Researcher Henry Jerison noted in 1973 that there was a consistent and evocative pattern in the brain-size relationship between predator and prey. The predator in any predator/prey partnership always had a bigger brain. Jerison concluded that the demands of catching another animal were far more exacting than eating foliage and avoiding being caught. Hunting required far more brains than running away.

The origin of thought is not about how big a brain humans might have needed to be successful hunters. What humans needed big brains for, as Geoffrey Miller has outlined, was to achieve sex opportunities. The same process that drove predator brain-size increases, in the case of humans, drove them to behave in ways that resulted in larger brains. What drove humans was the dance and the sounds that humans would make to accompany the dance, with the most evocative performers being picked more frequently as sexual partners.

A predator’s brain need be no bigger than what is required to catch prey. There is no biological incentive to add any more brain power than is absolutely necessary to survive. Humans, as far as we know, are the first species to revel in culture, thought, language and all its implications. Culture requires brains, and brains of that immense degree were never necessary in any other animal. The human act that demanded immense brain power was the act of art. This requirement is because in art there are no limits. Competition to exhibit artistic prowess in the form of dance drove the practitioners to behave in more and more beautiful and complex patterns, with the most successful dancers being chosen for sex. The genes of the best dancers got passed on. The better dancers had bigger brains, just as predators had bigger brains, because it takes bigger brains to behave in more physically or orally complex ways. Whereby a predator needs a brain only slightly bigger than its prey, an archaic human needs a bigger brain to perform better than his or her peers, a constantly escalating standard. The result was runaway sexual selection. The race was on.

It has been hypothesized that early humans were selected for an ability to travel long distances, running, to chase down wounded prey. Spuhler in 1979 outlined specific hormonal innovations that support this conjecture. I believe these glandular innovations are explained by dance as the driving force behind these changes. The theories devised since 1979 that hypothesize running as the driving force behind our evolution neglect dance as a theory that retains far more explanatory power. Again, with dance there are no limits. Massive brain growth is not explained by chasing down a predator. If that were the case, wolves would be our cultural superiors and we their pets.

Sexual selection based on an aesthetic increased the speed of evolution exponentially because the demonstrator of the art was practicing an intangible with no demonstrable ceiling in the exercising of the skill, a skill requiring increased brain mass. This sexual selection, specifically, is what never had occurred before in evolution. Peacock feathers evidence a runaway aesthetic, but no increased brain power was required. The verbal facility and plumage of parrots suggest that they may have experienced a similar selective dynamic, and indeed parrots, like humans, have two unusually large cerebral hemispheres with different-sized lobes. But parrots don’t exhibit complex dance. Humans sing and dance.

Still, we need to address an additional variable that explains why this type of selection occurred with humans and not another species earlier in time. Humans are not the first animal to dance or sing to achieve sex. Why didn’t this exponential brain growth occur in another animal over the course of the last billion or so years? Because the specific way that the human brain grew so large so quickly was by slowing down maturation rates, prolonging the time it took to reach adulthood. Adults with bigger brains achieved this result by spending more time in the childhood states where brain growth was most intense. When the best dancers and singers were picked for procreation, the ones picked were the humans most maturational delayed. It is a feature of only certain groups of species that they can easily increase or decrease their rates of maturation. Wolves and foxes, for example, exhibit a high degree of manipulation in this regard, and the large variation in dog appearances is directly related to this ability. Humans sexually selected to perform dance and song were coincidentally selected to mature slower. Slower maturers grow bigger brains because they spend more time at those early childhood states where brains are growing. The late, great Stephen J. Gould discussed this dynamic in detail over his 30-year career and used the word “neoteny,” earlier popularized by Bolk, to summarize its effects.

“To support the argument that we evolved by retaining juvenile features of our ancestors, Bolk provided lists of similarities between adult humans and juvenile apes: ‘Our essential somatic properties, i.e. Those which distinguish the human body form from that of other Primates, have all one feature in common, viz they are fetal conditions that have become permanent. What is a transitional stage in the ontogenesis of other Primates has become a terminal stage in man.’” (1926a, p. 468).

In his most extensive work, Bolk (1926c, p. 6) provided an abbreviated list in the following order:

1. Our “flat faced” orthognathy (a phenomenon of complex cause related both to facial reduction and to the retention of juvenile flexure, reflected, for example, in the failure of the sphenoethmoidal angle to open out during ontogeny).
2. Reduction or lack of body hair.
3. Loss of pigmentation in skin, eyes, and hair [Bolk argues that black peoples are born with relatively light skin, while ancestral primates are as dark at birth as ever].
4. The form of the external ear.
5. The epicanthic (or Mongolian) eyefold.
6. The central position of the foramen magnum (it migrates backward during the ontogeny of primates).
7. High relative brain weight.
8. Persistence of the cranial sutures to an advanced age.
9. The labia majora of women.
10. The structure of the hand and foot.
11. The form of the pelvis.
12. The ventrally directed position of the sexual canal in women.
13. Certain variations of the tooth row and cranial sutures.

To this basic list, Bolk added many additional features; other compendia are presented by Montague (1962), de Beer (1948, 1958) and Keith (1949). The following items follow Montague’s order (pp. 326-327) with some deletions and additions:

14. Absence of brow ridges.
15. Absence of cranial crests.
16. Thinness of skull bones.
17. Position of orbits under cranial cavity.
18. Brachycephaly.
19. Small teeth.
20. Late eruption of teeth.
21. No rotation of the big toe.
22. Prolonged period of infantile dependency.
23. Prolonged period of growth.
24. Long life span.
25. Large body size (related by Bolk, 1926c, p. 39, to retardation of ossification and retention of fetal growth rates).

Neoteny offers other bonuses besides an easy way to achieve rapid brain growth. These related or contingent features offer an astonishing variety of characteristics that contribute to our behavioral, physical and neurological make-up. These include upright posture, little body hair, small jaw relative to our progenitors, smaller teeth, longer life span and propensity to play and exhibit creativity and sexuality as an adult. The ability of our species to change maturational speeds in combination with art/sex as an accelerator in that process results in the unique species that we are today. But it is in revealing the biological process behind the change in maturation rates–what specifically physiologically causes maturation rates to change in humans–that truly whips away the curtain in this Wizard of Oz story of our unfolding. The biological engine (contrasted with art/sex as the social engine) behind maturational delay and acceleration is changing levels of testosterone.

Testosterone regulates maturation speed. It occurs on both a general and a specific scale. The story deepens.

General maturation speed is established at six weeks before birth and is based on the level of testosterone in the mother’s blood. Higher or lower levels influence males and females in the opposite direction. A mother with high testosterone will set her son’s maturation timer on slow, her daughter’s on fast. A mother with low testosterone levels will birth a son with fast maturation rates and a daughter moving at a slow pace. Environmental factors can have extremely powerful effects on mothers’ testosterone levels, radically amending maturation speeds. Autism and other disorders characterized by maturational delay or acceleration can and do result.

So testosterone can regulate speed through the mother setting the general speed of maturation. Testosterone can also regulate speed on a micro level, within a lifetime, by being increased or decreased within an individual. On obvious example is the eunuch. If testicles are removed before puberty, that individual will live several years longer than a normal male, in fact, as long as a female, because testosterone levels are dramatically reduced. In the West, this practice was used into the 20th century in order to create a permanent, ringing singing voice; in the East, it was used to provide a guard for the emperor’s females. The eunuch’s biological processes have been slowed down. Stress will increase testosterone levels and shorten life. Exercise will decrease testosterone levels and prolong life. As our testosterone levels change, so will the speed of our metabolism, which is connected to how slow or how fast we mature.

In humans, as we evolved from the primate progenitor over millions of years up and through homo erectus, sexual selection eventually utilized our ability to adjust maturation rates to increase brain size by slowing down our rates of maturation (by adjusting testosterone levels) in order to become better dancers and sound-makers to achieve more sex. It is a combination of these two processes–tightly focused sexual selection revolving around an aesthetic (also called runaway sexual selection) and neoteny (the ability of a species to unfold over time, prolonging its infant states into adulthood by slowing down maturation rates)–that propels humans through its unique trajectory.

The slow acceleration of the art/sex sexual selection feedback loop can be observed to become a high pitched fever and the center of society when the fossil record reveals males and females coming closer and closer in size. The specific change in dynamics toward the art/sex society was a shift away from the formerly necessary bonus of a relative larger male size and strength for the early hominid males to aggressively compete with one another. How do we know that this was the case? Fossil remains, though suggestive at best, reveal a high degree of sexual dimorphism between the emerging hominid male and female for at least 2 million or so years after diverging from our common ancestor with the chimpanzee. Australopithecus afarensis were highly dimorphic, with males almost twice the size of females. Homo erectus did not evidence this extreme difference. The similarity in size is due to the growth in the size of the female. Among living primate societies, increased sexual dimorphism is exhibited when there is violent intramale competition or frequent harsh contact with forces in the environment. These societies are usually patrifocal. It is unlikely that males and females would be mutually choosing each other as mates according to a physical aesthetic of dance and sound in a patrifocal society. A highly dimorphic social structure implies males physically threatening each other for procreation rights. The art/sex competition would only be a powerful dynamic in a promiscuous social structure where males cooperate to compete via the aesthetic as opposed to physical strength. Strength alone does not require brains. Survival of the fittest did not make us who we are.

And it is likely that the degree of focus in the matrifocal vs. patrifocal directions varied from band to band, region to region. The less hierarchical, dominant competition between males, the less necessary that males be large and strong and thus the less sexual dimorphism in the fossil record. And a single band or evolutionary pool may have evolved back and forth between the two poles over time. Eventually a line was crossed, probably more than once. Evidence that this crossing occurred is when the fossils show that males and females became far less sexually dimorphic–more similar in size. Matrifocal society had taken hold.

There are a number of repercussions and implications–not just in body size. When the first proto human bands transitioned into matrifocal tribes, we can assume that the environment was relatively benign or unthreatening because there was less need for the advantages conferred by sexual dimorphism. The life of the aesthetic could be concentrated on with less distraction. Bonobo chimpanzees exhibit societal relationships similar to what we’re discussing. The bonobo often pass band leadership through a dominant female’s son. Strong females are deferred to. Sex is a constant form of recreation and communication. There is little violent competition among males. Both males and females initiate sex. At the other great ape extreme are gorillas, where a single male dominates the band through the control of sex through a harem of females. Other males have sex opportunities, but at the leader’s discretion. In between these two extremes is the chimpanzee. Males compete for procreation opportunities by working their way up to alpha position, but all males can have sex with any willing female in estrus. Alpha males and their allies just get more access and at the times females are ovulating. Only males initiate sex. With chimpanzees and gorillas, food sharing is not associated with sexual behavior, whereas it is with bonobo. Predators have little effect on social structure in these three cases. Yet gorillas are highly sexually dimorphic because the most physically imposing males can more easily control sexual opportunities–intramale competition. Among bonobo where frequency of sex and amount of sperm produced determines paternity percentages, the males and females are closer in size. And, not incidentally, bonobo have a far larger penis and testicles than the gorillas, because their social structure demands these attributes. Bonobos exhibit maturational delay compared to both other chimps and gorillas and have larger relative brain size. Bonobos exhibit sexual swellings for 75% of their cycles as opposed to chimpanzee females showing these changes 50% of the time. It has been suggested that bonobo adult sexual behavior is not unlike the chimpanzee adolescent, an example of maturational delay, just as the bonobo skull is similar to the chimpanzee adolescent skull. Social structure determines relative size of the males and females to each other, relative size of male sexual organs and relative brain size. In the evolving human, with social structure having veered in the direction of a bonobo-like matrifocal highly sexed society, brain size (and likely penis and testicle size, though the fossil record is mum on this subject) was increasing at stunning rates with an extreme slowdown of maturation.

With the decrease in maturation rates and the continued revealing of more and more infant and child features into the adult phase of a species, there is an increase in both play and sexuality. Play and sex are closely related, with both evidencing themselves more frequently in the adult. In the course of just 20 years, experiments with foxes selected solely for signs of exhibiting tame behavior associated with maturational delay showed the females going into heat far more of the time. There is increased incidence of domesticity and playfulness when the same selective principle is applied to wolves as they morph into the dog. Again, note in the bonobo, compared to its close relatives the gorilla and chimpanzee, that evolution in a neotenous direction has resulted in dramatically increased sexuality. The female bonobo spends substantially more time in estrus than the female chimpanzee or gorilla.

There is another point concerning this social structure that best evidences the art/sex neoteny/brain size increase in hominid evolutionary trajectory. Male cooperative behavior relative to male competitive behavior is a feature of bonobo vs. gorilla social structures–matrifocal vs. patrifocal societies. A species that becomes more neotenous is inclined to become more cooperative. At the same time that art/sex is growing bigger brains and neoteny is reinforced, a host of other features (noted by Gould, Bolk and Montague) that contribute to individual and societal characteristics are also becoming evident. The ability of males to avoid the energy expended to control breeding opportunities by force and instead rely on neoteny to grow bigger brains to dance their way to the bedroom results in males with lower testosterone levels–males relatively comfortable around children. In a society where paternity is totally unknown and all descent is through the mother, low testosterone males are more likely to engender an environment where the infants and children are in less danger from male conflicts and infanticide, where males will kill infants not evidently their own. In addition, this is a society where males are more likely to hunt cooperatively because the high-testosterone, hierarchical posturing for position is far less evident. Many things are implied by this thesis, not the least of which is the reinforcement of cooperative behavior by the males of the species, creating further opportunities for the gathering of food, particularly foods high in fat needed to feed the evolutionarily expanding brain.


This entry was posted on Monday, November 17th, 2008 at 8:21 am and is filed under Art, Biology, Neoteny, Ontogeny, Play, Sexual Selection, Social Structure, Theory. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
2 Comments so far

  1. Evolution and Succession of Obsessions | Neoteny, sexual selection, cause of autism, human evolution, social transformation, left organizing and internet activism - how they all connect on October 6, 2009 7:38 am

    […] I’ve described two possible sources for human obsessional tendencies.  There is my story revolving around dance, song and gesture being the central focus and social matrix of Homo erectus and earlier hominid societies.  Geoffrey Miller in his The Mating Mind describes a runaway-selection feedback loop whereby hominid males and females select each other for features that compel procreation opportunities.  He and I have written about dance and song generating an exponential increase in brain size.  I have hypothesized that the Henry Jerison thesis of predator physical abilities always compelling a bigger brain than their prey applies to a dance-driven, sexual-selection feedback loop where adroit dancers are being reinforced to exhibit massive synapse production with no ceiling in escalation.  The brain mass created would be far more than is necessary to acquire food.  In other words, only aesthetics has the power to compel the kind of increases in brain size we’ve seen over the last three million years.  Only aesthetic-based obsession and compulsion, reinforced by a runaway-sexual-selection  feedback loop could evolve our species with such speed.  (Click here for more detail) […]

  2. Autism, performance and new communications technologies. | Neoteny, sexual selection, cause of autism, human evolution, social transformation, left organizing and internet activism - how they all connect on November 19, 2009 8:48 am

    […] and facile bodies, dancing up a storm to mate with discriminating members of the opposite sex (see Theory Summary), then perhaps performance consciousness is integral to who we were and what we are becoming.  If […]

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