, to the middle of the forearm.Considerably diminished in size,...

 (C7-C8).
The extensor pollicis longus
Extensor pollicis longus muscle
In human anatomy, the extensor pollicis longus is a skeletal muscle located dorsally on the forearm. It is much larger than the extensor pollicis brevis, the origin of which it partly covers, and acts to stretch the thumb together with this muscle....

 (EPL) originates on the dorsal side of the ulna and the interosseous membrane. Passing through the third tendon compartment, it is inserted onto the base of the distal phalanx of the thumb. It uses the dorsal tubercle on the lower extremity of the radius as a fulcrum
Lever
In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...

 to extend the thumb and also dorsiflexes and abducts the hand at the wrist. It is innervated by the deep branch of the radial nerve
Deep branch of the radial nerve
The deep branch of the radial nerve winds to the back of the forearm around the lateral side of the radius between the two planes of fibers of the Supinator, and is prolonged downward between the superficial and deep layers of muscles, to the middle of the forearm.Considerably diminished in size,...

 (C7-C8).
The extensor pollicis brevis
Extensor pollicis brevis muscle
In human anatomy, the extensor pollicis brevis is a skeletal muscle on the dorsal side of the forearm. It lies on the medial side of, and is closely connected with, the abductor pollicis longus.-Origin and insertion:...

 (EPB) originates on the ulna distal to the abductor pollicis longus, from the interosseus membrane, and from the dorsal side of the radius. Passing through the first tendon compartment together with the abductor pollicis longus, it is attached to the base of the proximal phalanx of the thumb. It extends the thumb and, because of its close relationship to the long abductor, also abducts the thumb. It is innervated by the deep branch of the radial nerve
Deep branch of the radial nerve
The deep branch of the radial nerve winds to the back of the forearm around the lateral side of the radius between the two planes of fibers of the Supinator, and is prolonged downward between the superficial and deep layers of muscles, to the middle of the forearm.Considerably diminished in size,...

 (C7-T1).
The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the anatomical snuff box
Anatomical snuff box
The anatomical snuffbox is a triangular deepening on the radial, dorsal aspect of the hand—at the level of the carpal bones, specifically, the scaphoid and trapezium bones forming the floor...

 (an indentation on the lateral aspect of the thumb at its base) The radial artery can be palpated anteriorly at the wrist(not in the snuffbox).

Intrinsic

There are four thenar muscles
Thenar eminence
The thenar eminence refers to the group of muscles on the palm of the human hand at the base of the thumb...

:

The abductor pollicis brevis
Abductor pollicis brevis muscle
The abductor pollicis brevis is a muscle in the hand that functions as an abductor of the thumb.-Structure:The abductor pollicis brevis is a flat, thin muscle located just under the skin. It is a thenar muscle, and therefore contributes to the bulk of the palm's thenar eminence...

 (APB) originates on the scaphoid tubercle
Scaphoid bone
The scaphoid bone is one of the carpal bones of the wrist. It is situated between the hand and forearm on the thumb-side of the wrist . The scaphoid bone is the largest bone of the proximal row of wrist bones, its long axis being from above downward, lateralward, and forward...

 and the flexor retinaculum
Flexor retinaculum of the hand
The flexor retinaculum is a strong, fibrous band that arches over the carpus, converting the deep groove on the front of the carpal bones into a tunnel, the carpal tunnel, through which the Flexor tendons of the digits and the median nerve pass.It is attached, medially, to the pisiform and the...

. It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by the median nerve
Median nerve
The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

 (C8-T1).
The flexor pollicis brevis
Flexor pollicis brevis muscle
The flexor pollicis brevis is a muscle in the hand that flexes the thumb. It is one of three thenar muscles. It has both a superficial part and a deep part.-Origin and insertion:...

 (FPB) has two heads. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium
Trapezium (bone)
The trapezium bone is a carpal bone in the wrist.The trapezium is distinguished by a deep groove on its palmar surface. It is situated at the radial side of the carpus, between the scaphoid and the first metacarpal bone...

, trapezoid
Trapezoid bone
The trapezoid bone is a carpal bone in tetrapods, including humans. It is the smallest bone in the distal row. It may be known by its wedge-shaped form, the broad end of the wedge constituting the dorsal, the narrow end the palmar surface; and by its having four articular facets touching each...

, and capitate. The muscle is inserted onto the radial sesamoid bone of the metacarpophalangeal joint. It acts to flex, adduct, and abduct the thumb, and is therefore also able to oppose the thumb. The superficial head is innervated by the median nerve
Median nerve
The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

, while the deep head is innervated by the ulnar nerve
Ulnar nerve
In human anatomy, the ulnar nerve is a nerve which runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest unprotected nerve in the human body , so injury is common...

 (C8-T1).
The adductor pollicis
Adductor pollicis muscle
In human anatomy, the adductor pollicis muscle is a muscle in the hand that functions to adduct the thumb. It has two heads: transverse and oblique....

 also has two heads. The transversal head originates along the entire third metacarpal bone, while the oblique head originates on the carpal bones proximal to the third metacarpal. The muscle is inserted onto the ulnar sesamoid bone of the metacarpophalangeal joint. It adducts the thumb, and assists in opposition and flexion. It is innervated by the deep branch of the ulnar nerve
Ulnar nerve
In human anatomy, the ulnar nerve is a nerve which runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest unprotected nerve in the human body , so injury is common...

 (C8-T1).
The opponens pollicis
Opponens pollicis muscle
The opponens pollicis is a small, triangular muscle in the hand, which functions to oppose the thumb. It is one of the three thenar muscles, lying deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis.-Structure:...

 originates on the tubercle of the trapezium and the flexor retinaculum. It is inserted onto the radial side of the first metacarpal. It opposes the thumb and assists in adduction. It is innervated by the median nerve
Median nerve
The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

.
The first dorsal interosseous
Dorsal interossei of the hand
The dorsal interossei of the hand are muscles that occupy the space between the metacarpals.-Structure:There are four dorsal interossei in each hand...

, one of the central muscles of the hand, extends from the base of the thumb metacarpal to the radial side of the proximal phalanx of the index finger.

Variation

"Hitchhiker's thumb" is an autosomal recessive trait more formally known as "distal hyperextensibility of the thumb". Homozygous carriers can extend the top of the thumb backwards nearly 90° when the thumb is extended in a "thumbs-up
Thumbs Up
A thumbs-up or thumbs-down is a common hand gesture achieved by a closed fist held with the thumb extended upward or downward in approval or disapproval, respectively...

". Cases of Hitchhiker's thumb are subject to a variation in range of motion. Some who have this condition are able to hyperextend thumbs backwards 90° while still able to bend thumbs forward partially or with an added normal forward range of motion. Those with a more traditional case of Hitchhiker's thumbs can hyperextend backwards 90° while unable to bend past a straight position of the thumb.
Malformations includes a triphalangeal thumb
Triphalangeal thumb
A triphalangeal thumb is a congenital malformation where the thumb has three phalanges instead of two. Besides the three phalanges, there can also be other malformations. The extra phalanx can vary from a very small phalanx to a full extra phalanx in a more finger-like thumb. It was first...

 and polydactyly
Polydactyly
Polydactyly or polydactylism , also known as hyperdactyly, is a congenital physical anomaly in humans, dogs, and cats having supernumerary fingers or toes....

.

Grips

One of the earlier significant contributors to the study of hand grips was orthopedic primatologist and paleoathropologist John Napier
John Napier (primatologist)
John Russell Napier, MRCS, LRCP, D.Sc. was a British primatologist, paleoathropologist, and physician, who is notable for his work with Homo habilis and OH 7, as well as on human and primate hands/feet...

 who proposed organizing the movements of the hand by their anatomical basis as opposed to work done earlier that had only used arbitrary classification. Most of this early work on hand grips had a pragmatic basis as it was intended to narrowly define compensable injuries to the hand, which required an understanding of the anatomical basis of hand movement. Napier proposed two primary prehensile grips: the precision grip and the power grip. The precision and power grip are defined by the position of the thumb and fingers where:
  • The power grip is when the fingers (and sometimes palm) clamp down on an object with the thumb makes counter pressure. examples of the power grip are gripping a hammer, opening a jar using both your palm and fingers, and during pullups.
  • The precision grip is when the intermediate and distal phalanges ("fingertips") and the thumb press against each other. Examples of a precision grip are writing with a pencil, opening a jar with the fingertips alone, and gripping a ball (as long as it's not tight against your palm).


Opposability of the thumb should not be confused with a precision grip as some animals possess semi-opposable thumbs yet are known to have extensive precision grips (Tufted Capuchins for example). Nevertheless, precision grips are usually only found in higher apes, and only in degrees significantly more restricted than in humans.

Evolution

The opposable thumb has helped the human species develop more accurate fine motor skill
Fine motor skill
Fine motor skills are the coordination of small muscle movements which occur e.g., in the fingers, usually in coordination with the eyes. In application to motor skills of hands the term dexterity is commonly used....

s. It is also thought to have directly led to the development of tools, not just in humans or their evolutionary ancestors, but other primates as well. The opposable thumb ensured that important human functions such as writing were possible. The thumb, in conjunction with the other fingers, makes human hands and those of other species with similar hands some of the most dexterous in the world.

A primitive autonomization of the first carpometacarpal joint
Carpometacarpal joint
The carpometacarpal joints are five joints in the wrist that articulates the distal row of carpal bones and the proximal bases of the five metacarpal bones....

 (CMC) may have occurred in dinosaurs. A real differentiation appeared perhaps 70 mya in early primates, while the shape of the human thumb CMC finally appears about 5 mya. The result of this evolutionary process is a human CMC joint positioned at 80° of pronation, 40 of abduction, and 50° of flexion in relation to an axis passing through the second and third CMC joints.
Opposable thumbs are shared by many primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s, including most simian
Simian
The simians are the "higher primates" familiar to most people: the Old World monkeys and apes, including humans, , and the New World monkeys or platyrrhines. Simians tend to be larger than the "lower primates" or prosimians.- Classification and evolution :The simians are split into three groups...

s, and some prosimian
Prosimian
Prosimians are a grouping of mammals defined as being primates, but not monkeys or apes. They include, among others, lemurs, bushbabies, and tarsiers. They are considered to have characteristics that are more primitive than those of monkeys and apes. Prosimians are the only primates native to...

s. The climbing and suspensory behaviour in orthograde
Orthograde posture
Orthograde is a term derived from Latin [ortho + gradi] that describes a manner of walking which is upright, with the independent motion of limbs....

 apes, such as chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...

s, has resulted in elongated hands while the thumb has remained short. As a result, these primates are unable to perform the pad-to-pad grip associated with opposability. However, in pronograde monkeys such as baboon
Baboon
Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae. There are five species, which are some of the largest non-hominoid members of the primate order; only the mandrill and the drill are larger...

s, an adaptation to a terrestrial lifestyle has led to reduced digit length and thus hand proportions similar to those of humans. Consequently, these primates have dexterous hands and are able to grasp objects using a pad-to-pad grip. It can thus be difficult to identify hand adaptations to manipulation-related tasks based solely on thumb proportions.
The evolution of the fully opposable thumb is usually associated with Homo habilis
Homo habilis
Homo habilis is a species of the genus Homo, which lived from approximately at the beginning of the Pleistocene period. The discovery and description of this species is credited to both Mary and Louis Leakey, who found fossils in Tanzania, East Africa, between 1962 and 1964. Homo habilis Homo...

, the forerunner of Homo sapiens. This, however, is the suggested result of evolution from Homo erectus
Homo erectus
Homo erectus is an extinct species of hominid that lived from the end of the Pliocene epoch to the later Pleistocene, about . The species originated in Africa and spread as far as India, China and Java. There is still disagreement on the subject of the classification, ancestry, and progeny of H...

(around 1 mya) via a series of intermediate anthropoid
Simian
The simians are the "higher primates" familiar to most people: the Old World monkeys and apes, including humans, , and the New World monkeys or platyrrhines. Simians tend to be larger than the "lower primates" or prosimians.- Classification and evolution :The simians are split into three groups...

 stages, and is therefore a much more complicated link.

It is possible, though, that a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of Homo habilis
Homo habilis
Homo habilis is a species of the genus Homo, which lived from approximately at the beginning of the Pleistocene period. The discovery and description of this species is credited to both Mary and Louis Leakey, who found fossils in Tanzania, East Africa, between 1962 and 1964. Homo habilis Homo...

preceded walking, with the specialized adaptation of the spine, pelvis, and lower extremities preceding a more advanced hand. And, it is logical that a conservative, highly functional adaptation be followed by a series of more complex ones that complement it. With Homo habilis
Homo habilis
Homo habilis is a species of the genus Homo, which lived from approximately at the beginning of the Pleistocene period. The discovery and description of this species is credited to both Mary and Louis Leakey, who found fossils in Tanzania, East Africa, between 1962 and 1964. Homo habilis Homo...

, an advanced grasping-capable hand was accompanied by facultative bipedalism, possibly implying, assuming a co-opted evolutionary relationship exists, that the latter resulted from the former as obligate bipedalism was yet to follow. Walking may have been a by-product of busy hands and not vice versa.

HACNS1 (also known as Human Accelerated Region
Human accelerated regions
Human accelerated regions , first described in August 2006, are a set of 49 segments of the human genome which are conserved throughout vertebrate evolution but are strikingly different in humans. They are named HAR1 through HAR49 according to their degree of difference between humans and chimpanzees...

 2) is a gene enhancer
Enhancer (genetics)
In genetics, an enhancer is a short region of DNA that can be bound with proteins to enhance transcription levels of genes in a gene cluster...

 "that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in the ankle
Ankle
The ankle joint is formed where the foot and the leg meet. The ankle, or talocrural joint, is a synovial hinge joint that connects the distal ends of the tibia and fibula in the lower limb with the proximal end of the talus bone in the foot...

 or foot
Foot
The foot is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made up of one or more segments or bones, generally including claws...

 that allow humans to walk
WALK
WALK may refer to:*WALK , a radio station licensed to East Patchogue, New York, United States*WALK-FM, a radio station licensed to Patchogue, New York, United States...

 on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

, HACNS1 has undergone the most change during the human evolution
Human evolution
Human evolution refers to the evolutionary history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids and mammals...

 since the chimpanzee-human last common ancestor
Chimpanzee-human last common ancestor
The chimpanzee-human last common ancestor is the last species, a species of African apes, that humans, bonobos and chimpanzees share as a common ancestor....

.

Other animals with opposable digits

Many animals also have some kind of opposable thumb or toe. An animal species is said to have opposable thumbs if the thumb is capable of bending in such a way that it can touch all the other digits on the hand. Most species do not have opposable thumbs. Opposable thumbs are a signature feature of the primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

 family, and played a large role in the ancient humans' invention and use of tool
Tool
A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

s.

Primates
  • Primate
    Primate
    A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

    s fall into one of four groups:
    • Nonopposable thumbs: tarsier
      Tarsier
      Tarsiers are haplorrhine primates of the genus Tarsius, a genus in the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes...

      s and marmoset
      Marmoset
      Marmosets are the 22 New World monkey species of the genera Callithrix, Cebuella, Callibella, and Mico. All four genera are part of the biological family Callitrichidae. The term marmoset is also used in reference to the Goeldi's Monkey, Callimico goeldii, which is closely related.Most marmosets...

      s
    • Pseudo-opposable thumbs: all strepsirrhines and Cebidae
      Cebidae
      The Cebidae is one of the five families of New World monkeys now recognised. It includes the capuchin monkeys and squirrel monkeys. These species are found throughout tropical and subtropical South and Central America.-Characteristics:...

    • Opposable thumbs: Old World monkey
      Old World monkey
      The Old World monkeys or Cercopithecidae are a group of primates, falling in the superfamily Cercopithecoidea in the clade Catarrhini. The Old World monkeys are native to Africa and Asia today, inhabiting a range of environments from tropical rain forest to savanna, shrubland and mountainous...

      s and all great ape
      Hominidae
      The Hominidae or include them .), as the term is used here, form a taxonomic family, including four extant genera: chimpanzees , gorillas , humans , and orangutans ....

      s
    • Opposable with comparatively long thumbs: gibbon
      Gibbon
      Gibbons are apes in the family Hylobatidae . The family is divided into four genera based on their diploid chromosome number: Hylobates , Hoolock , Nomascus , and Symphalangus . The extinct Bunopithecus sericus is a gibbon or gibbon-like ape which, until recently, was thought to be closely related...

      s (or lesser apes)


Darwinius masillae
Darwinius
Darwinius is a genus of Adapiformes, a group of basal primates from the Eocene epoch. Its only known species is Darwinius masillae, dated to 47 million years ago based on dating of the fossil site....

, an Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 primate fossil often described as a missing link between prosimian
Prosimian
Prosimians are a grouping of mammals defined as being primates, but not monkeys or apes. They include, among others, lemurs, bushbabies, and tarsiers. They are considered to have characteristics that are more primitive than those of monkeys and apes. Prosimians are the only primates native to...

 and simian
Simian
The simians are the "higher primates" familiar to most people: the Old World monkeys and apes, including humans, , and the New World monkeys or platyrrhines. Simians tend to be larger than the "lower primates" or prosimians.- Classification and evolution :The simians are split into three groups...

, had hands and feet with highly flexible digits featuring opposable thumbs and halluces.
Other placental mammals
  • Giant Panda
    Giant Panda
    The giant panda, or panda is a bear native to central-western and south western China. It is easily recognized by its large, distinctive black patches around the eyes, over the ears, and across its round body. Though it belongs to the order Carnivora, the panda's diet is 99% bamboo...

    s — five clawed fingers plus an extra-long sesamoid bone beside the true first digit that, though not a true digit, works like an opposable thumb.
  • Lophiomys — a genus of African cricetid (rodent), with only one extant species (Lophiomys imhausi), but several extinct species, with opposable halluces
    Hallux
    In tetrapods, the hallux is the innermost toe of the foot. Despite its name it may not be the longest toe on the foot of some individuals...

    .

Additionally, in many polydactyl cats, both the innermost and outermost ("pinky
Little finger
The little finger, often called the pinky in American English, pinkie in Scottish English , or small finger in medicine, is the most ulnar and usually smallest finger of the human hand, opposite the thumb, next to the ring finger.-Muscles:There are four muscles that...

") toes may become opposable, allowing the cat to perform more complex tasks.

Marsupials
  • In most phalangerid marsupial
    Marsupial
    Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

    s (a family of possum
    Possum
    A possum is any of about 70 small to medium-sized arboreal marsupial species native to Australia, New Guinea, and Sulawesi .Possums are quadrupedal diprotodont marsupials with long tails...

    s) except species Trichosurus and Wyulda the first and second digits of the forefoot are opposable to the other three. In the hind foot, the first toe is clawless but opposable and provides firm grip on branches. The second and third toes are partly syndactylous
    Dactyly
    In biology, dactyly is the arrangement of digits on the hands, feet, or sometimes wings of a tetrapod animal. It comes from the Greek word δακτυλος = "finger".Sometimes the ending "-dactylia" is used...

    , united by skin at the top joint while the two separate nails serve as hair combs. The fourth and fifth digits are the largest of the hind foot.
  • Similar to phalangerids though in a different order, koala
    Koala
    The koala is an arboreal herbivorous marsupial native to Australia, and the only extant representative of the family Phascolarctidae....

    s have five digits on their fore and hind feet with sharp curved claws except for the first digit of the hind foot. The first and second digits of the forefeet are opposable to the other three, which enables the koala to grip smaller branches and search for fresh leaves in the outer canopy. Similar to the phalangerids, the second and third digits of the hind foot are fused but have separate claws.
  • Opossums are New World marsupials with opposable thumbs in the hind feet giving these animals their characteristic grasping capability (with the exception of the Water Opossum
    Water Opossum
    The water opossum , also locally known as the yapok, is a marsupial of the family Didelphidae. It is the only member of its genus, Chironectes...

    , the webbed feet of which restrict opposability).
  • The mouse-like microbiotheres
    Microbiotheria
    The Monito del Monte is the only extant member of its family and the only surviving member of an ancient order, the Microbiotheria. The oldest microbiothere currently recognised is Khasia cordillerensis, based on fossil teeth from Early Palaeocene deposits at Tiupampa, Bolivia...

     were a group of South American marsupials most closely related to Australian marsupials. The only extant member, Dromiciops gliroides
    Monito del Monte
    The Monito del Monte The Monito del Monte The Monito del Monte (Spanish for "little mountain monkey", Dromiciops gliroides, is a diminutive marsupial native only to southwestern South America (Chile and Argentina). It is the only extant species in the ancient order Microbiotheria, and the sole New...

    , is not closely related to opossums but has paws similar to these animals, each having opposable toes adapted for gripping.


Reptiles
  • Troodon
    Troodon
    Troodon is a genus of relatively small, bird-like dinosaur from the Late Cretaceous period . Discovered in 1855, it was among the first dinosaurs found in North America...

     — a birdlike dinosaur — partially opposable thumbs.
  • Bambiraptor
    Bambiraptor
    Bambiraptor is a Late Cretaceous, 75 million year old, bird-like dromaeosaurid theropod dinosaur described by scientists at the University of Kansas, Yale University, and the University of New Orleans....

     — a small, predatory dinosaur — it could touch the outer two of its three digits together in an opposable grip.
  • Nqwebasaurus
    Nqwebasaurus
    Nqwebasaurus is the name given to a genus of dinosaur from the Tithonian to Valanginian . It was a basal coelurosaur related to Ornitholestes. Its fossils were found in South Africa in the Kirkwood Formation, which is called Nqweba in the native language of the Xhosa...

     — a coelurosaur with a long, three-fingered hand which included a partially opposable thumb (a "killer claw").


Most bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s have at least one opposable digit on the foot, in various configurations, but these are seldom called "thumbs".

Amphibians
  • Phyllomedusa
    Phyllomedusa
    Phyllomedusa is a genus of tree frog from Central and South America. It ranges from Costa Rica southward to Argentina. It has around thirty species.-Secretion:...

    , a genus of frogs native to South America.

External links

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