View source for Forelimb

{{Short description|One of the paired articulated appendages attached on the cranial end of a vertebrate's torso}}
{{About||anatomical details of the human arm|Upper limb}}
[[File:Handskelett MK1888.png|right|thumb|350px|Forelimbs in [[mammal]]s have varying functions but are all [[Homology (biology)|homologous]].]]
A '''forelimb''' or '''front limb''' is one of the [[bilateral symmetry|paired]] [[joint|articulated]] [[appendage]]s ([[limb (anatomy)|limb]]s) attached on the [[cranial]] ([[anterior (anatomy)|anterior]]) end of a [[terrestrial animal|terrestrial]] [[tetrapod]] [[vertebrate]]'s [[torso]]. With reference to [[quadruped]]s, the term '''foreleg''' or '''front leg''' is often used instead. In [[bipedal]] animals with an [[upright]] posture (e.g. [[humans]] and some other [[primate]]s), the term ''[[upper limb]]'' is often used.
A forelimb is not to be confused with a [[forearm]], which is a distal portion of the human upper limb between the [[elbow]] and the [[wrist]].
All vertebrate forelimbs are [[Homology (biology)|homologous]], meaning that they all [[evolution (biology)|evolved]] from the same structures. For example, the [[flipper (anatomy)|flipper]] of a [[turtle]] or of a [[dolphin]], the arm of a human, the foreleg of a horse, and the [[Bird wing|wings]] of both bats and birds are ultimately homologous, despite the large differences between them.<ref>{{cite web
| url= http://www.britannica.com/EBchecked/media/45466/Homologies-of-the-forelimb-among-vertebrates-giving-evidence-for-evolution
| title= Image Gallery: Homo Sapiens. homology: homologies of the forelimb among vertebrates
| publisher= Encyclopædia Britannica | access-date= January 27, 2013
}}</ref>
Specific uses of the forelimbs may be analogous if they evolved from different sub-structures of the forelimb, such as the flippers of turtles and dolphins, and the wings of birds and bats.<ref name=":1" />
== Evolution of forelimbs ==
Evolution of the forelimb may be characterized by many trends. The number of [[Finger|digits]], their characteristics, as well as the shape and alignment of [[Radius (bone)|radius]], [[ulna]], and [[humerus]], have had major evolutionary implications.
Changes in body size, foot posture, habitat, and substrate are frequently found to influence one another (and to connect to broader potential drivers, such as changing climate).<ref name=":0">{{Cite journal|last1=McHorse|first1=Brianna K.|last2=Biewener|first2=Andrew A.|last3=Pierce|first3=Stephanie E.|date=2019-09-01|title=The Evolution of a Single Toe in Horses: Causes, Consequences, and the Way Forward|journal=Integrative and Comparative Biology|language=en|volume=59|issue=3|pages=638–655|doi=10.1093/icb/icz050|pmid=31127281|issn=1540-7063|doi-access=free}}</ref>
=== Shape ===
A number of factors can influence the evolution of forelimb long bone shape, such as body mass, lifestyle, predatory behavior, or relative prey size. A general pattern is for heavier species to have more robust radii, ulnas, and humeri.<ref>{{Cite journal|last1=Fabre|first1=Anne-Claire|last2=Cornette|first2=Raphael|last3=Peigné|first3=Stéphane|last4=Goswami|first4=Anjali|date=2013-05-21|title=Influence of body mass on the shape of forelimb in musteloid carnivorans|journal=Biological Journal of the Linnean Society|volume=110|issue=1|pages=91–103|doi=10.1111/bij.12103|issn=0024-4066|doi-access=free}}</ref>
[[Mustelidae|Musteloid]] carnivorans that have an arboreal lifestyle tend to have long and slender forelimb long bones, which allow for improved movement and flexibility. Semi-fossorial and aquatic musteloid species tend to have short and robust forelimb long bones to deal with the strain from digging and swimming.<ref>{{Cite journal|last1=Fabre|first1=Anne-Claire|last2=Cornette|first2=Raphael|last3=Goswami|first3=Anjali|last4=Peigné|first4=Stéphane|date=2015-05-21|title=Do constraints associated with the locomotor habitat drive the evolution of forelimb shape? A case study in musteloid carnivorans|journal=Journal of Anatomy|volume=226|issue=6|pages=596–610|doi=10.1111/joa.12315|pmid=25994128|pmc=4450962|issn=0021-8782|doi-access=free}}</ref>
In the order [[Carnivora]], felids, which usually ambush and grapple with their prey, have shorter and more robust limbs. Their forelimbs are used for both short sprints and grappling, which means that they need to be flexible and durable. In contrast, canids, which often pursue their prey over greater distances, have longer, more gracile limbs. Running is pretty much the only use for their forelimbs, so they do not need to be adapted for anything else and can be less flexible.<ref>{{Cite journal|last1=Meachen-Samuels|first1=Julie|last2=Van Valkenburgh|first2=Blaire|date=June 2009|title=Forelimb indicators of prey-size preference in the Felidae|url=http://dx.doi.org/10.1002/jmor.10712|journal=Journal of Morphology|volume=270|issue=6|pages=729–744|doi=10.1002/jmor.10712|pmid=19123240|s2cid=20732642|issn=0362-2525}}</ref>
Predators hunting prey that is half their body weight or greater evolved shorter and more sturdy radii, ulnas, and humeri to decrease the likelihood of the bone breaking or fracturing while hunting. Predators hunting prey less than half their body weight tended to have longer and more slender forelimb long bones to improve energetic efficiency.<ref>{{Cite journal|last1=Michaud|first1=Margot|last2=Veron|first2=Géraldine|last3=Fabre|first3=Anne-Claire|date=2020-11-06|title=Phenotypic integration in feliform carnivores: Covariation patterns and disparity in hypercarnivores versus generalists|url=http://dx.doi.org/10.1111/evo.14112|journal=Evolution|volume=74|issue=12|pages=2681–2702|doi=10.1111/evo.14112|pmid=33085081|s2cid=224824184|issn=0014-3820}}</ref>
=== Polydactyly ===
{{See also|Polydactyly in early tetrapods}}
[[Tetrapod]]s were initially understood to have first developed five digits as an ancestral characteristic, which were then reduced or specialized into a number of uses. Certain animals retained 'primitive' forelimbs, such as [[Dactyly|pentadactylous]] (five-fingered) reptiles and primates. This has mostly held true, but the earliest tetrapod or "[[Elpistostegalia|fishapod]]" ancestors may have had more than five digits. This was notably challenged by [[Stephen Jay Gould]] in his 1991 essay "Eight (Or Fewer) Little Piggies".<ref>{{Cite web|url=http://www.stephenjaygould.org/library/gould_eight-piggies.html|title=Stephen Jay Gould "Eight (or Fewer) Little Piggies" 1991|author=Stephen Jay Gould|access-date=2015-10-02|archive-date=2010-01-11|archive-url=https://web.archive.org/web/20100111200502/http://www.stephenjaygould.org/library/gould_eight-piggies.html|url-status=dead}}</ref>
Polydactyly in early tetrapods should be understood as having more than five digits to the finger or foot, a condition that was the natural state of affairs in the very first tetrapods. Early groups like ''[[Acanthostega]]'' had eight digits, while the more derived ''[[Ichthyostega]]'' had seven digits, the yet-more derived ''[[Tulerpeton]]'' had six toes.
Tetrapods evolved from animals with fins such as found in [[lobe-finned fish]]es. From this condition a new pattern of limb formation evolved, where the development axis of the limb rotated to sprout secondary axes along the lower margin, giving rise to a variable number of very stout skeletal supports for a paddle-like foot.
=== Digit specialization ===
{{See also|Comparative foot morphology}}
Digits may be specialized for different forms of locomotion. A classic example is the horse's development of a single toe (monodactyly).<ref name=":0" /> Other hooves, like those of [[Even-toed ungulate|even-toed]] and [[odd-toed ungulate]]s, and even the hoof-like foot of extinct [[Hadrosauridae|hadrosaurs]],<ref>{{Cite journal|last=Zheng|first=R. ; Farke|date=2011|title=A Photographic Atlas of the Pes from a Hadrosaurine Hadrosaurid Dinosaur|url=https://www.smithsonianmag.com/science-nature/a-detailed-guide-to-a-hadrosaurs-foot-1530987/|journal=PalArch's Journal of Vertebrate Palaeontology|volume=8|issue=7|pages=1–12|issn=1567-2158}}</ref> may be regarded as similar specializations.
To bear their immense weight, [[Sauropoda|sauropods]], the most derived being [[Titanosauria|titanosaurs]], developed a tubular [[Manus (anatomy)|manus]] (front foot) and gradually lost their digits, standing on their metacarpals.<ref>{{Cite journal|last=Apesteguía|first=Sebastián|date=2005-01-01|title=Evolution of the titanosaur metacarpus|url=https://www.researchgate.net/publication/287912481|journal=Thunder-Lizards: The Sauropodomorph Dinosaurs|pages=321–345}}</ref> The stegosaurian forelimb has evidence for a sauropod−like metacarpal configuration<ref>{{Cite journal|last=Senter|first=Phil|date=2010|title=Evidence for a Sauropod-Like Metacarpal Configuration in Stegosaurian Dinosaurs|journal=Acta Palaeontologica Polonica|volume=55|issue=3|pages=427–432|doi=10.4202/app.2009.1105|issn=0567-7920|doi-access=free}}</ref> This was a different evolutionary strategy than megafaunal mammals such as modern elephants.
[[Therapsid]]s started evolving diverse and specialized forelimbs 270&nbsp;million years ago, during the Permian.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2019/03/190318151719.htm|title=Mammals' unique arms started evolving before the dinosaurs existed|website=ScienceDaily|language=en|access-date=2019-12-10}}</ref>
==== Opposable thumbs ====
Modern humans are unique in the musculature of the forearm and hand, though opposable thumbs or structures like them have arisen in a few animals.
In dinosaurs, a primitive autonomization of the first [[carpometacarpal joint]] (CMC) may have occurred. In primates, a real differentiation appeared perhaps 70 mya, while the shape of the human thumb CMC finally appears about 5 mya.
* [[Primate]]s fall into one of four groups:<ref>{{Cite book|title=Primate anatomy : an introduction|last=Ankel-Simons, Friderun.|date=2007|publisher=Elsevier Academic Press|isbn=978-0-08-046911-9|edition=3rd|location=Amsterdam|oclc=437597677}}</ref>
** Nonopposable thumbs: [[tarsier]]s and [[marmoset]]s
** Pseudo-opposable thumbs: all [[strepsirrhine]]s and [[Cebidae]]
** Opposable thumbs: [[Old World monkey]]s and all [[Hominidae|great apes]]
** Opposable with comparatively long thumbs: [[gibbon]]s (or lesser apes)
Pandas have evolved pseudo-opposable thumbs by extension of the [[sesamoid bone]], which is not a true digit.<ref>{{Cite journal|last1=Salesa|first1=Manuel J.|last2=Antón|first2=Mauricio|last3=Peigné|first3=Stéphane|last4=Morales|first4=Jorge|date=2006-01-10|title=Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas|journal=Proceedings of the National Academy of Sciences|language=en|volume=103|issue=2|pages=379–382|doi=10.1073/pnas.0504899102|issn=0027-8424|pmid=16387860|pmc=1326154|bibcode=2006PNAS..103..379S|doi-access=free}}</ref>
=== Pronation and supination ===
{{See also|Anatomical terms of motion}}
The ability to pronate the [[Manus (anatomy)|manus]] (hand) and forearm in [[Theria|therian mammals]] is achieved by a rounded head of the radius, which allows it to swivel across the ulna. Supination requires a dorsal glide of the distal radius and pronation a palmar glide in relation to the distal ulna.
[[Pronation]] has evolved multiple times, among [[mammal]]s, [[chameleon]]s, and [[Varanidae|varanids]].<ref name=":2" /> However, the more basal condition is to be unable to pronate. [[Dinosaur]]s were not capable of more than semi-pronation of the wrist,<ref>{{Cite journal|last=Hutson|first=Joel D.|date=2014|title=Quadrupedal Dinosaurs did not Evolve Fully Pronated Forearms: New Evidence from the Ulna|journal=Acta Palaeontologica Polonica|volume=60|issue=3|pages=599–610|doi=10.4202/app.00063.2014|issn=0567-7920|doi-access=free}}</ref> though bipedal origins of all quadrupedal dinosaur clades could have allowed for greater disparity in forelimb posture than often considered.<ref name=":2">{{Cite journal|last1=VanBuren|first1=Collin S.|last2=Bonnan|first2=Matthew|date=2013-09-18|title=Forearm Posture and Mobility in Quadrupedal Dinosaurs|journal=PLOS ONE|volume=8|issue=9|pages=e74842|doi=10.1371/journal.pone.0074842|issn=1932-6203|pmc=3776758|pmid=24058633|bibcode=2013PLoSO...874842V|doi-access=free}}</ref> [[Monotreme]]s have forearms that are not as dexterous as therians. Monotremes have a sprawling posture, and multiple elements in their pectoral girdles, which are ancestral traits for mammals.<ref>{{Cite book|title=Fins into limbs evolution, development, and transformation|last=Hall, Brian Keith, 1941-|date=2007|publisher=University of Chicago Press|oclc=928978489}}</ref>
In birds, the forearm muscles supinate, pronate, flex and extend the distal wing.<ref>{{Cite journal|last=Tobalske|first=Bret W.|date=2007-09-15|title=Biomechanics of bird flight|journal=Journal of Experimental Biology|language=en|volume=210|issue=18|pages=3135–3146|doi=10.1242/jeb.000273|issn=0022-0949|pmid=17766290|doi-access=free}}</ref>
=== Wings ===
All tetrapod forelimbs are homologous, evolving from the same initial structures in [[Sarcopterygii|lobe-finned fish]]. However, another distinct process may be identified, [[convergent evolution]], by which the wings of [[bird]]s, [[bat]]s, and extinct [[pterosaur]]s evolved the same purpose in drastically different ways.<ref name=":1">{{Cite web|url=https://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_09|title=Homologies and analogies|website=evolution.berkeley.edu|access-date=2019-12-09}}</ref> These structures have similar form or function but were not present in the [[Most recent common ancestor|last common ancestor]] of those groups.
Bat wings are composed largely of a thin membrane of skin supported on the five fingers, whereas bird wings are composed largely of feathers supported on much reduced fingers, with finger 2 supporting the [[alula]] and finger 4 the primary feathers of the wing; there are only distant homologies between birds and bats, with much closer homologies between any pair of bird species, or any pair of bat species.
=== Flippers ===
Marine mammals have evolved several times. Over the course of their evolution, they develop streamlined hydrodynamic bodies. The forelimb thus develops into a [[Flipper (anatomy)|flipper]]. The forelimbs of cetaceans, pinnipeds, and sirenians presents a classic example of convergent evolution. There is widespread convergence at the gene level.<ref>{{Cite journal|last1=Chikina|first1=Maria|last2=Robinson|first2=Joseph D.|last3=Clark|first3=Nathan L.|date=2016-09-01|title=Hundreds of Genes Experienced Convergent Shifts in Selective Pressure in Marine Mammals|url= |journal=Molecular Biology and Evolution|language=en|volume=33|issue=9|pages=2182–2192|doi=10.1093/molbev/msw112|pmid=27329977|pmc=5854031|issn=0737-4038}}</ref> Distinct substitutions in common genes created various aquatic adaptations, most of which constitute [[parallel evolution]] because the substitutions in question are not unique to those animals.<ref name=":3">{{Cite journal|last1=Zhou|first1=Xuming|last2=Seim|first2=Inge|last3=Gladyshev|first3=Vadim N.|date=2015-11-09|title=Convergent evolution of marine mammals is associated with distinct substitutions in common genes|journal=Scientific Reports|language=en|volume=5|issue=1|pages=16550|doi=10.1038/srep16550|pmid=26549748|pmc=4637874|issn=2045-2322|bibcode=2015NatSR...516550Z|doi-access=free}}</ref>
When comparing cetaceans to pinnipeds to sirenians, 133 parallel amino acid substitutions occur. Comparing and contrasting cetaceans-pinnipeds, cetaceans-sirenians, and pinnipeds-sirenians, 2,351, 7,684, and 2,579 substitutions occur, respectively.<ref name=":3" />
== See also ==
* [[Hindlimb]]
* [[Anatomical terms of motion]]
==Bibliography==
* {{cite book | title=Vertebrate zoology: an introduction to the comparative anatomy, embryology, and evolution of chordate animals | publisher=Sidgwick and Jackson | author=de Beer, Gavin | year=1956}}
{{notelist}}
==References==
{{reflist}}
== External links ==
* [https://www.academia.edu/5186625/Evolution_of_chameleon_locomotion_or_how_to_become_arboreal_as_a_reptile Evolution of chameleon locomotion: or how to become arboreal as a reptile]
{{fins, limbs and wings}}
[[Category:Limbs (anatomy)]]
[[Category:Vertebrate anatomy]]
[[Category:Animal morphology]]