thumb

{{more sources|section|date=April 2024}}

The English word finger has two senses, even in the context of appendages of a single typical human hand:

1) Any of the five terminal members of the hand.

2) Any of the four terminal members of the hand, other than the thumb.{{Cite web |date=2024-09-26 |title=Definition of FINGER |url=https://www.merriam-webster.com/dictionary/finger |access-date=2024-09-30 |website=www.merriam-webster.com |language=en}}

Linguistically, it appears that the original sense was the first of these two: {{Lang|mis|*penkwe-ros}} (also rendered as {{Lang|mis|*penqrós}}) was, in the inferred Proto-Indo-European language, a suffixed form of {{Lang|mis|*penkwe}} (or {{Lang|mis|*penqe}}), which has given rise to many Indo-European-family words (tens of them defined in English dictionaries) that involve, or stem 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 dorsal surface that features hair and a nail, and a hairless palmar aspect with fingerprint ridges

The thumb contrasts with each of the other four fingers by being the only one that:

• Is opposable to the other four fingers
• Has two phalanges rather than three. However, recently there have been reports that the thumb, like other fingers, has three phalanges, but lacks a metacarpal bone.Haeri, Seyed Mohammad Jafar, et al. "Human thumb consists of three phalanges and lacks metacarpal? A morphometric study on the long bones of the hand." Surgical and Radiologic Anatomy 44.8 (2022): 1101-1109.https://doi.org/10.1007/s00276-022-02986-9
• 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)
• Curls horizontally instead of vertically

and hence the etymology of the word: {{Lang|mis|*tum}} is Proto-Indo-European for 'swelling' (cf 'tumor' and 'thigh') since the thumb is the stoutest of the fingers.

==Opposition and apposition{{anchor|Other animals with opposable digits}}==

Anatomists and other researchers focused on human anatomy have hundreds of definitions of opposition.{{harvnb|van Nierop|van der Helm|Overbeeke|Djajadiningrat|2008|p=34}} Some anatomists{{harvnb|Brown|Freeman|Cuccurullo|Freeman|2004}} restrict opposition to when the thumb is approximated to the fifth finger (little finger) and refer to other approximations between the thumb and other fingers as apposition. To anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (the opponens pollicis and opponens digiti minimi 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 fingers during opposition.

Moving a limb back to its neutral position is called reposition and a rotary movement is referred to as circumduction.

Primatologists and hand research pioneers John and Prudence Napier defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with {{ndash}} or diametrically opposite to {{ndash}} the terminal pads of one or all of the remaining fingers." For this true, pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at the carpometacarpal joint).{{cite web

|title = Primates FAQ: Do any primates have opposable thumbs?

|publisher = Wisconsin Regional Primate Research Center

|url = http://www.primates.com/faq/

|access-date = 20 November 2010

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

File:BonoboFishing02 cropped.jpeg "fishing" for termites, an example of incomplete/"untrue" opposition{{cite news

|title = The Thumb is the Hero

|newspaper = The New York Times

|quote = The "fishing rod" a chimp strips of leaves and pokes into a termite nest to bring up a snack is as far as he'll ever get toward orbiting the planets.

|date = January 11, 1981 |access-date = 20 November 2010

|url = https://query.nytimes.com/gst/fullpage.html?res=9F04E3D8173BF932A25752C0A967948260

}}{{Better source needed|reason=Citation is from the New York Times which is not fully a scientific source. Additionally, nowhere in the article, does it explain why scientists call this an example of "incomplete" or "untrue" opposition|date=August 2024}}]]

• Primates fall into one of six groups:{{harvnb|Ankel-Simons|2007|p=345}}
• Thumbless: spider monkey and colobus
• Nonopposable thumbs: tarsiers (which are found in the islands of Southeast Asia), marmosets (which are New World monkeys)
• Pseudo-opposable thumbs: all strepsirrhines (lemurs, pottos and lorises) and Cebidae (capuchin and squirrel monkeys, which are New World monkeys)
• Opposable thumbs: Old World monkeys (Circopithecidae) except colobus, and all great apes
• Opposable with comparatively long thumbs: gibbons (or lesser apes)
• Yet to be classified: other New World monkeys (tamarins, Aotidae: night or owl monkeys, Pitheciidae: titis, sakis and uakaris, Atelidae: howler and woolly monkeys)

The spider monkey compensates for being virtually thumbless by using the hairless part of its long, prehensile tail for grabbing objects. In apes and Old World monkeys, the thumb can be rotated around its axis, but the extensive area of contact between the pulps of the thumb and index finger is a human characteristic.

Darwinius masillae, an Eocene primate transitional fossil between prosimian and simian, had hands and feet with highly flexible digits featuring opposable thumbs and halluces.{{harvnb|Franzen|Gingerich|Habersetzer|Hurum|2009|pp=15–18}}

• Giant pandas — five clawed fingers plus an extra-long sesamoid bone beside the true first finger that, though not a true finger, works like an opposable thumb.{{cite web

|title = The Panda's Thumb

|publisher = Athro

|url = http://www.athro.com/evo/pthumb.html

|year = 2000 |access-date = 21 November 2010

}}

• Most rodents have a partly opposable toe on each front paw, letting them grasp.{{cite book | title=The Rodent Order | last=Stefoff | first= Rebecca | year=2008 | publisher=Marshall Cavendish | isbn=978-0-7614-3073-5 | pages=62–63, 71–73 | url=https://books.google.com/books?id=7Gle7-L46TYC&pg=PA63 }}
• In some mice, the hallux ("big toe") is clawless and fully opposable, including arboreal species such as Hapalomys, Chiropodomys, Vandeleuria, and Chiromyscus; and saltatorial, bipedal species such as Notomys and possibly some Gerbillinae.{{Harvnb|Ellerman|1941|p=2}}
• The East African maned rat (Lophiomys imhausi), an arboreal, porcupine-like rodent, has four fingers on its hands and feet and a partially opposable thumb.{{cite book

|last = Grzimek |first = Bernhard

|title = Grzimek's Animal Life Encyclopedia, Vol 16, Mammals V

|edition = 2nd |page = 293

|editor1-last = Hutchins |editor1-first = Michael

|editor2-last = Kleiman |editor2-first = Devra G.

|editor3-last = Geist |editor3-first = Valerius

|display-editors = 3 |editor4-last = McDade |editor4-first = Melissa C.

|publisher = Gale Group |location = Farmington Hills, MI |year = 2003

|isbn = 978-0-7876-7750-3

|url = https://archive.org/details/grzimeksanimalli0002grzi_c1l6/page/293/mode/2up

}}

Additionally, in many polydactyl cats, both the innermost toe and outermost toe (pinky) may become opposable, allowing the cat to perform more complex tasks.{{Citation needed|date=August 2012}}

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| footer = Left: Opposable toes of the Sulawesi bear cuscus forelimb
Right: Opposable thumb on rear foot of an opossum

}}

• In most phalangerid marsupials (a family of possums) except species Trichosurus and Wyulda, the first and second toes 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, united by skin at the top joint while the two separate nails serve as hair combs. The fourth and fifth toes are the largest of the hind foot.{{harvnb|Nowak|1999|p=89}}
• Koalas have five toes on their fore and hind feet with sharp curved claws except for the first toe of the hind foot. The first and second toes 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 toes of the hind foot are fused but have separate claws.{{harvnb|McDade|2003|loc=vol 13, p. 44}}
• 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, the webbed feet of which restrict opposability).{{harvnb|McDade|2003|loc=vol 12, p. 250}}
• The mouse-like microbiotheres were a group of South American marsupials most closely related to Australian marsupials. The only extant member, Dromiciops gliroides, is not closely related to opossums but has paws similar to these animals, each having opposable toes adapted for gripping.{{harvnb|McDade|2003|loc=vol 12, p. 274}}

• The front feet of chameleons are organized into a medial bundle of toes 1, 2 and 3, and a lateral bundle of toes 4 and 5, and the hind feet are organized into a medial bundle of toes 1 and 2, and a lateral bundle of toes 3, 4 and 5.{{cite book | author= Anderson, Christopher V. | author2= Higham, Timothy E. | name-list-style= amp | chapter= Chameleon anatomy | editor1-last= Tolley | editor1-first= Krystal A. | editor2-last= Herrel | editor2-first= Anthony | title=The Biology of Chameleons | publisher=University of California Press | year=2014 | location=Berkeley | pages= 7–55 | isbn= 9780520276055 }}

• Dinosaurs belonging to the family of bird-like dinosaur Troodontidae had a partially opposable finger. It is possible that this adaptation was used to better manipulate ground objects or moving undergrowth branches when searching for prey.{{Cite journal |last1=Russell |first1=D. A. |last2=Séguin |first2=R. |year=1982 |title=Reconstruction of the small Cretaceous theropod Stenonychosaurus inequalis and a hypothetical dinosauroid |url=https://archive.org/details/syllogeus37nati |journal=Syllogeus |volume=37 |pages=1–43}}
• The small predatory dinosaur Bambiraptor may have had mutually opposable first and third fingers and a forelimb manoeuvrability that would allow the hand to reach its mouth. Its forelimb morphology and range of motion enabled two-handed prehension, one-handed clutching of objects to the chest, and use of the hand as a hook.{{Harvnb|Senter|2006}}
• Nqwebasaurus — a coelurosaur with a long, three-fingered hand which included a partially opposable thumb (a "killer claw").{{harvnb|de Klerk|Forster|Sampson|Chinsamy|2000|p=327}}. The left manus shows that the flexed digit I had the potential to partially oppose digits II and III.

In addition to these, some other dinosaurs may have had partially or completely opposed toes in order to manipulate food and/or grasp prey.

Image:Bird-feets-en.svg

{{See also|Dactyly#Anisodactyly}}

• Most birds have at least one opposable toe on the foot, in various configurations, though these are seldom called "thumbs". They are more often known simply as halluxes.

• The wukongopterid pterosaur Kunpengopterus bore an opposable first toe on each wing. The presence of opposable thumbs in this taxon is thought to be an arboreal adaptation.{{Cite journal|last1=Zhou |first1=X. |last2=Pêgas |first2=R. V. |last3=Ma |first3=W. |last4=Han |first4=G. |last5=Jin |first5=X. |last6=Leal |first6=M. E. C. |last7=Bonde |first7=N. |last8=Kobayashi |first8=Y. |last9=Lautenschlager |first9=S. |last10=Wei |first10=X. |last11=Shen |first11=C. |last12=Ji |first12=S. |title=A new darwinopteran pterosaur reveals arborealism and an opposed thumb |year=2021 |journal=Current Biology |volume=31 |issue=11 |pages=2429–2436.e7 |doi=10.1016/j.cub.2021.03.030 |pmid=33848460 |doi-access=free |bibcode=2021CBio...31E2429Z }}

• Phyllomedusa, a genus of frogs native to South America.{{cite journal|last=Bertoluci|first=Jaime|title= Pedal luring in the leaf-frog Phyllomedusa burmeisteri (Anura, Hylidae, Phyllomedusinae)|journal= Phyllomedusa: Journal of Herpetology|date=18 December 2002|volume=1|issue=2|doi=10.11606/issn.2316-9079.v1i2p93-95|page=93|doi-access=free}}

The skeleton of the thumb consists of the first metacarpal bone which articulates proximally with the carpus at the carpometacarpal joint and distally with the proximal phalanx at the metacarpophalangeal joint. This latter bone articulates with the distal phalanx at the interphalangeal joint. Additionally, there are two sesamoid bones at the metacarpophalangeal joint.

{{main|Muscles of the thumb}}

The muscles of the thumb can be compared to guy-wires supporting a flagpole; tension from these muscular 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.{{harvnb|Austin|2005|p=339}}

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 muscle bellies located in the hand proper.{{cite web

|title = Muscles of the thumb

|publisher = Eaton hand

|url = http://www.eatonhand.com/mus/mus131.htm

|access-date = 11 May 2010

}}

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| footer = Flexor pollicis longus (left) and deep muscles of dorsal forearm (right)

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A ventral forearm muscle, the flexor pollicis longus (FPL) originates on the anterior side of the radius distal to the radial tuberosity and from the interosseous membrane. It passes through the carpal tunnel in a separate tendon sheath, 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 of the median nerve (C7-C8){{harvnb|Platzer|2004|p=162}} It is a persistence of one of the former contrahentes muscles that pulled the fingers or toes together.

Three dorsal forearm muscles act on the thumb:

The abductor pollicis longus (APL) originates on the dorsal sides of both the ulna 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 (C7-C8).

The extensor pollicis longus (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 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 (C7-C8).{{harvnb|Platzer|2004|p=168}}

The extensor pollicis brevis (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 (C7-T1).

The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the anatomical snuff box (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).

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| footer = Thenar (left) and dorsal interossei (right) muscles

}}

There are three thenar muscles:

The abductor pollicis brevis (APB) originates on the scaphoid tubercle and the flexor retinaculum. It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by the median nerve (C8-T1).{{harvnb|Platzer|2004|p=176}}

The flexor pollicis brevis (FPB) has two heads. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium, trapezoid, 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, while the deep head is innervated by the ulnar nerve (C8-T1).

The opponens pollicis 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.

Other muscles involved are:

The adductor pollicis 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 (C8-T1).

The first dorsal interosseous, 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.{{harvnb|Platzer|2004|p=174}}

{{More|Hitchhiker's thumb|brachydactyly type D|triphalangeal thumb|polydactyly}}

File:Hitchhikers thumb.jpg ]]

There is a variation of the human thumb where the angle between the first and second (proximal and distal) phalanges varies between 0° and almost 90° when the thumb is in a thumbs-up gesture.{{cite web|url=http://udel.edu/~mcdonald/myththumb.html |title = Myth's of Human Genetics: Hitchhiker's Thumb|access-date=7 November 2012}}

It has been suggested that the variation is an autosomal recessive trait, called a hitchhiker's thumb, with homozygous carriers having an angle close to 90°.{{cite web|url=https://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=274200|title=Thumb, Distal Hyperextensibility of |work=OMIM|publisher=NCBI|access-date = 5 February 2010}} However this theory has been disputed, since the variation in thumb angle is known to fall on a continuum and shows little evidence of the bi-modality seen in other recessive genetic traits.

Other variations of the thumb include brachydactyly type D (which is a thumb with a congenitally short distal phalanx), a triphalangeal thumb (which is a thumb which has 3 phalanges instead of the usual two), and polysyndactyly (which is a combination of radial polydactyly and syndactyly).

{{multiple image

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| footer = Left: In a power grip the object is in contact with the palm.
Right: Cricketer Jack Iverson's "bent finger grip", an unusual pad-to-side precision grip designed to confuse batsmen.

}}

One of the earlier significant contributors to the study of hand grips was orthopedic primatologist and paleoanthropologist John Napier, who proposed organizing the movements of the hand by their anatomical basis as opposed to work done earlier that had only used arbitrary classification.{{harvnb|Slocum|Pratt|1946}}, {{harvnb|McBride|1942|p=631}} 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.{{harvnb|Napier|1956|pp=902–913}} 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 making 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 (only if the ball is not tight against the palm).

File:Distal-phalanges-thumb-index Journal.pone.0011727.g001.png

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).{{harvnb|Costello|Fragaszy|1988|pp=235–245}} Nevertheless, precision grips are usually only found in higher apes, and only in degrees significantly more restricted than in humans.{{harvnb|Young|2003|pp=165–174}}, {{harvnb|Christel|Kitzel|Niemitz|2004|pp=165–194}}, {{harvnb|Byrne|Byrne|1993|p=241}}

The pad-to-pad pinch between the thumb and index finger is made possible because of the human ability to passively hyperextend the distal phalanx of the index finger. Most non-human primates have to flex their long fingers in order for the small thumb to reach them.{{Harvnb|Jones|Lederman|2006|loc= Evolutionary Development and Anatomy of the Hand, p. 12}}

In humans, the distal pads are wider than in other primates because the soft tissues of the finger tip are attached to a horseshoe-shaped edge on the underlying bone, and, in the grasping hand, the distal pads can therefore conform to uneven surfaces while pressure is distributed more evenly in the finger tips. The distal pad of the human thumb is divided into a proximal and a distal compartment, the former more deformable than the latter, which allows the thumb pad to mold around an object.

In robotics, almost all robotic hands have a long and strong opposable thumb. Like human hands, the thumb of a robotic hand also plays a key role in gripping an object. One inspiring approach of robotic grip planning is to mimic human thumb placement.

{{cite journal |title=Robot grasp planning based on demonstrated grasp strategies |journal=The International Journal of Robotics Research|volume=34|pages=26–42|doi=10.1177/0278364914555544|year=2015 |last1=Lin |first1=Yun |last2=Sun|first2=Yu|s2cid=10178250}} In a sense, human thumb placement indicates which surface or part of the object is good for grip. Then the robot places its thumb to the same location and plans the other fingers based on the thumb placement.

The function of the thumb declines physiologically with aging. This can be demonstrated by assessing the motor sequencing of the thumb.{{Cite journal|last1=Bodranghien|first1=Florian|last2=Mahé|first2=Helene|last3=Baude|first3=Benjamin|last4=Manto|first4=Mario U.|last5=Busegnies|first5=Yves|last6=Camut|first6=Stéphane|last7=Habas|first7=Christophe|last8=Marien|first8=Peter|last9=de Marco|first9=Giovanni|date=2017-05-10|title=The Click Test: A Novel Tool to Quantify the Age-Related Decline of Fast Motor Sequencing of the Thumb|journal=Current Aging Science|volume=10|issue=4|pages=305–318|doi=10.2174/1874609810666170511100318|issn=1874-6128|pmid=28494715|url=https://hal.parisnanterre.fr//hal-02310540/file/Bodranghienetal.TheClickTestANovelTooltoQuantifytheAge-RelatedDeclineofFastMotorSequencingoftheThumbCurrentagingscience2017.pdf }}

A primitive autonomization of the first carpometacarpal joint (CMC) may have occurred in dinosaurs. A real differentiation appeared an estimated 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.{{harvnb|Brunelli|1999|p=167}}

Opposable thumbs are shared by some primates, including most catarrhines.{{Citation needed|date=December 2014}} The climbing and suspensory behaviour in orthograde apes, such as chimpanzees, 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 baboons, an adaptation to a terrestrial lifestyle has led to reduced finger 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.{{harvnb|Moyà-Solà|Köhler|Rook|1999 |loc= pp. 315–6}}

The evolution of the fully opposable thumb is usually associated with Homo habilis, a forerunner of Homo sapiens.{{Harvnb|Leakey|Tobias|Napier|1964|}}: "[In Homo habilis] the pollex is well developed and fully opposable and the hand is capable not only of a power grip but of, at least, a simple and usually well developed precision grip." This, however, is the suggested result of evolution from Homo erectus (around 1 mya) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.

Modern humans are unique in the musculature of their forearm and hand. Yet, they remain autapomorphic, meaning each muscle is found in one or more non-human primates. The extensor pollicis brevis and flexor pollicis longus allow modern humans to have great manipulative skills and strong flexion in the thumb.{{cite journal |last1 = Diogo |first1 = R. |last2 = Richmond |first2 = B. G. |last3 = Wood |first3 = B. |year = 2012 |title = Evolution and homologies of primate and modern human hand and forearm muscles, with notes on thumb movements and tool use |journal = Journal of Human Evolution |volume = 63 |issue = 1|pages = 64–78 |doi=10.1016/j.jhevol.2012.04.001 |pmid=22640954|bibcode = 2012JHumE..63...64D }}

However, a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of Homo habilis 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, 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.{{harvnb|Harcourt-Smith|Aiello|2004}} Walking may have been a by-product of busy hands and not vice versa.

HACNS1 (also known as Human Accelerated Region 2) is a gene enhancer "that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in the ankle or foot that allow humans to walk on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human genome, HACNS1 has undergone the most change during the human evolution since the chimpanzee–human last common ancestor.{{cite web

|url = http://www.sciencecodex.com/gene_enhancer_in_evolution_of_human_opposable_thumb

|title = HACNS1: Gene enhancer in evolution of human opposable thumb

|publisher = Science Codex

|date = September 4, 2008 |access-date = December 16, 2009

}}

• Pollicization
• Prehensility
• Thumb signal
• Thumb twiddling
• Thumb war

{{Reflist|30em}}{{Reflist|group=upper-alpha}}

{{Refbegin|30em|indent=yes}}

• {{Cite journal

|last1 = Almécija |first1 = S. |last2 = Moyà-Solà |first2 = S. |last3 = Alba |first3 = D. M.

|title = Early Origin for Human-Like Precision Grasping: A Comparative Study of Pollical Distal Phalanges in Fossil Hominins

|journal = PLOS ONE |volume = 5 |issue = 7 |pages = e11727 |year = 2010

|doi = 10.1371/journal.pone.0011727 |pmid = 20661444 |pmc = 2908684

|bibcode = 2010PLoSO...511727A

|doi-access = free }}

• {{cite book |year = 2007

|last = Ankel-Simons |first = Friderun

|title = Primate Anatomy

|chapter = Chapter 8: Postcranial Skeleton

|page = 345 |edition = 3rd

|publisher = Academic Press |isbn = 978-0-12-372576-9

}}

• {{cite book

|last = Austin |first = Noelle M.

|title = Joint Structure and Function: A Comprehensive Analysis |edition = 4th

|chapter = Chapter 9: The Wrist and Hand Complex

|editor1-first = Pamela K. |editor1-last = Levangie

|editor2-first = Cynthia C. |editor2-last = Norkin

|publisher = F. A. Davis Company |location = Philadelphia

|isbn = 978-0-8036-1191-7 |year = 2005

}}

• {{cite book

|last1 = Brown

|first1 = David P.

|last2 = Freeman

|first2 = Eric D.

|last3 = Cuccurullo

|first3 = Sara

|last4 = Freeman

|first4 = Ted L.

|editor-last = Cuccurullo

|editor-first = Sara

|title = Physical Medicine and Rehabilitation Board Review

|chapter = Upper Extremities—Hand Region: Range of Motion of the Digits

|publisher = Demos Medical Publishing

|year = 2004

|chapter-url = https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=physmedrehab&part=A4492#A4530

|isbn = 978-1-888799-45-3

}} (NCBI)

• {{cite book

|year = 1999

|last = Brunelli

|first = Giovanni R.

|editor1-last = Brüser

|editor1-first = Peter

|editor2-last = Gilbert

|editor2-first = Alain

|title = Finger bone and joint injuries

|chapter = Stability in the first carpometacarpal joint

|publisher = Taylor & Francis

|isbn = 978-1-85317-690-6

|chapter-url = https://books.google.com/books?id=-pzGRMvXFzAC&pg=PA167

}}

• {{Cite journal |doi=10.1002/ajp.1350310402 |pmid=31936992 |year=1993 |last1=Byrne |first1=R.W. |last2=Byrne |first2=J.M.E. |title=Complex Leaf-Gathering Skills of Mountain Gorillas (Gorilla g. beringei): Variability and Standardization |journal=American Journal of Primatology |volume=31 |issue=4 |pages=241–261 |s2cid=84429453 |url=http://www.st-andrews.ac.uk/~www_sp/people/personal/rwb/publications/1993%20Byrne_Byrne_AJP.pdf |issn=0275-2565 |url-status=dead |archive-url=https://web.archive.org/web/20090920100630/http://www.st-andrews.ac.uk/~www_sp/people/personal/rwb/publications/1993%20Byrne_Byrne_AJP.pdf |archive-date=September 20, 2009 }}
• {{Cite journal

|last1 = Christel |first1 = Marianne I.

|last2 = Kitzel |first2 = Stefanie

|last3 = Niemitz |first3 = Carsten |author-link3=Carsten Niemitz

|title = How Precisely do Bonobos (Pan paniscus) Grasp Small Objects?

|journal = International Journal of Primatology |volume = 19 |issue = 1 |pages = 165–194

|date = 30 November 2004

|doi = 10.1023/A:1020319313219

|s2cid = 23567551

}}

• {{Cite journal

|last1 = Costello |first1 = Michael B.

|last2 = Fragaszy |first2 = Dorothy M.

|title = Prehension in Cebus and Saimiri: I. Grip type and hand preference

|journal = American Journal of Primatology |volume = 15 |issue = 3 |pages = 235–245

|date = March 1988

|url = http://www3.interscience.wiley.com/cgi-bin/fulltext/110487513/PDFSTART

|doi = 10.1002/ajp.1350150306

|pmid = 31968893 |s2cid = 86556774

}}{{dead link|date=January 2025|bot=medic}}{{cbignore|bot=medic}}

• {{cite journal

|last1 = de Klerk

|first1 = W.J.

|last2 = Forster

|first2 = C.A.

|last3 = Sampson

|first3 = S.D.

|last4 = Chinsamy

|first4 = A.

|author5 = Ross, C.F.

|title = A new coelurosaurian dinosaur from the Early Cretaceous of South Africa

|journal = Journal of Vertebrate Paleontology

|year = 2000

|issue = 2

|volume = 20

|pages = 324–332

|url = http://rosslab.uchicago.edu/publications/De%20Klerk%20et%20al.%202000.pdf

|archive-url = https://wayback.archive-it.org/all/20120130211657/http://rosslab.uchicago.edu/publications/De%20Klerk%20et%20al.%202000.pdf

|url-status = dead

|archive-date = 2012-01-30

|doi = 10.1671/0272-4634(2000)020[0324:ancdft]2.0.co;2

|s2cid = 128622530

}}

• {{cite journal

|last1 = Diogo |first1 = R

|last2 = Richmond |first2 = BG

|last3 = Wood |first3 = B

|title = Evolution and homologies of primate and modern human hand and forearm muscles, with notes on thumb movements and tool use

|journal = Journal of Human Evolution |year = 2012 |volume = 63 |issue = 1 |pages = 64–78 |doi=10.1016/j.jhevol.2012.04.001 |pmid=22640954

|bibcode = 2012JHumE..63...64D

}}

• {{Cite book |last = Ellerman |first = John Reeves |title = The families and genera of living rodents. Vol. II. Family Muridae |publisher = British Museum (Natural History) |year = 1941 |location = London |url = https://archive.org/stream/familiesgeneraof02elle#page/2/mode/2up/search/Lophiomyidae }}
• {{cite journal

|last1 = Franzen |first1 = JL

|last2 = Gingerich |first2 = PD

|last3 = Habersetzer |first3 = J

|last4 = Hurum |first4 = JH

|author5=von Koenigswald, W |display-authors=etal

|title = Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology

|journal = PLOS ONE |year = 2009 |volume = 4 |issue = 5 |pages = e5723

|doi = 10.1371/journal.pone.0005723 |pmid=19492084 |pmc=2683573 |bibcode=2009PLoSO...4.5723F |editor1-last = Hawks |editor1-first = John

|doi-access = free

}}

• {{cite journal

|last1 = Harcourt-Smith |first1 = W E H

|last2 = Aiello |first2 = L C

|title = Fossils, feet and the evolution of human bipedal locomotion

|volume=204 |issue=5 |pages=403–16 |date=May 2004 |journal = Journal of Anatomy |pmid=15198703 |pmc=1571304 |doi=10.1111/j.0021-8782.2004.00296.x }}

• {{cite journal |doi=10.1016/S0021-9290(08)70148-9 |last=Hsu |first=Ar-Tyan |author2=Meng-Tsu Hu |author3=Fong Ching Su |title=Effect of Gender, Flexibility and Thumb Type on Thumb Tip Generation |journal=Journal of Biomechanics |date=July 2008 |volume=41 |issue=Supplement 1 |pages=S148 |url=http://www.jbiomech.com/article/S0021-9290%2808%2970148-9/abstract }}
• {{cite book

|last1 = Jones

|first1 = Lynette A.

|author1-link = Lynette Jones

|last2 = Lederman

|first2 = Susan J.

|author-link2 = Susan Lederman

|title = Human hand function

|url = https://archive.org/details/humanhandfunctio0000jone

|url-access = registration

|publisher = Oxford University Press US

|year = 2006

|isbn = 9780195173154

}}

• {{cite journal |last1 = Leakey |first1 = LSB |author1-link = Louis Leakey |last2 = Tobias |first2 = PV |author2-link = Phillip V. Tobias |last3 = Napier |first3 = JR |author3-link = John Napier (primatologist) |title = A New Species of Genus Homo from Olduvai Gorge |journal = Nature |date = April 1964 |volume = 202 |pages = 7–9 |url = http://purple.niagara.edu/wje/Bio121/Leaky%201964%20habilis.pdf |pmid = 14166722 |doi = 10.1038/202007a0 |bibcode = 1964Natur.202....7L |issue = 4927 |s2cid = 12836722 }}{{Dead link|date=February 2020 |bot=InternetArchiveBot |fix-attempted=yes }}
• {{Cite book

|year = 1942

|last = McBride

|first = Earl Duwain

|title = Disability evaluation: principles of treatment of compensable injuries

|publisher = Lippincott

|page = 631

|url = https://books.google.com/books?id=YIpgQgAACAAJ

}}

• {{cite book

|last = McDade |first = Melissa C.

|title = Grzimek's animal life encyclopedia: Volumes 12–16, Mammals I–V |edition = 2nd

|chapter = Koalas (Phascolartidae)

|editor1-last = Hutchins |editor1-first = Michael

|editor2-last = Kleiman |editor2-first = Devra G.

|editor3-last = Geist |editor3-first =Valerius

|display-editors = 3 |editor4-last = McDade |editor4-first = Melissa C.

|location = Farmington Hills, MI |publisher = Gale Group |year = 2003

}}

• {{cite journal |doi = 10.1073/pnas.96.1.313 |last1 = Moyà-Solà |first1 = Salvador |pmc = 15136 |last2 = Köhler |first2 = Meike |pmid = 9874815 |last3 = Rook |first3 = Lorenzo |title = Evidence of hominid-like precision grip capability in the hand of the Miocene ape Oreopithecus |journal = PNAS |date = January 5, 1999 |volume = 96 |issue = 1 |pages = 313–317 |url = http://www.pnas.org/content/96/1/313.full.pdf |bibcode = 1999PNAS...96..313M |doi-access = free }}
• {{Cite journal

|last = Napier |first = John Russell

|title = The prehensile movements of the human hand |pmid = 13376678

|journal = J Bone Joint Surg Br |volume = 38 |issue = 4 |pages = 902–913

|date = November 1956

|doi = 10.1302/0301-620X.38B4.902 |doi-access = free }}

• {{cite book

|last = Nowak

|first = Ronald M.

|title = Walker's mammals of the world, Volume 2

|edition = 6th

|publisher = JHU Press

|year = 1999

|isbn = 978-0-8018-5789-8

|url = https://books.google.com/books?id=T37sFCl43E8C&pg=PA89

}}

• {{cite book |year = 2004

|last = Platzer |first = Werner

|title = Color Atlas of Human Anatomy, Vol. 1: Locomotor System

|publisher = Thieme |isbn = 3-13-533305-1

|edition = 5th

}}

• {{Cite journal

|last = Senter |first = Phil

|title = Comparison of forelimb function between Deinonychus and Bambiraptor (Theropoda: Dromaeosauridae)

|journal = Journal of Vertebrate Paleontology |year = 2006 |volume = 26 |issue = 4 |pages = 897–906

|doi = 10.1671/0272-4634(2006)26[897:COFFBD]2.0.CO;2

|s2cid = 85919882

}}

• {{Cite journal

|year = 1946

|last1 = Slocum

|first1 = D.B.

|last2 = Pratt

|first2 = D.R.

|title = Disability Evaluation for the Hand

|journal = Journal of Bone and Joint Surgery

|volume = 28

|issue = 3

|pages = 491–5

|url = http://www.ejbjs.org/cgi/reprint/28/3/491.pdf

|pmid = 20992193

}}{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }}

• {{cite journal |last1 = van Nierop |first1 = Onno A. |last2 = van der Helm |first2 = Aadjan |last3 = Overbeeke |first3 = Kees J. |last4 = Djajadiningrat |first4 = Tom J.P. |title = A natural human hand model |journal = Visual Comput |volume = 24 |year = 2008 |issue = 1 |pages = 31–44 |doi = 10.1007/s00371-007-0176-x |url = http://www.ece.uvic.ca/~btill/papers/mocap/vanNierop_etal_2008.pdf |doi-access = free }}
• {{Cite journal |date=January 2003

|last = Young |first = Richard W.

|title = Evolution of the human hand: the role of throwing and clubbing

|journal = Journal of Anatomy |volume = 202 |issue = 1 |pages = 165–174

|pmid = 12587931

|pmc=1571064

|doi = 10.1046/j.1469-7580.2003.00144.x

}}

{{Refend}}

• {{Commons category-inline|Thumbs}}
• {{wiktionary-inline|thumb}}

{{Human regional anatomy}}

{{Authority control}}

Category:Fingers