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The Evolution of the Human Hand From an Anthropologic Perspective

Published:August 23, 2021DOI:https://doi.org/10.1016/j.jhsa.2021.07.006
      Coupled with the developing brain and freed from ambulatory responsibilities, the human hand has experienced osteologic and myologic changes throughout evolutionary time that have permitted manipulative capacities of social, functional, and cultural importance in modern-day human life. Hand cupping, precision gripping, and power gripping are at the root of these evolutionary developments. It is in appreciation of the evolutionary trajectory that we can truly understand how ‘form is function.’ The structure of the human hand is distinct in many ways from that of even our closest relatives in the primate order (ie, chimpanzees). We present some of the key anatomic changes and evolutionary anatomic remnants of the human hand. The human hand is truly an amazing organ—the product of millions of years of selective changes.

      Key words

      The pentadactylous upper limb is not unique, having appeared in fossil records from over 350 million years ago, long before the emergence of humans.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      Yet, when combined with the development of the human brain, the human hand has evolved to permit incredible function contributing to social, functional, and cultural importance. As with many aspects of anatomy, form is function. There is a plethora of orthopedic literature surrounding the anatomy of the human hand, its functions, and its pathology. However, there is little written in the medical literature regarding the evolutionary basis underlying the development of the form of the human hand. The structure of the human hand, particularly the osteology and myology, is distinct in many ways from that of even our closest relatives in the primate order (ie, chimpanzees). Anatomic changes have largely been the result of the transition of the hand’s principal functions over evolutionary time. The primary role of the forelimb was initially a support element for quadrupedalism (4-legged walking). It then evolved to that of a grasping structure for maximum arboreal locomotive efficiency. Finally, with the freedom that arose with bipedalism, it transitioned to a structure primarily used for manipulating objects.
      • Marzke M.W.
      Upper-limb evolution and development.
      Despite its unique functions, many of the human hand’s core anatomic features are plesiomorphic traits, meaning they have been expressed by many different species throughout evolutionary time. For example, the ability to oppose the thumb is often thought of as uniquely human and the major contributor to the successful development of our species. However, this function, made possible by a hypermobile saddle joint between the trapezium and first metacarpal, is found present across most species of apes dating back to over 23 million years ago.
      • Marzke M.W.
      Upper-limb evolution and development.
      Given that both the general dimensions and specific anatomical features of the human hand have existed before, the question remains as to how the human hand has evolved an unprecedented aptitude for object manipulation. The leading hypothesis for this is that while many single anatomic features of the human hand have existed before, they have never all existed together in the unique combination in which they are expressed today.
      • Diogo R.
      • Richmond B.G.
      • Wood B.
      Evolution and homologies of primate and modern human hand and forearm muscles, with notes on thumb movements and tool use.
      Importantly, the human hand was also given the opportunity to specialize further than in its ancestors because it was connected to a more powerful brain and was freed from locomotive responsibility by the rise of bipedalism.
      • Almécija S.
      • Smaers J.B.
      • Jungers W.L.
      The evolution of human and ape hand proportions.
      • Alba D.M.
      • Moyà-Solà S.
      • Köhler M.
      Morphological affinities of the Australopithecus afarensis hand on the basis of manual proportions and relative thumb length.
      • Hartwig W.C.
      • Doneski K.
      Evolution of the hominid hand and tool making behavior.
      This brief review will outline key anatomical structures and functions of the human hand and discuss their development from an evolutionary perspective.

      Importance of Grip and Tools

      One cannot discuss the evolution of the human hand without emphasizing the importance of the ability of the human hand to perform unique grips and ultimately use tools and weapons. Three primary grip functions of the human hand have been described: cupping, precision grip, and power grip. These grips are optimized by the presence of several anatomic adaptations, such as shorter second through fifth rays relative to the thumb, a compartmentalized distal palmar pad, a strong independent flexor pollicis longus (FPL) muscle, large intrinsic muscles, a trapeziometacarpal saddle joint, and a unique pollical distal phalanx.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      ,
      • Marzke M.W.
      Upper-limb evolution and development.
      ,
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.

      Cupping

      The first important grip is cupping of the hand, allowing for conforming the hand to natural, variably shaped objects and exhibiting controlled, forceful use of those objects, such as in the making of tools (Fig. 1).
      • Marzke M.W.
      Evolutionary development of the human thumb.
      ,
      • Marzke M.W.
      Upper-limb evolution and development.
      Cupping is made possible by distinctive metacarpal head and base shapes allowing for metacarpal convergence during metacarpophalangeal flexion. Additionally, the second and third digits pronate as they flex, and the fourth and fifth digits supinate as they flex, allowing the hand to accommodate a spherical object.
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.
      This capability is paired with a strong, hyper-flexible thumb on a relatively flat trapezium, allowing for manipulation of a cupped object.
      • Marzke M.W.
      Upper-limb evolution and development.

      Precision grip

      The second critical grip for human hand development is the precision grip, also referred to as the throwing or baseball grip (Fig. 2). The comparatively longer thumb length relative to the other digits, along with a strong FPL muscle, allows the thumb to forcefully oppose each digital pad to achieve this grip.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      ,
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.
      The fingertips also come equipped with an extensive sensory network, greatly overrepresented on the homunculus allowing for precision grip and release of a thrown object.

      Power grip

      The third important grip is the power grip, also referred to as the clubbing or hammer grip (Fig. 3). This grip is best illustrated by a person holding a cylindrical object (such as a club) diagonally across the palm and supported by opposing forces between the thumb/thenar eminence and flexed digits. Large extrinsic and intrinsic thumb muscles, larger moment arms in their respective tendons, and a deep palmar fat pad contribute to a stronger clubbing grip as compared to other species such as chimpanzees.
      • Marzke M.W.
      Upper-limb evolution and development.
      ,
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.
      Other anatomic features that contribute to this grip include the free rotation of the fifth metacarpal toward an opposed thumb metacarpal and relative thumb length allowing for overlapping the other digits during grasp.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      ,
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.
      Additionally, uncoupling of the ulna from the carpal bones, facilitated by changes in the pisiform and the ulnar flexor and extensor muscles, allows for a greater degree of ulnar deviation, further aiding a hammering motion.
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.
      The fifth metacarpal is relatively thick with an enlarged head, an adaptation beneficial to sustaining the intense forces that accompany repeated clubbing.
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.
      Similarly, modified carpal bones on the radial side of the wrist (ie, the human scaphoid) help dissipate stresses generated in the thumb.
      • Young R.W.
      Evolution of the human hand: the role of throwing and clubbing.
      While these important gripping abilities offered evolutionary advantage, Marzke
      • Marzke M.W.
      Evolutionary development of the human thumb.
      describes anatomical consequences of these grips that frequently present to modern-day orthopedic clinics. While the structure and function of the trapeziometacarpal saddle joint is critical to each of the described grips, its unique structure contributes to the development of arthritic changes at the joint as well as subluxation during pinching activities.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      Unlike in other species, the surface of the human trapezium is not entirely flat and not entirely curved. It is hypothesized that species with a flatter trapezial surface were prone to subluxation but likely avoided arthritic issues of the joint. In contrast, species with a more curved trapezial surface were likely prone to higher rates of arthritis. Thus, it seems that the modern human trapezium has evolved with a compromise of sorts to maintain critical grip function while minimizing the consequential pathology of either extreme.
      • Marzke M.W.
      Evolutionary development of the human thumb.

      Thumb Osteology

      Over time, the thumb lost the stability of a digit that could strictly flex and extend and evolved to a digit that could accommodate the various positions required for prehension, opposition, and circumduction.
      • Almécija S.
      • Moyà-Solà S.
      • Alba D.M.
      Early origin for human-like precision grasping: a comparative study of pollical distal phalanges in fossil hominins.
      ,
      • Leversedge F.J.
      Anatomy and pathomechanics of the thumb.
      Humans have the longest mean thumb length relative to index finger length of primates, with the increase in thumb length and strength facilitating the ease of opposition for manipulating objects.
      • Marzke M.W.
      • Marzke R.F.
      Evolution of the human hand: approaches to acquiring, analysing and interpreting the anatomical evidence.
      ,
      • Napier J.
      The evolution of the hand.
      Multiple other thumb features have evolved to allow for efficient manipulation of objects, such as a FPL that is separate from the flexor digitorum profundus (FDP), the pollical distal phalanx (PDP) being broad and having 2 compartments with ungual spines and a tuft, relatively large and strong intrinsic thumb musculature, and a biconcave-convex trapeziometacarpal joint with a flat saddle trapezial surface.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      The most refined human manipulative abilities use pad-to-pad interactions between the pulp of the thumb and the pads of the digits for precise grip and manipulation.
      • Almécija S.
      • Moyà-Solà S.
      • Alba D.M.
      Early origin for human-like precision grasping: a comparative study of pollical distal phalanges in fossil hominins.
      Species that lack this refined human anatomy, such as chimpanzees, fail to perform these critical pad-to-pad interactions and rather primarily manipulate objects between the pad of the thumb and the side of the second digit.
      • Marzke M.W.
      • Marzke R.F.
      Evolution of the human hand: approaches to acquiring, analysing and interpreting the anatomical evidence.
      Of these thumb osteologic and myologic features, the PDP is the most unique to modern humans.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      Important features of the human PDP are the pronounced insertion of the FPL deviated to the radial side, the presence of an ungual fossa, and 2 ungual spines.
      • Marzke M.W.
      Evolutionary development of the human thumb.
      The asymmetry of these structures causes the thumb to pronate when flexed, importantly facing the pad directly toward the pads of the other digits, facilitating strong pad-to-pad interactions. Previous evolutionary analysis of variable PDP anatomy suggested the evolution of the human PDP was related to selective pressures of tool use.
      • Susman R.L.
      Hand of Paranthropus robustus from Member 1, Swartkrans: fossil evidence for tool behavior.
      • Susman R.L.
      Fossil evidence for early hominid tool use.
      • Susman R.L.
      Hand function and tool behavior in early hominids.
      However, human-like PDPs were discovered in ancient australopiths, suggesting that our PDP structure (and the fine manipulative skills it affords) was present up to 20 million years ago, long before the first tools were made.
      • Almécija S.
      • Moyà-Solà S.
      • Alba D.M.
      Early origin for human-like precision grasping: a comparative study of pollical distal phalanges in fossil hominins.
      Much like previously discussed aspects of hand anatomy, pad-to-pad manipulation is plesiomorphic, likely having been expressed in many different species long before the human hand. However, all other living apes lack these distinct PDP features, under the hypothesis that locomotive evolutionary pressures kept them from developing.
      • Almécija S.
      • Moyà-Solà S.
      • Alba D.M.
      Early origin for human-like precision grasping: a comparative study of pollical distal phalanges in fossil hominins.
      Thus, the PDP anatomy we see in the human hand today is, again, the result of the evolutionary freedom to specialize afforded by bipedalism.
      • Almécija S.
      • Moyà-Solà S.
      • Alba D.M.
      Early origin for human-like precision grasping: a comparative study of pollical distal phalanges in fossil hominins.

      Forearm and Wrist

      Scaphoid

      The forearm and wrist have also substantially changed over the course of human evolution. For example, the modern human scaphoid form is the result of a fusion between the scaphoid bone and the os centrale.
      • Ladd A.L.
      Upper-limb evolution and development: skeletons in the closet. Congenital anomalies and evolution’s template.
      Humans and chimpanzees both display this fusion, which is unlike most other mammals where the 2 bones remain separated.
      • Marzke M.W.
      Upper-limb evolution and development.
      Based on prior evolutionary studies, the fusion of these bones increased rigidity and stability of the wrist and functioned to increase support during knuckle walking. A more recent study supported this theory by demonstrating that Orangutans (closely related to both humans and chimpanzees but primarily arboreal) have a nonfused, mobile os centrale.
      • Orr C.M.
      Knuckle-walking anteater: a convergence test of adaptation for purported knuckle-walking features of African Hominidae.
      ,
      • Orr C.M.
      Kinematics of the anthropoid os centrale and the functional consequences of scaphoid-centrale fusion in African apes and hominins.
      While knuckle walking is no longer a part of everyday human life, the fused os centrale in the human hand continues to stabilize the hand and create a rigid wrist that is resistant to various shearing stresses.
      • Orr C.M.
      Kinematics of the anthropoid os centrale and the functional consequences of scaphoid-centrale fusion in African apes and hominins.
      This evolutionary remnant provides insight into why the scaphoid is such a critical carpal stabilizer.

      Palmaris longus muscle

      Another feature of the wrist that offers a glimpse into human evolutionary development is the palmaris longus (PL) muscle. Recent studies have measured PL muscle belly size, length, and fiber arrangement among various arboreal and terrestrial apes. Results show arboreal species have a higher prevalence of PL as well as features correlating with PL function and strength.
      • Aversi-Ferreira R.A.
      • Bretas R.V.
      • Maior R.S.
      • et al.
      Morphometric and statistical analysis of the palmaris longus muscle in human and non-human primates.
      In humans however, the PL has widely varying rates of absence (64% of the Indian population, 37.5% of the Serbian population, 4.5% of the Chinese population, and in as few as 1.5% of the population in Zimbabwe).
      • Ceyhan O.
      • Mavt A.
      Distribution of agenesis of palmaris longus muscle in 12 to 18 years old age groups.
      • Erić M.
      • Koprivčić I.
      • Vučinić N.
      • et al.
      Prevalence of the palmaris longus in relation to the hand dominance.
      • Sankar K.D.
      • Bhanu P.S.
      • John S.P.
      Incidence of agenesis of palmaris longus in the Andhra population of India.
      • Sebastin S.J.
      • Puhaindran M.E.
      • Lim A.Y.
      • Lim I.J.
      • Bee W.H.
      The prevalence of absence of the palmaris longus—a study in a Chinese population and a review of the literature.
      While an important metacarpophalangeal flexor in arboreal primates, the considerable variation of PL prevalence in humans suggests that it is unnecessary for modern hand and wrist function and is likely a vestigial structure, signifying our descent from a common tree-dwelling ancestor.

      Thumb myology

      Other unique features of the human forearm and wrist compared to other existing primates are the muscles of the thumb. These classically include: (1) the presence of a true FPL, (2) a deep head of flexor pollicis brevis, (3) a volar interosseous of Henle, and (4) an extensor pollicis brevis.
      • Diogo R.
      • Richmond B.G.
      • Wood B.
      Evolution and homologies of primate and modern human hand and forearm muscles, with notes on thumb movements and tool use.
      The presence of a true FPL is unique in that the FPL of most nonhominoid primates not only sends a tendon to the thumb but also is connected to the FDP tendons, causing synchronous flexion among the digits and the thumb. This interconnection between the FPL and FDP, now commonly referred to as Linburg-Comstock anomaly, is present bilaterally in only 14% of human specimens (31% unilaterally).
      • Linburg R.M.
      • Comstock B.E.
      Anomalous tendon slips from the flexor pollicis longus to the flexor digitorum profundus.
      The independence of FPL and FDP is thought to substantially contribute to the manipulative function of the human hand, including allowing for an efficient squeeze of cylindrical objects for clubbing/pounding. Electromyographic studies have shown that the independent function of the FPL and FDP allows for efficient tool manipulation and creation by allowing strong flexion of the FDP without strong recruitment of the FPL.
      • Williams E.M.
      • Gordon A.D.
      • Richmond B.G.
      Hand pressure distribution during Oldowan stone tool production.

      Anconeus epitrochlearis muscle

      Another evolutionary remnant of forearm myology is the anconeus epitrochlearis muscle. This short forearm supinator is estimated to be present in 1% to 30% of humans and has been reported clinically as a cause of ulnar nerve compression.
      • Abdala V.
      • Diogo R.
      Comparative anatomy, homologies and evolution of the pectoral and forelimb musculature of tetrapods with special attention to extant limbed amphibians and reptiles.
      ,
      • Nellans K.
      • Galdi B.
      • Kim H.M.
      • Levine W.N.
      Ulnar neuropathy as a result of anconeus epitrochlearis.
      While rare in hominoids (with the exception of chimpanzees), this muscle is most often found in reptiles, amphibians, mammals, and birds.
      • Abdala V.
      • Diogo R.
      Comparative anatomy, homologies and evolution of the pectoral and forelimb musculature of tetrapods with special attention to extant limbed amphibians and reptiles.
      Given this diversity, the anconeus epitrochlearis is likely a curious anatomic remnant of a very distant common ancestor.
      Coupled with the developing brain and freed from ambulatory responsibilities, the human hand has experienced osteologic and myologic changes throughout evolutionary time that have permitted manipulative capacities of social, functional, and cultural importance in modern-day human life. Hand cupping, precision gripping, and power gripping are at the root of these evolutionary developments. It is in appreciation of the evolutionary trajectory that we can truly understand how ‘form is function.’ We have presented some of the key anatomic changes to the human hand, as well as presented some evolutionary remnants. The human hand is truly an amazing organ—the product of millions of years of selective changes.

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