INTRODUCTION TO THE AGE OF THROWERS

 

By Richard W. Young

                                                                             

If you are interested in human origins and evolution, you have a treat in store for you in what follows--Eduard Kirschmann’s unusually creative and provocative new book, available for the first time in English, thanks to Susan Way’s lucid translation from the German. 

The queries, “where did we come from?” and “what made us the way we are?” represent two of the most profound questions that seem to haunt the human mind.  All cultures have developed myths and beliefs that seek to provide the answers.  These myriad tales, so varied in their style and content, that seek to account for our origins and characteristics, reveal the range and creativity of the unfettered human mind. With the development of science, a new way of answering questions about human origins became possible–a fettered or disciplined method of thinking in which ideas and explanations are constrained by what can be observed, recorded and measured.  By the 19^th century there was sufficient evidence to support the conclusion that plants and animals had evolved from earlier forms (rather than being the result of a single act of creation).  In 1859 Charles Darwin described the mechanism by which this occurs and in 1871 he presented his argument that humans had evolved by this same process.  Today biological scientists universally agree that “nothing makes sense in biology except in the light of evolution”.

Eduard Kirschmann’s seminal work, The Age of Throwers, is thoroughly Darwinian and remarkably original.  It supplies a fresh new way of thinking about human origins and human evolution that provides explanations for several of the most significant features of our species.

The Throwing Hypothesis.  Kirschmann begins with a plausible and stunningly simple assertion: the human lineage began when a population of chimpanzee-like apes began to throw stones more frequently to ward off predators.  It is an assertion that is certainly reasonable, when we consider that throwing is a natural human behavior.  Children do it without training or encouragement.  Adults are inclined to throw objects in self defense or with aggressive purposes.  We pay enormous salaries to professional throwers so we can watch them perform.  Human aimed throwing is an exceedingly complicated act, involving the entire body under exquisite control by the brain and no other animal can do it.  We are unique in this regard.  We are the greatest throwers of all time!  Considering these observations, it seems clear that human throwing had a long evolutionary history that profoundly affected our brain and body. This is the Throwing Hypothesis, defined by Kirschmann as “the assumption that very demanding and extensive adaptations to aimed throwing occurred during the course of hominid evolution” (Chapter 1.4).  If true, our ancestors must have depended importantly on this behavior for survival and success in leaving descendants.   Nevertheless, the origins of human throwing prowess have never  received an evolutionary explanation.  This is a  gap in our understanding of major  proportions.   Herein lies a measure of the magnitude of what Kirschmann has accomplished in this book.  Not only does he provide such an explanation, he makes throwing the basis of a new theory of human evolution. It is an ambitious and refreshingly original attempt to explain how we began, and how we came to be the way we are.

The Throwing Hypothesis is the Core Principle of a Theory.  According to Lewin (1997), any theory of human evolution must explain how it was that an apelike ancestor, equipped with powerful jaws and long, daggerlike canine teeth and able to run at speed on four limbs, became transformed into a slow, bipedal animal whose natural means of defense were at best puny.  Add to this the power of intellect, speech, and morality and one has the complete challenge to evolutionary theory.  This book meets this challenge.  It provides new insights and stimulating answers to all these questions.

Kirschmann does not assert that he is presenting a theory.  He speaks of a “model” that is based on his throwing hypothesis.  Nevertheless, his proposal has both the structure and function of a theory, and thus–call it what you will–it meets the definition of a theory.

Theories are explanatory structures.  As Nagel (1961) emphasized, it is the desire for explanations controlled  by factual evidence that generates science; and it is the organization of knowledge based on explanatory principles that is the distinctive goal of science.  A theory offers a systematic account of widely diverse phenomena.  It is the most inclusive of scientific explanatory structures.  Founded on a small number of principles that explain a large number of empirical laws--regularities that emerge from analysis of observations–a theory is more elegant when it has a minimum of clearly defined principles and can account for a large body of information.  The aim is simplicity in structure and enormity in scope.  Theories explain regularities and provide a deeper and more accurate understanding of phenomena by showing that they are instances of general rules which are manifestations of the underlying principles (Goudge, 1961; Hempel, 1966; Nagel, 1961).

Darwin’s theory is a classic example.  His basic principles–variation and natural selection–are parsimonious and the scope of their application encompasses all of biology.  (A theory of human evolution is but a tiny subdivision of Darwin’s grand theory, but it is of special importance because it concerns our species!)  From this perspective, theories are the highest goal of science. Those who denigrate them as too speculative (“Darwinian evolution is just speculation; it’s only a theory”) overlook that a proper scientific theory can be tested by evidence and thereby strengthened or weakened–and even falsified.

 In my judgement, Kirschmann’s Age of Throwers is the best theory of human evolution since Darwin’s 1871 classic.  This may sound extravagant, but it can be justified.  First, how does Kirschmann’s theory rank in comparison to existing theories of evolution?  Astonishingly, there are none!   There has not been one since the synthesis of genetics and Darwinism took place over 50 years ago.  In the modern era, despite a substantial increase in the volume of evidence in all related fields of science, only a collection of isolated “hypotheses” concerning restricted aspects of human evolution has emerged.  Thus, we have a killer ape hypothesis, a hunting hypothesis, a scavenging hypothesis, a gathering hypothesis, a cooking hypothesis, a food-sharing hypothesis, a nuclear-family hypothesis, numerous bipedalism hypotheses, hypotheses to account for human sexuality, coalition enforcement, the enlarged brain and aspects of modern morality and mentality.  One of these, the killer ape hypothesis, has been classified as a “myth” (de Waal, 2001); another, the hunting hypothesis, has been branded as a fable (Cartmill, 1993).  None has garnered widespread support.  (According to Cartmill, even Darwin’s 1871 classic was only nominally “Darwinian”, since it did not represent human traits as adaptations to anything!)

In the absence of a comprehensive modern theory of human evolution, Kirschmann’s book is an anthropological landmark. It is founded upon a single principle (“the throwing hypothesis”), is explicitly Darwinian, and extensive in scope.  It derives its explanatory power from adaptation to a behavior that is oddly missing from any of the hypotheses cited above.  Kirschmann asserts that the human lineage began with a throwing specialization.  As he phrases it, “the weapon characteristic was transferred from the teeth to the hands at the beginning of hominid evolution” (Chapter 3.3.2).  This approach gains credibility because the basic postulate--that humans are specialized throwers--is supported by evidence commonly available to every one of us.  Kirschmann explains how we gained this unprecedented prowess and shows how this behavior can help account for many of our other unique features.

Darwin’s View: 1871.  Some of the ideas developed by Kirschmann can be traced back to the great man himself.  In Darwin‘s 1871 treatise, he sought primarily to demonstrate the probability that humans had evolved by the same process of evolution he had described for other living species. A detailed narrative of how humans evolved was not possible.  Existing knowledge of the cause of variation and the nature of the hereditary process was essentially nil and evidence of fossil hominids too scanty to be useful.  Nevertheless, his brilliant mind provided hints, intimations and ideas of how certain human traits might have arisen. Among these was the suggestion of a linkage between throwing, weapons, the structure of the hand and bipedalism, a concept which Kirschmann expands and elaborates.  Here is what Darwin said:  In throwing a stone or spear a man must stand firmly on his feet (p. 138).  Ape hands are good for climbing trees but are less perfectly adapted for diversified uses.  The hands and arms could hardly have been come perfect enough to have manufactured weapons, or to have hurled stones and spears with true aim as long as they were used for locomotion or climbing trees. From these causes alone it would have been an advantage to man to have become a biped (p. 141).  They would thus have been better able to have defended themselves with stones or clubs, or have attacked their prey, or otherwise obtained food.  The best constructed individuals would in the long run have succeeded best, and have survived in larger numbers (p. 142).  As they acquired the habit of using weapons, they would have used their jaws and teeth less, and these would have become smaller (p. 144).

A few pages later (pp. 155-156) Darwin refers to the Duke of Argyll’s criticism that the human body has diverged from the structure of “brutes” in the direction of greater physical helplessness and weakness, which seems impossible to ascribe to natural selection.  He adduces the naked and unprotected state of the body, the absence of great teeth or claws for defense, the lack of physical strength, the slow speed in running, the slight sense of smell by which to discover food or avoid danger and the reduced ability of quickly climbing trees to escape from enemies.   It does sound like a recipe for extinction.  How does Darwin reply?

Surprisingly, he says nothing about throwing stones and spears or swinging clubs!  Instead, he plays the “intelligence” card:  The slight corporeal strength, his little speed, his want of natural weapons &c., are more than counterbalanced, firstly by his intellectual powers, through which he has whilst still remaining in a barbarous state formed for himself weapons, tools, &c., and secondly by his social qualities which lead him to give aid to his fellow-men and to receive it in return.  The earliest progenitors of men were no doubt inferior in intellect and social disposition, but they might have existed or even flourished, if, whilst they gradually lost their brute-like powers, such as climbing trees &c., they at the same time advanced in intellect (p. 157).

Thus, a connection between throwing, weapons, the hand and bipedalism can be traced to Darwin, but he did not develop the concept, and when it seems he could have used it, he chose not to do so.  Now, 131 years later, Kirschmann answers the Duke of Argyll’s protestation by playing the “throwing” card.         

Modern Darwinism. In current perspective it is behavior that drives evolutionary change when it affects reproductive success. Heritable variations are irrelevant to evolution if they are not reproduced. A satisfactory theory of evolution must identify an innovative behavior which caused the earliest hominids to branch off from the ancestral apes because it yielded reproductive advantages. Furthermore, these benefits must persist for a prolonged period during which adaptation to the new behavior can occur. The initial behavioral change and each subsequent stage in its improvement must provide reproductive advantages for adaptation to continue.  This requirement was a serious problem when bipedalism was thought to be the inaugural hominid behavior. It proved an insurmountable obstacle to show how bipedalism would lead to increased reproductive success during the millions of years of body remodelling required to bring upright gait to its current (still unimpressive) levels of efficiency.

Aimed throwing of rocks avoids this impasse.  On the reasonable assumption that this behavior would give apes who used it an immediate advantage in reproductive competition with conspecifics who did not–by increasing their survival, access to food, defense against predators, opportunities to breed, etc.–we can readily see that this advantage would be open-ended.  In each generation thereafter, those who were the most effective rock throwers would continue to have a reproductive advantage.  As long as this advantage was maintained, any heritable variation that enhanced the behavior would tend to be selected.  This is essentially the process envisioned by Kirschmann.  During adaptation to aimed throwing, variations were selected that improved this behavior when they yielded a net reproductive advantage.  This led to augmented throwing proficiency and gradually brought about a redesigned body and a more capable brain.

Highlights of “The Age of Throwers”. Kirschmann proposes that several million years ago, some chimpanzee-like apes initiated the hominid lineage when males began to throw rocks at predators for self defense.  Reduced predation was the main advantage gained by the earliest stone-throwers.  Hominid males carried rocks wherever they went.  It was their “master tool”.  When thrown it was a weapon.  It was a useful hammerstone for cracking bones so the marrow could be eaten.  It was a nutcracker.  It could be thrown against a boulder to get sharp-edged fragments for cutting.  (Eventually it would be struck against another rock to obtain sharp edges in a way that didn’t ruin its other functions).  Hominids gradually gained the upper hand in their competition with predators due to their increasing adaptation to rock-throwing.  When, due to millions of years of abuse, predators became wary of the hominids late in the Australopithecine stage, hominids started chasing them from their kills and eating what was left of the carcasses.  This was followed by the era of Homo erectus, the peak period of the Age of Throwers.  With the predator problem now overcome, the hominid population was free to expand.  This led to conditions in which males in rival groups competing for scarce resources began to throw rocks at each other.  This, in turn, resulted in changes in skull shape and increased skeletal robusticity which reduced the incidence of fractured bones from rock projectiles.  By this stage, adaptation of the skeleton to throwing was largely completed, but further fine-tuning of throwing mechanisms in the brain may have continued.

The proposal that the throwing adaptation had a profound effect on the evolution of brain is among the most exciting and potentially fertile of Kirschmann’s many creative ideas.  He also shows that an adaptation to throwing behavior can account for several additional human characteristics that have until now lacked a compelling evolutionary explanation, including bipedalism and the structure of the hand.  These are weighty issues, and Kirschmann has tied them together in a provocative new way. Here are a few examples:

The Hominid Ancestor. Any theory of human origins must have a starting point–the ancestor from which the hominid line began.  In chapter 2 Kirschmann examines this topic.  He chooses for the hominid ancestor our nearest relative, Pan troglodytes, the common chimpanzee.  The habitat of chimpanzees resembles that in which the earliest hominids are believed to have lived and many aspects of their behavior remind us of ourselves. The most ancient hominid fossils strikingly resemble chimpanzees.  More is known about the anatomical structure and behavior of chimpanzees than of any non-human primate. Thus, this species provides an extensive data base of objective scientific evidence with which to begin. Humans, according to Kirschmann, can be thought of as “chimpanzees that have become optimized for throwing” (1.4).

His account begins when the ancestral apes advanced from a habitat on the edge of the savannah into a drier zone. Climatic changes that caused their woodland habitat to shrink may have played a role in this migration (3.3.2). On the grasslands they responded to the increased danger of predation by using sticks and stones to defend themselves (1.4). This resembles chimpanzee behavior and thus follows naturally from the chimpanzee model. Hand-to-hand fighting is risky. The Australopithecine may have used sticks as clubs, but stone projectiles had clear advantages as distance weapons (3.2).

Genetic Advantages of Throwing. In open country, when our ancestors found themselves in dangerous situations, weapons took on increased importance for their fitness (3.3.2)--that is, for their genetic reproductive success (2.2). Evolution is an integrative process in which all the advantages and disadvantages of a variation for the fitness of the bearer undergo simultaneous consideration (3.1). Throwing adaptations only make sense for evolutionary considerations if this activity had a consistent, high importance for the reproductive success of the hominids. The capacity in the use of weapons should, therefore, have exercised enormous influence on their fitness (3.4).  In addition to the advantages resulting from predator defense, successful  reproduction often means prevailing over members of one’s own species and especially one’s own sex (4.3).  In chimpanzees, and presumably early hominids, rank order among men had a strong influence on reproductive success (5.4).   Marked sexual selection in a multi-male society was the most important factor in the evolution of humans.  Through sexual selection the most imposing males were preferred by females. The best throwers belonged to the most imposing males of the group, with correspondingly good prospects of high rank and above average reproductive success. Intellectual abilities developed because of throwing adaptations also had a direct high importance for rank and thus for reproductive success (6.6).  I have suggested elsewhere some possible additional routes to reproductive success from effective throwing and clubbing (Young, 2002a).  Adding these to the list strengthens Kirschmann’s argument.

Bipedalism. One of the great enigmas of human evolution is the question of why our ancestors broke from the mammalian pattern and began walking on their hind legs.  What conceivable advantage could they have gained by doing that?  Kirschmann offers a new and fascinating explanation: upright gait was linked to upright throwing. 

An important reason must have existed for the development of bipedalism.  Specialization for the use of weapons has the potential to be such a reason (3.2). Throwing for defense against predators is viewed as sufficient explanation for the transition to walking upright (3.6), but this isn’t the only reason.  The transport of stones for defensive weapons also required upright locomotion (1.4).  Furthermore, as the throwing adaptation led to increased mobility of the wrist, this would have hindered the use of hands in walking.  Upright orientation of the upper body for throwing could have also been one of the causes of adopting a vertical gait.  “My interpretation of the transition to bipedality”, he writes, “thus rests upon multiple advantages of walking upright for the application of weapons” (3.3.2).

The Human Hand. 19^th century scientists were awed by the perfection of the human hand.  Some thought it so perfect it stood as evidence of a supernatural designer.  Yet none of the authorities on human evolution since then–not Keith, not Elliott, not Dart, nor anyone after them–has developed a compelling explanation for the unique structure of the human hand.  (Recently it has been suggested that the later stages of hand evolution may have been influenced by stone tool-making).  All previous hypotheses to explain bipedalism are mute about the evolution of the hand.  This seems odd, because “hominids became bipedal in order to free the hands” is a classic anthropological dictum (Pilbeam, 1970; Landau, 1991).  Since tottering on two feet would seem to offer disadvantages, the hands must have been doing something extremely valuable to compensate for the reduced balance, speed and endurance that accompanied bipedalism. What the hands were doing seems to be crucial to understanding why hominids became bipedalists, and the structure of the human hand should provide clues about that behavior.  Indeed, if Kirschmann is right, the human hand should be adapted for throwing.

Kirschmann’s breakthrough idea identifies the causal factor in the development of bipedalism, connects it with a behavior involving the hands, and relates that to hand structure.   Bipedalism evolved because of the reproductive benefits of throwing, and it was adaptation to this behavior that led to modifications that produced our “perfect” hands.  The precision of the grip and the sensitivity of the fingers are critical to throwing.  Both are associated with the large areas of the motor and sensory cortex in which the hand is represented (1.4; 3.3.1). The long, opposable thumb is essential for the throwing grip (3.1).  I believe Kirschmann is right–but only half-right–in attributing human hand structure to a throwing adaptation. There are two unique human grips, identified by John Napier (1956), who called them the “precision grip” and the “power grip”.  I have presented evidence that the precision grip is a throwing grip but the power grip is a clubbing grip (Young, 2002b).  This suggests that our ancient ancestors were throwing and clubbing.

Handedness is also a uniquely human trait (ninety percent of us are right-handed).  This ancient hominid trait has not yet received a satisfactory evolutionary explanation (Young, 2002a). Once again, the throwing hypothesis provides an insight. Kirschmann relates handedness to the rotation of the upper body around its long axis in the throwing motion. Because the earliest stone tools seem to have been made predominantly by right-handed stone-knappers 2.5 million years ago, Kirschmann concludes that by then hominids were capable of rotating the upper body in this manner and had integrated it into the acceleration procedure of the throwing motion (3.3.2; 7).

Evolution of the Human Brain.  Few issues in paleoanthropology excite greater interest than the remarkable increase in size and capability of the human brain during human evolution.   Kirschmann’s analysis makes it seem likely that throwing was involved, and in proposing this novel approach he opens a new research domain. This is certainly one of the major accomplishments of his book.  With increasing adaptation to aimed throwing, not only did decisive changes take place in the human body plan, the brain entered new realms of achievement (1.4). He argues convincingly that the act of aimed throwing is extremely complex, and most of the complexity is lodged in the brain.  He gives a provocative glimpse into the wonders in store for researchers who begin to isolate the underlying brain networks and processes that make possible the astonishing feat of long-distance, accurate, targeted throwing of high-speed missiles.

The human throwing motion entails an unusually effective coiling and uncoiling of the body that begins in the legs and feet, then travels through the hips, torso, shoulder, upper arm, lower arm, wrist and fingers, generating a cumulative packet of kinetic energy that is transferred to the missile, which must be released at a precisely controlled instant to attain an accurate trajectory toward the target (Young, 2002a).  Kirschmann characterizes it as consisting of six different rotations plus the release, combined in an intricately coordinated movement that is ballistic (1.4).  A regulated motion can be controlled as it takes place, but a ballistic motion is completed too quickly for any regulation to occur. This puts throwing in a very special category of human behavior. All the variables affecting the throw have to be worked out in advance and inserted into an action protocol (3.3.1).

The capacity to perform this unusual, coordinated, exceedingly rapid, sequential series of movements is part of the normal developmental sequence.  It is more advanced in boys (Gesell, et al.1940) and in men compared to women, as predicted by Kirschmann’s assessment that this and certain other sexually dimorphic traits can be explained by attributing them to the specialization of men for throwing in the course of evolution (3.3.1). Possibly no greater feat of coordination than the human throwing motion has ever evolved, and it must have involved highly significant modifications of the brain (3.3.1). Beginning with analysis of sensory information that precedes the throw, Kirschmann emphasizes the importance of calculating target distance (1.4). He believes that distance calculation for accurate throwing was a major force in remodelling brain structure and causing brain expansion. Cognitive performance evolved as a solution to one task can easily be applied to other tasks (1.4). The highly developed human ability to construct complex scenarios originally stood in the service of evaluating distances (3.3.4; 6.8; 7).  This, combined with the evolved thrower’s abilities to manipulate sequential data, may underlie human ability for advanced planning and play a role in the development of language (7).

In his analysis of determining target distance, Kirschmann discusses the mechanisms by which the human brain reconstructs the third dimension from a two-dimensional retinal image and concludes that only the known size of observed objects offers a basis for calculating absolute distances beyond 3 meters. Distance is determined from a comparison of the (remembered) size of objects with the size of the visual angle (the dimensions of the object’s image on the retina) (3.3.4).  This means that the ability of the human brain to perceive distances is significantly dependent upon memory. This, in turn, contributes importantly to the high level of human recognition abilities. Objects first must be identified, then estimates of size and shape can be called up from memory. These brain mechanisms are all part of the throwing adaptation.

Evaluating the distance to a target (which may be moving in any direction), calculating the required trajectory of the missile to reach the target and intercept it if it is moving, deciding what to throw and when to throw it based on previous experience, activating the throwing-motion action-protocol after adjusting it according to analysis of sensory input, coordinating the contractions of all the involved muscle groups to generate maximum kinetic energy, and timing the release of the missile precisely–all these are unique abilities that developed during human evolution.  They support Kirschmann’s assertion that cognitive capacity evolved as part of the throwing adaptation should be accorded great significance (3.3.1).

Kirschmann maintains that adaptation to throwing is sufficient to explain the growth of the brain in Australopithecus and its subsequent expansion in Homo habilis (7). The increase in brain volume at least through early Homo erectus is mainly due to specializations for targeted throwing (6.1). The transition to H. erectus, when life in the trees was abandoned, marks the conclusion of physical adaptations to throwing (4.3). When, at this time, hominids began to throw at each other, the estimation of large distances gained new importance driving an additional expansion of brain performance (4.1; 7). Further encephalization in subsequent hominids, on the other hand, can be attributed mainly to selection for language and raised requirements on the brain of social behavior within groups (4.3; 7).

Beyond the Throwing Hypothesis. Towards the end of this thought-provoking book, in Chapters  5 and 6, Kirschmann explicitly ventures beyond the scope of the “pure” thrower hypothesis and grapples with several additional issues for human evolution, such as why we fall in love, human reproductive strategies and male homosexuality.  (Even here he points out possible connections between throwing rocks and human sexual features such as reduced testicular size, loss of the penis bone, and the development of protuberant female breasts).  He then takes up the question of cultural evolution (including speech and morality) (6.5). Cultural information, he points out, is not selected to optimize the reproductive success of single individuals, and it is stored and transmitted differently than genetic information. Cultural features can develop a life of their own, and become distant from reality. (The scientific method provides a remedy). Even when he goes beyond the limits of his core principle, he always seems to have something new and provocative to say about each issue he examines, and even when his explanations may not be totally compelling, they are always interesting.  

Predictions.  In an unusual and bold departure from most attempts to explain features of human evolution, Kirschmann offers several predictions based on his model. You will find them in Chapters 3.3.2; 3.3.4; 4.1 and 7.

Conclusion.  It seems clear that Kirschmann has identified something very basic about the manner of our evolution and has opened the door to a vast new field of scientific research--investigation of the effects of a throwing adaptation on the human mind and body. His basic theme is plausible, Darwinian, and consistent with a wide array of evidence. It states that the hominid lineage began when a group of apes began to throw rocks as weapons in conflict situations, a behavior which brought such important and sustained improvement in reproductive success that during subsequent millenia they became increasingly adapted  to this behavior. This simple assertion not only accounts for the unrivaled human proficiency at powerful and accurate throwing, the unique throwing  motion that appears as part of the developmental sequence, our fascination with the feats of great throwers, and our natural inclination to throw objects in self defense or anger, it also offers a route to explaining an impressively long list of other human characters, including upright stance, bipedal gait, the unprecedented form of our hands and the expanded size and capacities of our brains.  The specialized use of hand-held weapons provides a simple, elegant perspective for rethinking the classic problems of human origins and the process of how we came to be the way we are.

 Kirschmann remarks that culture, unlike biology, offers the opportunity for a sudden change of major importance.  In this book he has provided the basis for a significant advance in our understanding of human evolution.

Richard W. Young

Los Angeles, California

October, 2002

 

 

                                                            REFERENCES CITED

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de Waal, F. B. M.  2001 Apes from Venus: Bonobos and human social evolution.  In: Tree of  Origin, F. B. M. De Waal, editor.  Harvard University Press, Cambridge, pp. 41-68.

Gesell, A., H. M. Halverson, H. Thompson, F. L. Ilg, et al. 1940 The First Five Years of Life. Harper Brothers, N.Y.

Goudge, T. A.  1961  The Ascent of Life.  George Allen & Unwin, London.

Hempel, C. G. 1966  Philosophy of Natural Science.  Prentice-Hall, Inc. Englewood Cliffs, N. J.

Landau, M. 1991 Narratives of Human Evolution, Yale University Press, New Haven.

Nagel, E. 1961 The Structure of Science.  Harcourt, Brace & World, Inc., N.Y.

Napier, J. R. 1956  The prehensile movements of the human hand.  Journal of Bone and Joint Surgery, 38B: 902-913.

Pilbeam, D. 1970  The Evolution of Man.  Funk and Wagnalls, N.Y.

Young, R. W.  2002a Human evolution: The role of throwing and clubbing.  Submitted to Current Anthropology.

Young, R. W.  2002b Evolution of the human hand: The role of throwing and clubbing. Journal of Anatomy (2003) 202, pp165-174.