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Posted By Robert DePaolo On November 19, 2010 @ 12:56 pm In Psychology | No Comments


 In the course of human evolution a sequence of occurrences led to homo-indigenous traits and behaviors regarding sexuality, gender interactions, child rearing and the origin of a family ethic.  The evolutionary changes appear to have been congruent, in the sense that if one occurred and others had not the changes might well have been maladaptive. Whether these changes  occurred simultaneously or in some proximal sequence is unknown. Nor is it apparent which change occurred first, or what were the time frames in which they emerged.  Even more unclear is the issue of when these traits and habits were first incorporated into social mores and human values. It presents a puzzle with regard to human evolution and invites speculation. That provides a pivot point for the following discussion.


Before beginning the discussion, it might help to point out that “evolutionary congruence” is an arbitrary term used here for purposes of making a point. It is not a scientific term and it is not the same as “co-evolution” – the latter of which denotes an evolutionary trend whereby two separate creatures or biochemical entities evolve in ways that lead to a cooperative, adaptive interaction. (Futuyma & Slatkin, 1983), (Thompson, 1994). Congruence refers to the fact that in human evolution certain traits and behavioral/emotional dispositions had to occur either simultaneously or in close temporal proximity for them to jell into adaptive sexual, parental and social-familial dynamics.

 Describing these aspects of human biosocial evolution makes it possible to assess not only how congruous changes in the brains of infants, the socio-sexual habits of parents and cognitive  byproducts of such evolutionary phenomena occurred, but also to see how disastrous the human condition might have been had these changes occurred in irregular or temporally disparate sequence.

The first step in discussing evolutionary congruence will be to describe the pieces of the puzzle. The second step will address cultural change emanating from biological/evolutionary phenomena.





In the evolutionary transition from hominids to humans, the brain-to-body ratio increased dramatically. Just why this occurred is not clear. Some have opined that human brain expansion resulted from the shift to a meat diet (McBroom 1999); a trend that might have begun with hominids like homo habilis, whose brain mass was considerably greater than that of the australopithecine bipeds. Another idea put forth suggests brain expansion resulted from a nomadic life style made possible by a heat resistant bipedal posture, an enhanced venous cranial cooling system and a concomitant need to adapt to varying environments via anticipatory thought and decision making skills. (Falk 1986). Still another theory holds that specific pressures from the environment – particularly with respect to the need to secure meat via hunting skills – led to a gradual expansion of the cerebral cortex, a part of the brain that integrates inputs and can thus enhance focusing, aiming and throwing skills (Calvin 1990).  Another hypothesis is that brain expansion was the result of neoteny, a process in which development continues well beyond the previous endpoint of maturation, due in part to hormonal changes resulting from heavy and continuous exposure to sunlight (DePaolo 2009). Whether any of these theories is correct it is clear that the hominid and subsequently the human brains did expand rapidly by evolutionary time scales.


As beneficial as brain expansion might have been, it also created problems.  For the offspring of  large-brained humans to inherit this trait meant their brains had to be larger at birth. That meant human females had to evolve an expansion of the pelvic cavity and girdle. That did occur, and to a maximum degree. In fact any further expansion of the human female pelvis would have led to a virtual incapacity to walk – a notion often referred to as the “Obstetric Dilemma” (Merry, 2005). The question is; did brain and pelvic expansion occur simultaneously? If so, the process would have been congruent. If not, it could have led to an extraordinarily high mortality rate for both infants and mothers and threatened continuity of the species.

Just why pelvic expansion occurred is also unclear. One precipitating factor might have been a permanent change in estrogen levels resulting from prolonged exposure to sunlight. In any case, since the human species is alive and well, one can assume pelvic expansion did not occur in random Darwinian fashion. It’s possible that some sort of bio-chemical trigger caused both pelvic and brain expansion to occur at roughly the same time.

That notion will probably seem uncomfortably deterministic to a true-blue advocate of natural selection, perhaps even Lemarckesque. On the other hand, some biologists have suggested that the chance factor in evolution presents an improbable scenario because there was simply not enough time or skewing of bio-information to produce a wide variety of life forms (Morris 2005),  (Hardin 1961), (Halstead 1980).


Once pelvic expansion occurred, other factors came into play. When an infant is born with a large brain, the probabilities of its neurons being functionally connected at birth are slim. Thus it would be born virtually helpless.  Human mothers would then have to devote more time to the infant, not just in terms of physical exertion but also in anticipating the needs of a non-verbal, non-ambulatory creature through an enhanced empathic ability. Such “prolonged concern” would often leave the mother drained physically and emotionally. Thus the energy expended on child rearing for the mother would have been exorbitant.

Nature did provide a partial solution to the mother-and-child problem. Other than with the first generation of highly encephalized infants, mothers would also have had a large brain with vast interconnections. Since the sheer volume of that circuitry went functionally beyond the pragmatics of perception, motor control and language, some of the added brain tissue was freed up to observe and monitor other parts.  Much of the excessive brain tissue came in the form of frontal lobe expansion. Due to its functional detachment, it had the advantage of being able to engage in rehearsals, ie internal playbacks and a re-structuring of actions and experiences – a process now referred loosely as “operational”, or imaginative thought. This is not to suggest the frontal lobes are singularly responsible for imaginative thought. It is to suggest that due to their vast connections with other parts of the brain, the frontal lobes can provide an inhibitory version of experiences arising from limbic, cerebellar and cortical areas. 

Such covert, inhibitory mental skills would have been easily transferrable to the social and parental domains. As a result, a mother would have a capacity to “feel” for someone else, particularly her infant.  Yet this too had a sensitive time table. Had offspring developed brain expansion prior to the mother (in other words if singular mutations occurred among infants in chance fashion, rather through direct inheritance) the results could have been disastrous. Lacking sufficient brain mass, the mother would not have had the empathic ability to care for her helpless child. Once again, that could have led to a high infant mortality rate.

In that context one might presume that a proximal time table provided congruence in the co-expansion of the mother’s and infant’s brain, whereas chance might not have allowed for such an evolutionary agreement.




Since a late-developing infant is difficult to care for it sure would help if the father could get involved, particularly when it came to protection, procurement of foods and the provision of shelter. This could have been done through a male-female division of labor or in complementary fashion, whereby he did what she could not do at the moment and vice versa.  That’s where the pelvis came into play.

In order to expand to its birth-size during pregnancy (and not grow so large as to produce injury to internal organs) the pelvis had to be larger to begin with. In other words, women needed a gestational/morphological head start for this mutation to work. For that reason the human female pelvis became permanently enlarge. One byproduct of this was that an enlarged pelvis sends a signal to males that females are in a state of estrus. Thus to males it began to appear that mating season was year ‘round.

Sexual attraction was not the only lure in keeping males around. Certainly a female who appeared to be in permanent estrus would have provided a compelling reason for the male to stick around, especially once the female’s breasts also became permanently enlarged (another sign of fertility in the primate world). Yet since the male’s brain had also expanded frontally and beyond functional necessity it too could monitor, mimic and empathize.  Knowing that he had such a capability the female could submit cues and prompts designed to stimulate both the male’s sexual impulses and empathic inner core in what amounted to a two-fold family preservation strategy – she now had at her disposal a “feminine mystique.”  The seeds of pair-bonding were thus planted.  As a result the odds on infant survival improved.

That didn’t completely resolve the issue, because a complex sexual dynamic would inevitably arise from this. While males would tend to react primordially to the females’ enlarged breasts and pelvises the female was, despite her evolutionary make-over, not always in estrus. It presented a conflict for both him and her and one that had to be resolved for the pair bond to endure. One way this could have been resolved was through the excess brain tissue that allowed each to put themselves in the other’s shoes. At times she might not be interested in mating (due to lowered hormonal levels), but she could draw on her brain’s frontal lobes and its vast connections with emotional components of the brain to “do it for his sake.” If he got pleasure, she got pleasure – albeit in a vicarious sense. By the same token, the male could draw on his “frontal reserves” to accommodate her in various ways. As a result of the capacities for compromise, empathy and agreement, a kind of emotional contract was implicitly ratified. More interesting, and more congruent, was that the same trait enabling mothers and fathers to feel for one another was transferrable to their offspring. In other words excessive frontal brain tissue in effect paved the road to the human family and arguably to modern civilization.

That brings yet another aspect of congruence. It revolves around the question of whether frontal brain expansion had to occur in close temporal proximity to pelvic and breast expansion for it to be adaptive. Once again, that implies that a causal or lawful process overrode chance in the evolution of humans and of the family unit.


In modern times compromise is not something one takes for granted. Due to the confusion resulting from the female’s fertile appearance and her actual receptivity the same was probably true in the earliest negotiations between men and women. To empathize, resolve conflict and formulate agreements requires enhanced communication skills. While most primates resolve conflict through physical means, by hugging, posturing, aggressing and copulating, that might not suffice in a human pair-bonding situation. Somehow the noise making capabilities previously enhanced by lowering of the larynx and neocortical associative and integrative capacities resulting from brain expansion had to be fine-tuned.

Language is obviously used for other than mating purposes. For example it would be an important strategic tool in hunting expeditions. Yet while human hunters have always communicated with one another in pursuit of prey it has most often taken the form of quiet, signals combined with manual gestures. It has none of the trappings of a sexual dialectic. To communicate in a sexual context requires a weighing of ideas, a sense of the other person’s point of view, a capacity for logic (even if self-serving) that could lead to an apportionment of wants, needs and emotions. 

Miller (2000) has proposed that sex was a prime factor in the evolution of human speech, that a human male has to express himself to pass muster and ultimately gain sexual favors because the female uses his language as a measure of fitness and sense of commitment. Given the various ways language is used, for example for self-prompting, enhancing attention faculties, guiding motor activities and stimulating imagination that might be too simplistic. Yet one can assume since most skills acquired through evolution must ultimately pass the fitness test by improving the chances for survival and propagation sexuality might have provided an evolutionary ratchet. 






Language is a funny thing. On one hand, so voluminous, on the other so structurally simplistic. It has a basic format from which we seldom deviate. This format provides a template for all manner of expression, whether poetic or mundane. It is called the sentence. While distinctly human it is not at all mysterious. Language is merely a symbolic way of reflecting what goes on in the natural world. It has subjects (creatures or things) predicates (actions) objects (things or creatures on the receiving end of those actions) and descriptions of those objects and actions. Certainly there are connecting parts such as conjunctions and prepositions but those merely provide bridges from one idea to the next.  In other words human language is simply a way to describe who-is doing-what-to-whom (and how). In some sense, all creatures have such a cognitive sequencing capacity – even if they lack the vocal control to express it in the way we do. For example lionesses on the hunt have to conceptualize their activity via an implied sentence structure in order to succeed. Their cognitive grammar might consist of the following sequence… Zebras are running in that direction. Some are very fast. We see a slow runner. Some of us chase the slow runner. Others surround it. We nip at its legs to prevent it from running away. We bring it down. We cut off its air supply with bites to the neck. It stops breathing. We eat.

Language has such generative potential that it can be used to create an entire culture, as it did for homo sapiens.  Yet due to its simple structure it can be transmitted to many people so that alphabets, contractual laws, delineations of crimes and punishments, religious beliefs can be developed and implemented. Most of all, it enables large groups of people to live together in communities, which potentiates production, industry and prosperity. The latter effect is most intriguing, because prosperity changes the game, especially with regard to pair bonding, parenting and economic functions.  

Some have argued that with cultural advancements, increased prosperity and cultural diversity  habits pertaining to pair-bonding, parenting, child dependency and group cohesion will tend to change. In many instances one can expect wealthier individuals and families to detach from dependent relationships. That might explain why the question of whether humans are inherently monogamous has never been answered definitively. Extra-marital sex has always been a fact of human existence. So have bigamous practices, monogamy and various other relationship formats. In answering that question is might be most accurate to say that while   evolution disposes us toward certain traits and behaviors they can often be converted to other uses, depending on environmental conditions.



Answers to the question of why human evolution seems to feature congruence come hard.   The parallels among human anatomical and behavioral changes are so profound and interwoven that unless they could be assumed to occur simultaneously or contemporaneously,  some force other than chance might be presumed to have had a hand in the process. Just what that process might be is difficult to conceptualize at this point. Is evolution an algorithmic process which puts limitations on anatomical traits and behaviors, thus presenting a problem similar to that faced by the quantum physicist, i.e. having one view of nature based on ostensibly empirical evidence, but at the same time knowing that lurking beneath is some “quantum-like” reality that in its own probabilistic way governs the order of the biological world? Given the scope of that question, it might become necessary at some point to abandon the mutually exclusive tenets of creationism and science, and instead seek knowledge of a new factor (be it physical or metaphysical) that overrides chance and entails an a-priori model of “good fit” by which it guides the evolutionary process.














Futuyma, D.J. & Slatkin, M (1983) Co-evolution Sinauer Associates, Sunderland, Mass.

Thompson, J.L. (1994) The Co-evolutionary Process, Chicago, University of Chicago Press

McBroom, P (1994) Meat Eating was Essential for Human Evolution. Public Affairs, University of California, Berkley News Release

Stanford, C.B. (1999) The Hunting Apes:  Meat Eaters and the Origin of Human Behavior. Princeton University Press

Falk, D. (1986) Evolution of cranial blood drainage in hominids: enlarged occipital/marginal sinuses and emissary foramina. American Journal of Physiology and Anthropology 70; 311-324

DePaolo, R. The Evolution of the Human Brain: Dilemmas and Possible Solutions, Article in Scienceray Dec. 29, 2009

Merry, C.V. (2005) Pelvic Shape-Mind:  Primary Cause of Human Evolution. Trafford Publishing

Halstead, L.B. (1980) Museum of Errors. Nature, Vol. 288. P 208

Morris, S.C. (2005) Life’s Solution: Inevitable Humans in a Lonely Universe. Cambridge University Press

Hardin, G.H. (1961)  Biology: Its Principles and Implications. Freeman Press  p. 251

Miller, G (2000) The Mating Mind:  How Sexual Choice Shaped the Evolution of Human Nature. Doubleday/Heinemann

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