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Corresponding author: Nathan T. Morrell, MD, University of New Mexico, Department of Orthopaedics & Rehabilitation, 1 University of New Mexico, MSC10 5600, Albuquerque, NM 87131.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
The fifth metacarpal is relatively thick with an enlarged head, an adaptation beneficial to sustaining the intense forces that accompany repeated clubbing.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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).
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.
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.
Comparative anatomy, homologies and evolution of the pectoral and forelimb musculature of tetrapods with special attention to extant limbed amphibians and reptiles.
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.
Comparative anatomy, homologies and evolution of the pectoral and forelimb musculature of tetrapods with special attention to extant limbed amphibians and reptiles.
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