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The hand is a five-fingered extremity located on the end of the forelimb of primates. The human hand is distinct from other primates due to its ability to firmly grasp objects resulting in increased precision and maneuverability. The effects of bipedalism and tool-making in early hominin species are believed to have led to these evolutionary innovations. The hand is differentiated from the paw as it includes an opposable thumb. The human hand consists of four fingers and an opposable thumb connected to the palm. Opposability is defined as the ability to independently connect the tips of each finger to the tip of the thumb.

The Hand Bones
The human hand contains twenty-seven bones, the eight carpals make up the wrist and the 5 metacarpals make up the palm. Each of the four fingers contains proximal phalanges, which are closest to the palm; intermediate phalanges; and distal phalanges, which are farthest from the palm. The thumb only contains proximal and distal phalanges without the intermediate phalange.

Primate Hands
The earliest ancestors of primates lived 100 million years ago and resembled the tree shrews of today. These mammals were quadrupedal and lived in the forests of Africa. In these ancestors, the eyes shifted from the side of the skull to the front. This allowed for stereoscopic vision that would have been important for the evolution of the grasping hand later on.

In humans, the hand is directly controlled by the central nervous system, which allows the individual to deliberately use it to make gestures that convey meaning. This was made possible due to an innovation in the early lineage of primates where direct connections between neurons in the cortical motor areas and spine developed This gave the cerebral cortex mono-synaptic control of the hand motor neuron muscles.

Humans’ closest genetic relative alive today is the chimpanzee, and it is estimated that the human and Pan (chimpanzee) lineages diverged 5-7 million years ago. Chimpanzees are also the extant species that most resembles the fossils of the earliest hominins.

The metacarpal and carpal bones of the chimpanzee hand are elongated, but also small, weak, and largely immobile. Yet, since chimpanzees’ primary means of terrestrial locomotion is knuckle-walking, the third and fourth metacarpals are very robust so as to absorb the high compression. The proximal and intermediate phalange finger bones are not straight and instead curve towards the palm in order to better withstand stress from gripping tree limbs when climbing.

Human Hands
There are several traits of the human hand that are derived from primates: for example, having five-fingers, and the lack of hair on the palms and fingers. The os central bone is found in prosimians, apes and human embryos. In humans and our closest living relatives, the chimpanzees and gorillas, the os central fuses to the scaphoid bone but may stay separate in some individuals.

The human hand is unique among primates due to its longer thumb and fingers that can be precisely controlled. The proportions of humans hands are more similar to the proportions of apes that lived in the Miocene than other primate species that exist today. The typical primate hand has a small thumb with long curved fingers. The thumb of the human hand is larger and more muscular, while the fingers are straight and shorter than other primates. This arrangement results in a thumb that is fully opposable to the fingers. In humans there has been a shift of strength to the thumb, second and third fingers. The human thumb contains three muscles that provide added strength and control. The most powerful human thumb muscle is the flexor pollicis longus muscle, which is absent in chimpanzees.

Humans have short palms, while those of chimpanzees and other existing ape species are elongated. Chimps and gorillas likely evolved metacarpal bones that were elongated as a consequence of knuckle-walking locomotion.

The wrist moves from being extended to being flexed when objects are thrown, and when clubbing deviates between radial (twisted towards the thumb) and ulnar (twisted towards the little finger) positions. The chimpanzee wrist is not as efficient at making these movements.

When flexed, human fingers rotate towards the centre of the hand so that the tips of the fingers and thumb meet, which allows for two types of grip unique to humans: the precision and power grips. The power grip is for applying force to objects and the precision grip is for movements that need to be precise or delicate, such as with small tools and instruments. Throwing an object requires a grip where the thumb must be long enough to oppose the finger.

Together, the fingers and thumb form a vice that can squeeze objects against the palm. This is crucial for the ability to hold tools, such as rocks or clubs, in the hand. Effective clubbing would require a secure grip. This maximizes the time that the time force is applied, minimizes the recoil, and allows individuals to use the tool or club again immediately.

Overall, in comparison other primates, the human hand has a longer thumb; shorter palm; shorter, straighter fingers.

Bipedalism
The earliest evidence for bipedalism places it at approximately 5-6 million years ago. However, the process of becoming fully bipedal was gradual and it wasn't until 2 million years ago where evidence of Homo erectus indicates that there was a species that walked fully upright.

Bipedalism resulted in the hominin forearms no longer being required for locomotion. This allowed for the opportunity of innovation such as carrying, holding, and manipulating objects with great precision to evolve.

Tool Use
The making and use of tools is theorized to have facilitated the evolution of traits in the human hand that differentiate it from the hands of other primates and even some early hominins. The first evidence of stone technology by hominins dates to 2.6 million years ago. This technology would have allowed hominins to access previously unobtainable resources. The lineage would have become dependent on their use for important aspects of acquiring resources and ultimately survival. The use of tools would have been selected for.

Stone knapping involves the use of a hard hammerstone, a soft hammer fabricator (made of wood, bone or antler), to detach stone flakes from a lump of tool stone. Flakes are detached in sequence in order to obtain sharp flakes, on which a variety of tools can be made, or to rough out a blank for later refinement into a projectile point, knife, or other object. The hammerstone and hammer fabricator are held in different hands when producing flakes.

The complex human wrist allows for the trapeziun, trapezoid, and captitate carpal bones to be connected. When the thumb and fingers are used to manipulate objects such as in stone-knapping, the broad trapezoid-capitate complex works to relieve stress.

The features of the human hand also allow for cupping, which is important when holding objects for an extended period of time. Asymmetry between the 2nd and 5th metacarpals allows the index finger to rotate towards the little finger. A joint at the base of the 5th metacarpal allows the little finger to rotate towards the thumb and index finger.

In order to grip a stone tool and secure it to the hand, is important to be able to apply the distal segment of the thumb to the opposing side of the tool as the fingers. Those individuals that would have been more able to effectively grip objects would have had an advantage by being better able to use tools to acquire food. Experiments have shown that individuals with larger, stronger grips are more efficient at using stone flake tools in cutting tasks than individuals with smaller, weaker grips.

A ligament that connects to the 3rd metacarpal is missing in most other hominins, and may serve to stabilize against large external forces. These kinds of forces could be caused by the use of hammer-stones by later hominins to produce stone tools 2.6 million years ago. However, hand specimens of A. afarensis dating to 3.2 million years ago show these features, which predates the earliest evidence of stone tools. In fact these features may have already been adapted for the use of clubbing and throwing objects, and would have proved well-suited to later innovation in making stone tools.

Before stone tools, tool use may have involved hand-held weapons that would have provided advantages when fighting. These could simply have been hand-held rocks and/or wooden clubs. The male individuals in a primate or early hominin community that proved to be more proficient in throwing and clubbing would rise higher in the dominance hierarchy relative to those who were less adept at throwing and clubbing. The more dominant males would have acquired more opportunities to breed with females, and have increased reproductive success. These innovations would also provide better defense from predators and allow for predators to be driven away from carcasses to increase scavenging. By providing advantages when fighting, individuals would be better able to survive to reproduce. The selection for improved throwing and clubbing over millions of years would have led to many anatomical changes in the body and the hand. Sexual Selection may have also favoured these adaptions. The hominin males would be better able to dominate other males, take over the best areas for feeding and obtain more meat, and be better able to protect other hominins, resulting in females being more likely to select these males for reproduction. The new body motions would have improved balance and made a more upright stance more efficient, thus, resulting in increased bipedalism. Increased bipedalism would have in turn provided the hand with increased opportunities to do other tasks without being required for locomotion.