Evolution of the human hand: the role of throwing and clubbing
Abstract
It has been proposed that the hominid lineage began when a group of chimpanzee-like apes began to throw rocks and swing clubs at adversaries, and that this behaviour yielded reproductive advantages for millions of years, driv- ing natural selection for improved throwing and clubbing prowess. This assertion leads to the prediction that the human hand should be adapted for throwing and clubbing, a topic that is explored in the following report. It is shown that the two fundamental human handgrips, first identified by J. R. Napier, and named by him the ‘precision grip’ and ‘power grip’, represent a throwing grip and a clubbing grip, thereby providing an evolutionary explana- tion for the two unique grips, and the extensive anatomical remodelling of the hand that made them possible. These results are supported by palaeoanthropological evidence.
Key words handgrips; manual structure; palaeoanthropology.
Introduction
The typical primate hand is characterized by a diminu- tive thumb in combination with long, curved fingers (Midlo, 1934). In contrast, the human hand has a much larger, more muscular, mobile, and fully opposable thumb combined with fingers that have shortened and straightened. This striking exception to the primate pattern clearly requires an evolutionary explanation (Marzke & Marzke, 2000; Fig. 1). Although no compre- hensive account has been offered, there is general agreement that the anatomical reconstruction of the hand during human evolution was somehow linked with tool behaviour. This approach is consistent with evidence that an early hominid (hominin) behaviour was bipedal gait, which would have ‘freed the hands’ for greater use of tools. However, basic questions remain: what kind of tools? In what manner and for what purpose were they used? How did such behaviour provide reproductive advantages sufficient to drive natural selection during the millions of years required to transform the ancestral ape hand into the human hand?
A proposal that offers an answer to these questions is provided in the following report. It is suggested that the tools were hand-held weapons that were hurled or swung as bludgeons at adversaries during disputes, providing the aggressors with advantages that in vari- ous ways promoted reproductive success. The resulting selection for improved throwing and clubbing prowess, prolonged over millions of years, led to numerous ana- tomical changes throughout the body, including those that characterize the evolution of the human hand.
The demonstration of a compelling reproductive advantage is essential to any argument that purports to identify a behaviour that leads to evolutionary change. The behaviour must be able to increase the proportion of genes in the breeding population of those who are most adept at it. There are numerous ways that aggressive use of weapons could have led to this result.
The best throwers and clubbers in a community would rise in the male dominance hierarchy and thereby obtain more breeding opportunities. Use of rocks or clubs would provide an advantage in territorial hostilities with unarmed outgroup conspecifics, yielding improved access to breeding females and food, which promote reproductive success. In conflicts between armed hominid communities, those with the most adept warriors would be more likely to prevail. Defence against predators would be enhanced, and opportunities for scavenging would increase when predators could be driven from carcasses. Weapons would have made hunting more effective. Meat obtained by males through hunting and scavenging could be traded for sex with females.
Females who aggressively used weapons would have protected themselves and their children better and increased their access to disputed food resources. A further reproductive benefit could occur through female mate selection. Because hominid males who were skilled throwers and clubbers were more likely to rise to higher ranks, wield more power, dominate other males, commandeer the best feeding sites, obtain more meat, and protect women and children better, females would be more likely to select such males for mating when- ever the opportunity arose. Generation after genera- tion, natural selection would have enhanced the anatomical basis of throwing and clubbing prowess.
This scenario accounts for the unprecedented ability of modern humans to throw missiles and swing clubs with power and accuracy. Selection for improved throwing and clubbing produced an innovative, instinctive, whole-body motion performed from an upright stance that begins with a thrust of the legs. Improved dynamic upright balance on more powerful legs and resilient feet in the service of throwing and clubbing would have made upright locomotion more efficient, leading to its increasing use and eventually culminating in habitual bipedalism. (Several other unique human anatomical and behavioural features can also be accounted for by this approach: Young, 2002.)
The throwing and clubbing motion that begins in the legs progresses through the hips, torso and arms and ultimately imparts accumulated kinetic energy to the hand or hands holding the weapon. The entire body is involved, but the role of the hands is crucial. Natural selection must have acted strongly on the hands from the outset of aggressive throwing and clubbing behav- iour. Indeed, analysis of the evolution of the human hand provides an opportunity to falsify or lend cred- ence to the throwing-and-clubbing proposal.
Grasping a spheroid and precisely controlling its release, required for accurate throwing, demands a grip that differs from one that can firmly grasp a cylindrical club-handle and absorb the reaction force of impact without release of the weapon. This implies that the human hand should manifest two unique grips – one specialized for throwing, the other for clubbing. The following report will show that the two predicted grips are the two fundamental human handgrips first iden- tified by the British anatomist, Napier (1956).
The chimpanzee hand
The chimpanzee hand will be taken as a model for the hand of the hominid ancestor. The most ancient hominid fossils closely resemble chimpanzees, who are genetically our nearest relatives (Sibley, 1992; Ruvolo, 1997). Pan and human lineages diverged 5 –7 million years ago (Mya), about the time the first hominid specimens appear in the fossil record (Klein, 1999). The fingers, metacarpal and carpal bones of the chimpanzee hand are elongated, but in typical primate fashion the thumb is small, weak and relatively immobile (Figs 1 and 2). The third and fourth metacarpals, which absorb the highest compression during knuckle-walking, are especially robust (Lewis, 1977; Susman, 1979). Both proximal and middle phalanges are curved toward the palm to withstand stress from gripping limbs during arboreal locomotion (Susman & Creel, 1979; Susman, 1987, 1994). The finger tips are cone-shaped, and lack
broad apical tufts (Napier, 1960; Susman, 1988b, 1991). Owing to the transverse arrangement of the meta- carpo-phalangeal articulations, there is a transverse skin crease across the palm (Napier, 1960, 1993; Lewis, 1977; Fig. 1). Thumb phalanges and metacarpals are slender and short (Susman, 1994; Fig. 2) and the intrinsic muscles of the thumb, underlying the thenar region of the palm, are small (Marzke et al. 1992).