|A book by|
William H. Calvin
UNIVERSITY OF WASHINGTON
SEATTLE, WASHINGTON 98195-1800 USA
The Throwing Madonna|
Essays on the Brain
Copyright 1983, 1991 by William H. Calvin.
You may download this for personal reading but may not redistribute or archive without permission (exception: teachers should feel free to print out a chapter and photocopy it for students).
Scanned, OCR'ed, and webbed -- but NOT proofread (14 Jan 97)
ETHOLOGY AND EVOLUTION
He who understands baboon would do more toward metaphysics than Locke.CHARLES DARWIN
Get used to thinking that there is nothing Nature loves so well as to change existing forms and to make new ones like them.MARCUS AURELIUS
(Antoninus, Roman emperor A. D. 161 -180),
Natural selection is not always good, and depends (see Darwin) on many caprices of very foolish animals.GEORGE ELIOT
A 1982 Gallup poll of American adults revealed that:
9% agreed with "Man has developed over millions of years from less advanced forms of life. God had no part in this process."
38% agreed with "Man has developed over millions of years from less advanced forms of life, but God guided this process, including man's creation."
44% (a quarter of whom were college graduates) said they accepted the statement that "God created man pretty much in his present form at one time in the last 10,000 years."
9% had no opinion on creationism.
If we were specially created, then why were our genes created in the image of the African apes?
VINCENT SARICH, commenting on the
99 percent identity of human and
chimpanzee genetic material
The Throwing Madonna
Dux femina facti. [A woman was the leader of the enterprise.]
VIRGIL, The Aeneid
Their manner of writing is very peculiar, being neither from the left to the right, like the Europeans; nor from the right to the left, like the Arabians; nor from up to down, like the Chinese; nor from down to up, like the Cascagians, but aslant from one corner of the paper to the other, like ladies in England.JONATHAN SWIFT, Gullivers Travels
Whenever a man stands up and proclaims himself as something special, unequaled in the world, we naturally become a little wary. Yet humankind as a whole has long been doing exactly that, proclaiming ourselves as special creatures for a long list of reasons. There have, of course, been some rude setbacks to our self-esteem. The sun turned out not to revolve around us after all, but we got used to that. And then Darwin explained how one animal species evolves into a new type of animal--and did it so convincingly that many educated people have put aside their seemingly natural notions that humans were specially created in a manner quite unlike ordinary animals. There is something built into our brains which makes us curious about our origins. We make up stories and try them out for size. We (at least in science) eventually discard most of those stories in favor of other ones that fit the facts better.
Not, of course, that this has prevented the Arkansas state legislature from trying to make inconvenient facts go away-- though, to put their recent promotion of biblical creationism in perspective, rumor has it that about fifty years ago, this same August body declared that, for the convenience of schoolchildren within Arkansas, the irrational number pi (3.14159 . . . ) would henceforth be equal to exactly 3. (I have always imagined a capitol rotunda hanging over them, whose architect followed their rule in computing the dome's circumference from its diameter--and thus left a large crack.) Delusions about evolutionary biology have also been known to lead to catastrophe. Remember the "master race"? Or Lysenko's doctrinaire genetics and its disasters for Soviet agriculture?
The list of attributes that separate humans from our distant cousins, the great apes, has been shrinking as our study of animal behavior has intensified. Culture, toolmaking, language, "consciousness"--you name it, and some aspect of ethological research can probably provide an example from another animal. Instead of a quantum jump in ability separating us from the apes, we usually find that differences are more a matter of degree that humans have developed some attributes (say, language-related attention span) to a greater extent.
Handedness, however, still survives on the human uniqueness list. No other animal species consistently prefers the same hand for certain skilled actions (well, let us say "vertebrate" so that we don't have to worry about the lobster or snapper shrimp with its one giant crushing claw). Individual rats and monkeys may prefer to use their right hand, but there are always enough other rats and monkeys preferring left hands for similar actions that it all averages out--the populations as a whole lack a side preference. So how did human handedness arise in evolution? And why right-handedness as the predominant preference rather than left-handedness--chance, or just another outcome of natural selection operating upon our ancestors?
If asked whether we are right-handed or left-handed, most of us would reply according to what hand we use for handwriting. Only 0.34 percent of us claim to be able to use either hand for writing a legible message. While 10.60 percent of us use the left hand, 89.06 percent write with the right. But if one tests other manual skills, the percentages are quite different. Ballistic skills, such as throwing a ball or swinging a hammer, are strongly right-handed, while fine motor skills without a rapid sequence of movements are considerably less specialized. For example, threading a needle is only 77 percent right-handed.
Surely some skills, such as throwing stones, are more primitive than others--say, threading needles. Indeed, our prototype of handwriting must itself be a secondary use of some more primitive aspect of handedness, having existed only since the invention of writing some 5000 years ago. Considering the literacy rate for most of those centuries, handwriting skills have been very lightly exposed to the selection pressures that shape genes in the Darwinian manner. Perhaps the head scribe could have supported more children than an illiterate blacksmith, but it seems a marginal argument at best. So one might expect the right/left aspects of reading and writing to provide us some good examples of how new lateralizations arise from old ones. Thus, the following warm-up exercise in historical ethology before the main event.
Written symbols (abstract, as opposed to cartoonlike pictograms) are thought to derive from the tokens used to keep tax records. A small bullet-shaped stone represented a bushel of grain, a long cylindrical stone stood for a herd animal, and so forth (the ancient Sumerians used at least eighteen such tokens in 3000 B.C., just before writing took off). The tokens were likely kept in jars by the tax collector and, like the coins in a modern piggy bank, had to be removed to be counted each time. Besides this disadvantage, there were probably instances of fraud, moving a few "goats" or "bushels" from the jar representing one person's tax payments over into another's jar.
Some Sumerian genius solved these problems by pressing the tokens into a slab of soft clay and then setting it out in the sun to dry and harden. The tokens probably didn't stick very well, but even if they fell out, the impression showed which token had been there. So soon the impressions of the tokens came to be used instead, merely "read" from a permanent tablet. And at some point, another genius just drew the shape of the token-in the clay with a stylus, perhaps after breaking the last token of the desired shape. Thus, writing without typesetting evolved (the reinvention of movable type some 4500 years later was, of course, a major event).
So far, there is nothing in this somewhat fanciful reconstruction of events to suggest a right/left factor. The earliest tablets could probably be read from right to left just as easily as from left to right, since reading was just a matter of recognizing a token's shape and then counting them (mathematicians conjecture that this was the last stage of quantity counting which preceded the development of abstract numbers).
But, given the ease of forgery of clay tablets, someone tried chiseling the symbols into good hard rock (though, of course, some other side of human nature might equally well have provided the motivation--say, the need to aggrandize an "eternal" king by recording his wealth on a monument).
Our do-it-yourself ethology exercise even provides some real exercise for right-handed readers: Try holding a chisel and hammer (imaginary ones will do if real ones are not handy) up in front of you, chipping away delicately at the right side of a "tablet"--and then moving as far left as you can. Then try chipping left to right instead.
Done? If most of the ancients were equally right-handed, they too would have preferred to start from the easy side and work toward the more awkward side, rather than vice versa--and thus (so the story goes) the right-to-left sequence of such ancient written languages as Hebrew.
But most written languages scan from left to right; the usual rationale, again based on right-handedness, is that this avoids smearing the ink with the right hand, which might occur if working from right to left instead. Thus the switch from right-to-left to the predominant left-to-right ordering of symbols on a line, wherever the old traditions were not too firmly entrenched.
While such "just-so" stories suggest explanations for the right/left aspects of reading and writing, those aspects derive from the human predominant right-handedness--and force us to consider why hammering is easiest to do with the right hand for 88.24 percent of us. And whether hammering is itself just a secondary use of some more ancient skill originally shaped by Darwinian natural selection. The right-handedness of writing and hammering is exceeded only by swinging a racket (89.34 percent) and, strongest of all, by throwing a ball (89.47 percent). Both of the latter involve rapid overarm movements and both have suitably ancient counterparts (clubbing and stoning); indeed, hammering looks like a refined version of the very same shoulder and elbow motions employed in both clubbing and throwing. Might modern handwriting be a further refinement employing the same neural machinery?
Neither clubbing nor throwing is necessarily a one-handed skill. Clubbing can often best be done using both hands and swinging the club over the head. But chimps certainly engage in one-handed hammering to crack open nuts (and, as we shall see in Chapter 3, with surprising precision). Chimpanzees throwing large rocks during threat displays often use both hands--but chimps also throw one-handed, both underhand and overarm using the same motions as humans (though their aim is often almost random).
Our best prototype for early individual handedness is probably throwing because advanced throwing is distinctly one handed--and because it has a sustained growth curve, with longer throwing distances repeatedly rewarded by natural selection in a manner hard to envisage for advanced hammering. The successful hunting of small mammals and birds (too fleet to chase, except via a good fast rock) is particularly important, as Chapter 4 elaborates, because it may have allowed our foraging ancestors to live in many new habitats with minimal forage--thus supporting a hominid population explosion.
But why might one-handed throwing develop a right-sided preference? Even if some individuals developed a right-handed habit for throwing, others would have settled on the left hand--so that throwing should distribute about fifty-fifty in the population. But it doesn't. To what can we attribute the modern 89.47 percent species preference for the right hand in rock throwing? Might it have been a random genetic decision? Maybe, but first we must try to relate it to some other right/left aspect of our world--which is, on the surface at least, quite symmetrical. But under the surface of our bodies lie some asymmetries, such as the unpaired internal organs. The right side's liver and the left side's spleen are silent and hidden, seemingly unrelated to hands and arms--but the heart's left-sided aspect does indeed show some promise. My colleague Joan Lockard and I have come up with some evolutionary musings about how the heart could be related to both individual handedness and species right-handedness They are speculations rather than tentative conclusions, but they illustrate the nature of the problem and what a proper solution might entail.
Actually, the heart is pretty much located in the center of the chest, contrary to folklore--but the sound of the heartbeat is loudest on the left (this is because the more muscular left ventricle creates much higher blood pressures than the right ventricle, thus engendering more turbulence and valve-closing noise). So, just listening, one identifies the heart with the left side of the chest.
But who listens to the heart? Infants do. And hence our throwing madonna.
Infants cry and fuss much less if they are allowed to listen to a tape recording of a heart beating (indeed, this electronic pacifier is now used in some hospital nurseries). Since they had nine months to get used to that rhythm, perhaps they do not appreciate being weaned from it. Lacking the marvels of modern machinery, one can always just hold the infant against one's chest. And guess which side is better?
So it is hardly surprising that three out of every four mothers observed in shopping centers are carrying their infant with their left arm. This has been going on for centuries before shopping centers: a survey of madonna-with-child paintings in European art galleries (indeed, of over 400 such artworks from four cultures) showed left-armed infant carrying in 80 percent of the cases. And it may be a very, very old practice indeed.
It could be argued (perhaps this has occurred to you already) that left-sided infant carrying could just be a trivial consequence of right-handedness, to leave the right hand free for doing its thing. If that were true, then most fathers should carry infants on the left side too, since they are almost as right-handed as the mothers, on the average. But men seem as likely to carry an infant with the right arm as with the left; they exhibit no preference at all. And mothers who do not begin cradling their baby shortly after birth (separation due to illness or prematurity, etc.) do not, as a group, develop the strong left-armed preference; this not only adds weight to the weaning-from-the-heartbeat theory but suggests that perhaps the infant trains the mother to carry on the left, by crying more otherwise. If we cannot uncover a trivial explanation for the maternal left-armed infant carrying (and arm strength and cultural factors seem unlikely from recent research findings), perhaps we should concede it as primitive and ask if right-handedness is instead a consequence of the "left-armedness" of maternal infant carrying.
But how might one-handed throwing (our best candidate for handedness thus far) be related to maternal left-armedness? Via a hungry mother. We tend to think of gatherer-hunter bands, back before agriculture, with everyone paired off and specialized in certain roles--the men hunting, and the women gathering while tending their noisy infants. But our penchant for dichotomy and optimal arrangements may mislead us. It seems likely that before things became that specialized, back a few million years ago before the brain started its rapid enlargement, women also hunted. Indeed, they still do in some primitive tribes, mostly small game incidental to gathering. And, contrary to our fixation upon "big game" and cooperative hunting enterprises, it may have all started with small game and individuals.
One requirement would be to keep an infant quiet while stalking game. Distraction ("see the rabbit?"), or holding the infant up to smile at it (called en face in the literature), would hardly seem appropriate to the situation, besides requiring both parental hands. But newborns can also be temporarily quieted by such maneuvers as a nipple in the mouth (or a substitute; even a fold of blanket will serve if the infant is not too hungry), crossing the infant's arms across the chest and pressing them (this is said to work because the posture creates memories of the womb!), and wrapping the infant securely in a blanket (again, back to the womb). One-armed cradling of the infant beneath a breast borrows from all of these maneuvers (though one must be careful not to press on the infant's face while shadowing its eyes; that sets off a struggling, noisy reflex which serves to protect breathing space). None of these maneuvers is intrinsically left-or righthanded. But heartbeat pacification is.
Now, chasing an animal is even more tiresome than usual if you are carrying along an infant. It would certainly encourage a mother to practice throwing stones at nearby rabbits and birds. But one mustn't scare the prey away, and so women hunters would have tried to keep their infants quiet. And there is an advantage to cradling them against the left chest. So it would usually have been right-handed throwing that was most successful. Q.E.D.
But, you may say, surely this is a neo-Lamarckian argument (mother's acquired right-handed skills being somehow passed on genetically to a subsequent baby, a possibility considered quite unlikely on genetic grounds). No, there is indeed a standard Darwinian explanation for how natural selection could have shaped a predominantly right-handed population.
One side of the brain usually has better neural machinery than the other for orchestrating rapid movement sequences (other than locomotion, as has become apparent in the physiological study of some foundations of language skills). If a hunter's left brain was the better rapid sequencer, the right-handed throws should usually be faster (and thus go farther) than lefthanded throws. If it was the right brain which was the faster sequencer, then left-handed throwing (and hammering, etc.) would be better.
Who would be the more successful hunters, the left-or the right-handed throwers? For hominid men not encumbered with infants, there might be no consistent side preference when averaging across the population. But mothers with left-brain sequencers should be better hunters (faster throws and quieter infants) than those mothers who had to hold their infants on the right side in order to use their best throwing arm. More of the infants carried by right-handed mothers (and in turn often carrying their right-handed genes) would survive than those of lefthanded mothers.
This argument does not assume a female predisposition for left-armed infant carrying (though that may well exist too; as described in Chapter 5, facial emotions are best judged when they are seen with the left visual field); as noted earlier, the infant may merely train the mother to carry nearer the sound of her heart, by crying otherwise. So the only innate behavior assumed is that, in effect, the infant doesn't like being weaned from the sound of the heartbeat to which it became accustomed in utero.
Thus crying babies would cause more right-handed genes-- really left-brain movement sequencing genes--to survive into subsequent generations. Like compound interest, even small differences grow when exposed to continuous selection pressures (punctuated evolution, rather than Darwinian gradualism, only means that such selection forces are most effective when acting upon a small, isolated inbreeding population about the time a new species evolves). Given at least 100,000 generations in the last several million years, much but not all of today's population could have left-brain rapid sequencing--approximately what we today call right-handedness in 89 percent of human population. The evidence for predominant right-handedness goes back at least a half-million years to the grips on the flaked tools found with Homo erectus in Zhoukoudian, China.
Some such mechanism is needed to explain how handedness got started, how brain enlargement occurred, how lateralizations of functions came about. The human brain is distinguished from the brains of the great apes by an extraordinary extent of lateralization of function. Language on the left, visual-spatial on the right is an easy-to-remember dichotomy which hardly does justice to the mosaic of lateralized functions, but does serve to remind us just how much humans have departed from the usual primate arrangement of homologous hemispheres and duplicated "head offices" for most functions (though there are hints here and there of various asymmetries: For example, rat brains have thicker cortex on the right side). And rapid motor sequencing is a convenient candidate for an early lateralization because it seems like a good foundation on which many language specializations could be constructed, as discussed in Chapters 4 and 16.
Our common heritage, shared with the great apes, may turn out to include several of the essential ingredients: a tendency of one hemisphere to be better than the other for rapid motor sequencing and even the practice (whether innate or acquired) of left-armed infant carrying. But unless mothers used that neural sequencing machinery for an important reason during infant carrying, there might be no environmental selection for left-brain sequencing. And thus (as we shall explore in Chapter 4) perhaps no selection for bigger brains and language. Did the great apes miss the bigger-brain bandwagon because mothers don't throw?
Yet one cannot draw such conclusions without a lot of detailed facts about primate behavior and motor systems neurophysiology. Primate ethology is a kind of behavioral archaeology. To quote George Eliot again,
Our deeds will travel with us from afar And what we have been makes us what we are.
And you don't have to visit a hot, dusty dig to see this kind of archaeology in action: just look around, the next time you visit the zoo, for someone with a clipboard and a stopwatch who is intensely studying the mannerisms and play of the monkeys and apes--and perhaps comparing them to the visitors, too. Things like handedness and infant-carrying preferences are among our major clues to a smaller hominid brain that no longer exists, and careful study of them reveals far more than the casual glance and the cocktail party truism.
While this maternal hunting possibility for right-handedness is perhaps an improvement on its predecessor (an anatomically marginal hypothesis which few believed: a warrior holding his shield in his left hand to better "protect his heart"), most hypotheses turn out to be wrong or only part of the truth. At their best, such hypotheses inspire and provide a new focus for research: into why there is left-armed infant carrying, into why righthandedness is stronger for ballistic skills than fine motor skills, into the corresponding abilities of the great apes (for example, the relation of chimp throwing abilities to their better-developed hammering skills discussed in Chapter 3), into the neurophysiology of throwing and other ballistic movements.
So, caution. Yet, maternal-infant hypotheses have an inherent strength when arguing natural selection: for example, Lovejoy's theory that the upright posture allowed mothers to forage for food while carrying infants who couldn't cling to parental body hair in monkey/ape fashion. And skill in mothering is under very strong selection due to the often high infant mortality rate. Jane Goodall found that inexperienced chimpanzee mothers lost over half of their infants though experienced mothers lost only 17 percent (surviving human childhood became the norm only in the present century, and only in the developed countries). While silent hunting skills may be an important survival skill for mothers themselves, the infant is even more exposed to selection than the mother: the survival of the infant is threatened by even minor diminution in the mother's health or fitness (absent the backup provided by kinship and/or societal care).
Infant pacification and maternal hunger would seem a likely part of the evolutionary history of handedness--but whether this fascinating aspect of natural selection will prove to be the key factor, only more work will tell. Still, consider the possibilities. Today, only the portraits of twentieth-century male pitchers hang in the Baseball Hall of Fame. Yet perhaps someday--just possibly--a "Museum of Human Origins" might have its entrance hall dominated by a large painting depicting a turning point in hominid evolution: showing a mother with infant, a rock in flight, and a rabbit. Its title might read: The Throwing Madonna.
The Throwing Madonna:
Essays on the Brain (McGraw-Hill 1983, Bantam 1991) is a group of 17 essays: The Throwing Madonna; The Lovable Cat: Mimicry Strikes Again; Woman the Toolmaker? Did Throwing Stones Lead to Bigger Brains? The Ratchets of Social Evolution; The Computer as Metaphor in Neurobiology; Last Year in Jerusalem; Computing Without Nerve Impulses; Aplysia, the Hare of the Ocean; Left Brain, Right Brain: Science or the New Phrenology? What to Do About Tic Douloureux; The Woodrow Wilson Story; Thinking Clearly About Schizophrenia; Of Cancer Pain, Magic Bullets, and Humor; Linguistics and the Brain's Buffer; Probing Language Cortex: The Second Wave; and The Creation Myth, Updated: A Scenario for Humankind. Note that my throwing theory for language origins (last 3 essays) has nothing to do with the title essay: THE THROWING MADONNA is a parody (involving maternal heartbeat sounds!) on the typically-male theories of handedness.
Many libraries have it (try the OCLC on-line listing, which cryptically shows the libraries that own a copy), and used bookstores may have either the 1983 or the 1991 edition.
- Powell's Books in Portland lists used copies in their web database.
Email || Home Page || Table of Contents || End notes for this chapter || Continue to read next chapter