Sex, Time, and Power

by Leonard Shlain

  Few species of animals see colors. Some fish, amphibians, and reptiles have color vision, as do the majority of birds, but polychromatic vision is poorly represented among mammals. Most of the planet’s creatures see the world in gray scale, but their lack of color vision does not pose a handicap because smell (or some other sense) is so superior. Primates possess extraordinary color vision. The vital need to judge distances accurately when swinging through trees made the discernment of slightly different shades of color critical to an arboreal, wingless creature’s survival.

  A significant number of primates are frugivorous, meaning that their diet’s primary staple is fruit. Being able to distinguish a brightly colored orange mango or yellow plantain among distant foliage and gauge its degree of ripeness by its tint is an indispensable visual skill.

  Since color vision was so crucial to our ancestors’ survival, why would 8 percent of males, but not females, fail to develop it to its fullest potential? Is there a male-centric activity for which a visual color defect could be turned to an advantage?

  One of the best defensive shields an animal possesses is an appearance that blends in with its natural environment. Some creatures have perfected this camouflage so well that even when they are pointed out, they remain difficult to isolate from their surroundings when they stand still. But spotting an animal with a coat that is shaded to blend in with its familiar habitat is considerably less difficult for a hunter who does not see the same colors as the others in his band. One color-deficient male within a hunting party would be a valuable asset indeed.

  His worth in the Pleistocene can be extrapolated from the way color-deficient soldiers were deployed in recent history. Intelligence corps on both sides in the two world wars actively recruited them to serve as front-line spotters because they, better than their color-visioned comrades-in-arms, could see right through the enemy’s efforts to camouflage guns and tanks. Having one fellow out of twelve who could easily spot predators lurking, prey hiding, or a camouflaged enemy would incrementally increase the success of that group.

  Over time, selective pressures would have encouraged the spread of a male color-deficient gene in a small segment of the male population. A point of diminishing returns would soon be exceeded if more than one out of eleven or twelve men were color-deficient—then the hunting effectiveness of the entire band diminished—hence the persistence of the 8 percent ratio. Because women rarely engaged in full-scale cooperative hunting or warfare, no selective advantage existed for them to evolve a color deficiency.

  The third anomalous skewed masculine quirk is that 8 percent of men are left-handed. One would expect that handedness would be evenly distributed, with 50 percent preferring the right hand and 50 percent preferring the left. This is decidedly not the case. Surveys from widely differing cultures confirm the odd 92-to-8 ratio of righties to lefties the world over.*

  Many theories attempt to explain why we, as a species, have such a strong predilection to use the right hand. The one most commonly accepted has to do with the connection between language and handedness. It served Natural Selection to position speech (which is a highly sequential fine motor activity) in the hemisphere that also controlled the preferred hand, which would execute highly sequential fine motor movements—overwhelmingly, the left hemisphere. The need to manipulate the environment concretely through the use of handmade tools and abstract words was economically packaged in the hemisphere that best sequenced information in a linear fashion. The right hemisphere, nearly devoid of language skills, could then devote its entire attention and storage space to processing information in a mode opposite to the left. The right hemisphere typically excels at those tasks that require gestalt, all-at-once awareness, such as face recognition or map reading.

  If right-handedness conferred a significant advantage for humans, one might expect that all humans would be right-handed. But the pesky problem of having to explain the 8 percent who are left-handed persists. Left-handedness, like ESSP, color-deficient vision, and baldness, is more common in males than females. But here the disparity between the sexes is not as great.11 Most surveys find that 8 percent of men are left-handed compared with only 5 percent of women. Nevertheless, this gender skewing remains consistent enough that an explanation for the inequality should be sought.

  As I have hypothesized about the previous two male traits, the answer lies in hunting and survival. The presence of one left-handed male hunter among eleven or twelve other, right-handed hunters would have markedly increased their overall success.

  Archaic hunting required three essential skills—location of prey, accurate aiming, and effective throwing—all of which depend on excellent vision. A feature of the human visual system would have made the presence of one left-hander out of twelve a distinct asset. The right and left eyes, separated by the bridge of the nose, see the same scene from ever-so-slightly different angles. Because of the partial crossing over of fibers from each optic nerve within the brain to the opposite cortex, however, the image reconstructed in the right and left visual cortex located in the back of the skull is markedly different from what the right and left eyes see.

  The right visual cortex reconstructs what is before both eyes using information presented only to the left-hand side of their full-on visual field. Conversely, the left visual cortex processes only visual information presented to the right-hand side of their full-on visual field. Right-handed people detect movement, judge distances, and estimate measurements best in their right visual cortex, which is far superior in visio-spatial skills to their left hemisphere.

  Although the brain of a left-handed person is not simply the mirror image of a right-handed person’s, it comes close enough so we can make the following observation. Left-handers judge distances, orient spatially, and distinguish visual clues best in the half of the world appearing on their right. Their left hemisphere is far superior in visio-spatial skills to their right hemisphere.

  An advancing group of right-handed hunters would judge distances, orient spatially, and distinguish visual clues best in the half of the scene that appeared on their left. They would be able to target prey more accurately by throwing right-handed toward their left than by throwing right-handed toward their right. (Observing a right-handed quarterback attempting to hit a receiver who is running to his right demonstrates how awkward and inaccurate throwing in that direction can be.)

  Imagine a vertical line dividing each eye’s field of vision into a right side and a left side. The left hemisphere’s visual cortex in the back of the brain processes visual information from only what both eyes see to the right of the two vertical lines; the right hemisphere processes visual information from only what both eyes see to the left of the two vertical lines.

  A small but significant advantage would accrue to a band consisting of twelve hunters working in concert if one member of the band had a left visual cortex that saw best what was approaching (or fleeing) to the group’s right. Adding to his value would be this hunter’s ability to throw left-handed toward the right with great accuracy. (Although it may have been more advantageous to have six right-handers and six left-handers in the hunting group, Natural Selection had to balance the demand that language localize in the left hemisphere against the success of the hunting band. The solution was to have only 8 percent of males prefer their left hand.)

  In swordfights (or any other hand-to-hand combat), a left-handed swordsman has a slight advantage over a right-handed one, because the latter has trained to fight against an opponent who typically uses his right arm. Right-handed swordsmen called those using their left hand “sinister,” from a Latin word that is also the source of the English word meaning left-sided, “sinistral.” Its alternative meaning is “evil” and “deceitful.” If the hunting band were a war party about to engage in battle with other mostly right-handed adversaries, a left-handed warrior would be their secret weapon.

  In baseball, right-handed batters fear the slight advantage a southpaw pitcher enjoys over them. Thirty perc
ent of Hall of Fame major-league pitchers are southpaws. The same edge holds true for a batter hitting lefty against a pitcher throwing righty. Half of the Hall of Fame batters are either left-handed or switch-hitters.12

  Our species is very young as species go, being only 150,000 years old. Not enough time has passed to alter significantly traits that served to increase our survival when we lived in a hunter-gatherer mode. The addition of one left-handed hunter-warrior in hunting bands could explain the persistence of this ratio among males all across the spectrum of cultures, as well as explain why left-handedness is more prevalent in men than it is in women.

  Baldness is the last peculiar 8 percent trait that almost exclusively affects human males. Over their lifetime, 40 percent of men will experience a significant loss of hair from the crown of their head. Eight percent of men, however, experience extensive loss of hair by the time they are in the prime of their lives.* Typically, they lose the entire crown, leaving a fringe around the sides and rear to form what has been called a “Hippocrates wreath.” Hair loss, with a few very rare exceptions, does not involve the eyebrows, beard, or mustache, nor is hair lost from a man’s legs, chest, underarms, or pubic area. No one that I am aware of has come forth with a credible theory for why this trait appears so often in men, and why baldness is almost (but not quite) unknown among women.

  The prime of a man’s life is the period between twenty and forty-five, when he applies earlier-acquired critical skills to his primary work in life. The primary work of men for a vast span of our history as a species was hunting. It seems a strange coincidence that the 8 percent of men destined to lose their hair did so within the phase of their life when they would have become the most skilled and experienced hunters. And their baldness was confined to the very top of the head.

  Male patterned baldness is virtually absent in other mammals.* Why don’t healthy dogs, cats, seals, horses, and cows experience baldness? A trait as prominent and persistent as male patterned hair loss, I propose, must be present in the human genome for a reason related to either survival or reproduction.

  Sexual selection is the other powerful force molding species besides the drive to survive. Hair frames the face, a human’s most distinguishing visual feature. Men who lose their hair are obsessively concerned that they will be less attractive to women. Women, for their part, prefer men with a full head of hair, but they do not discriminate against bald men as much as men believe they do. Men either wear hats or often resort to rugs, plugs, or drugs to conceal from women that they have lost their hair. Yet bald men do not have less chance to mate and have children than men with a full head of hair. (If they did, the gene controlling baldness would be leached out of the genome.) Sexual selection does not seem to be the reason that 8 percent of men in their prime have a dramatically altered appearance.

  Testosterone levels have been linked to baldness. If this was the culprit, then one would expect baby boys to emerge bald, because they have had such high levels in utero. Instead, the shock of newborn hair is the same for boys as it is for girls. If testosterone was a factor, why wouldn’t teenage boys, experiencing the greatest surge in testosterone after puberty, be at the greatest risk for developing baldness? Instead, a male’s hair is never more luxurious and full than when he is adolescent. Testosterone levels are essentially the same between bald and nonbald men. The peculiar distribution of male patterned baldness, the similiar reproductive rates between bald and nonbald men, its unisexual distribution, and its peak in a man’s prime of life are features that suggest it was built into the male genome because it somehow advanced the survival chances of the species.

  Yet baldness would have been extremely disadvantageous to a man trying to compete on the Pleistocene’s Serengeti. The brain is the most metabolically energetic part of a human’s anatomy. It generates more heat than any other organ, thus requiring the evolution of a sophisticated cooling system to prevent it from overheating. The scalp has an extensive network of small arteries and veins, as anyone trying to stanch the bleeding from even the smallest laceration of the scalp can attest. The blood supply to this patch of skin is particularly robust. Dean Falk and her co-workers hypothesize that the overabundance of veins and arteries in the scalp present in humans serves the same function as circulating radiator water does in cooling an automobile engine. By constantly running cooler blood over the scalp, the blood vessels carrying “coolant” reduce the temperature of this vital organ.13

  Besides the miles of both large and small scalp veins, the peculiar human distribution of body hair also plays a critical role in moderating the brain’s temperature. The densest patch of hair growth on the human body is on the crown of the skull. When our ancestors stood up, they markedly cut down the amount of body surface exposed to the glare of a noontime sun. Unlike animals that can distribute the sun’s heat over the length of their body, humans have the very top of the head receiving the full brunt of sunlight during the hottest part of the day. Selective pressure provided ample protection to this vulnerable area by covering it with a thick tuft of crown hair. The abundant amount of air trapped under and between individual hairs provides a natural form of insulation against the heat. The density of scalp hair also serves to slow the evaporation of scalp perspiration and adds to the efficiency of the brain’s natural air conditioner.

  Peoples living near the equator have frizzier hair than people who live in cooler climes. Its microscopic structure ensures that more air will be sequestered closer to the scalp, thus better insulating their brain from excessive ambient heat. When the temperature drops, scalp hair serves the same function by protecting the brain against excessive cold.

  The loss of scalp hair would expose a bald person to the very real risk of heatstroke, not to mention the less serious but troublesome problem of protecting the exposed skin from ultraviolet light. Bald men do not venture out in the hot sun without some form of protection. One would predict that the gene for baldness would have been culled out of the gene pool, because men who had it would be more likely to die of heatstroke than men who retained their hair. This would have been a much more important factor when our ancestors left the shade of the forest and began making a living on the shimmering yellow immensity of the African savanna.

  Yet baldness occurs in nearly every population of males around the globe, indicating that it is a very old adaptation that must have been performing some beneficial function for the human species. What significant advantage did male baldness confer on the human species that offset the removal of nature’s key protection for the brain against overheating?

  Whatever the reason our hominid ancestors decided, five million years ago, to rear back on their haunches, stand up, and start heel-to-toe walking, achieving greater speed was not among them. There is virtually no fish, bird, amphibian, reptile, or mammal that a bipedal human can chase and easily catch.* If eating the flesh of other animals became the key to increasing human intelligence, then Homo sapiens would first have to get close enough to kill something before his prey sensed his presence and bolted.

  In the cat-and-mouse game played for 99.9 percent of hominid existence, male hunters had to sneak up on unsuspecting prey. The bow and arrow did not come into use until fifteen thousand years ago. Stealth was the critical component of early Homo sapiens’ hunting strategy, since only very crude weaponry was available to inflict a fatal wound.

  Prey, over time, must learn to recognize their enemies, for the simple reason that those that cannot distinguish between friend and foe will not live long enough to reproduce. Therefore, one of the most effective defenses a prey animal can evolve is the instinct called “flight distance.” Attuned to the smell, sound, or sight of a familiar predator, prey animals flee at the approach of one. If part of a larger group, an alert individual prey can sound the alarm signaling the others to escape en masse. Indisputably, the most dangerous predator is a human. Animals that did not learn to recognize and avoid us paid the price of being hunted to extinction.†

  Among t
he many explanations proffered for why humans adopted bipedalism is the Theory of Sentinel Behavior. African meercats, squirrels, and rabbits, along with many other wary four-footed prey animals, exhibit what is called sentinel behavior to increase alertness. Habitually locomoting on all fours, they periodically pause to stand on their hind legs. The vertical position allows them to see over obstacles and gain a larger field of vision. Many primates—for example, gorillas and baboons—have descended from their habitat in the trees and spend most of their time on the ground. Ground-dwelling primates typically engage in frequent sentinel behavior. Temporarily standing on two legs serves as an adjunct to survival.

  Any animal desiring to peer over an obstacle must first show the top of its head. The peerless expert at sentinel behavior is the only truly bipedal mammal, Homo sapiens. Animals that have survived the predations of humans have been programmed by instinct or have learned by experience to identify the typical silhouette of a human head peeking from behind a shrub. Minus a periscope, there is no way that a hunter can sneak a peek without first having to first expose his pate.

  Anthropologists observing the techniques of present-day hunters such as the !Kung San of the Kalahari note the elaborate lengths tribesmen go to disguise themselves as they try to get within striking distance of prey. Hunters know to approach prey downwind, crouch low, and maintain silence. If the prey identifies them too soon, escape will surely follow, and the hunters will return empty-handed. They must do everything they can to prevent any stimulus that will activate their quarry’s flight-distance response. Experienced African hand Michael Crawford comments, “The idea of an upright primate scoring by being able to peer over the tops of the grasses is an appealing one—to anyone who has no experience in hunting. In reality, the main difficulty facing any hunter is not spotting his prey, but preventing his prey from spotting him. The art lies in the stalking.”14