Imagine that to begin with, there was only one sex.
  This was because there was only one kind of mind. This mind had no space nor time sense,
  and was only aware of mass and energy. A brain that would develop such a mind would have
  no ganglia, but only a single whole brain neural network and its attendant biochemistry. 
  As creatures with brains of this limited nature
  evolved, those with greater processing capacity survived to pass on that quality down the
  evolutionary line. It may be that this would occur with organisms that had but a handful
  of brain cells. As long as the cells constituted a single neural network in a single
  biochemistry, a minimal system of response would exist. There would be memory, for this
  kind of brain could learn (based on the billiard ball example in the section above). There
  would also be pre-conscious, for this kind of brain would allow the organism to habituate
  or sensitize to various stimuli over time. There would not be, however, a subconscious or
  conscious, as there was no spatial or temporal capacity to anticipate or put things in
  context. 
  When it came to survival situations where context
  or anticipation would increase the odds, this simple organism could not take advantage of
  them. But, through random variation, it is not unlikely that some mutated variety of brain
  might develop more than one neural network, and in so doing, enjoy two new dimensions of
  awareness: space sense and time sense. 
  At this stage, the creature's mind would be
  perfectly balanced between the two appreciations. Through natural variation, a creature
  might develop lobes of the brain that favored space and time appreciations respectively.
  So, although it could now employ experience and apply it to the future, when situations
  arose where the context was at odds with anticipation, the creature's mind would go into
  brain-lock. This poor organism could not longer respond to immediate stimuli, and was
  trapped between what the bigger picture of surroundings indicated versus what the
  progressive direction indicated. Like a deer, frozen in the headlights of a car, this
  early thinking machine would stand in the road until it was run over, or more likely,
  eaten. 
  Through random mutation, some creatures might have
  more or simply more efficient neurons when it came to producing excitatory
  neurotransmitters or inhibitionary neurotransmitters. (Both are natural requirements for
  the model explained in the section above.) Those that had more exciters favored the
  neurology. Those that had more inhibitors favored the biochemistry. As a result, these
  minds were no longer balanced, but could actually pay more attention, with more accuracy
  to one appreciation or the other. 
  The downside of this kind of split, is that while
  more attention is being placed in one direction, it is being deprived from another
  direction. So, each kind of mind could be "blind-sided" in the area it saw less
  clearly that its adversary. Those creatures that tended to group together in binary pairs,
  would have a greater chance of survival than an unbalanced individual alone. Each could
  see clearly into the other's blind side, and together they could peer more deeply into
  space and time, context and anticipation, than any single organism could by itself. 
  The more specialized the minds of the bonded pair
  became, the more successful the survival value. But, of course, this required the
  organisms to be attracted to one another, for if they were not, their line died out in
  favor of those that were. Spatial and temporal creatures became attracted and bonded
  together for survival. 
  The mechanism for attraction was sensory
  stimulation that indicated an opposing bias in neurotransmitter production. The sex
  hormones Testosterone and Estrogen each have an impact on the production of
  neurotransmitter. Testosterone triggers the creation of more exciter neurotransmitters
  (such as Seratonin). Estrogen triggers the creation of more inhibitor neurotransmitters
  (such as Dopamine). In addition, each ganglion of the brain has "L" cells and
  "R" cells, which are suspected of producing exciters and inhibitors. 
  Let's jump ahead to the human species and see where
  things ended up. At the twelfth to fourteenth week of pregnancy, a flush of Testosterone
  either floods the brain of the developing fetus or it doesn't. If it does, for a two week
  period the brain is bathed in exciters. If it doesn't, the brain is bathed in a greater
  ratio of inhibitors. This hormone wash affects the production efficiency of the L and R
  cells. During this time, the lifetime efficiency of these two kinds of cells is
  established by the wash and set into the cells. From this time forward, the biochemistry
  in the ganglia will favor Seratonin or dopamine - space or time. 
  The hormone wash recedes at the fourteenth week.
  The newborn human, therefore, is almost nearly balanced in appreciation of space and time,
  with only a slight bias toward one or the other. This helps the youngster who is below
  reproductive age to function as efficiently as possible as a balanced individual in behalf
  of its own survival. 
  Once the infant reaches the age of puberty,
  hormones in the body simultaneously do several things. Secondary sexual characteristics
  show up that make the spatial and temporal minds more physically attractive to each other
  so that they will bond. Reproduction is enabled. And, a second flood of hormones enters
  the blood stream and the brain to tip the nearly balanced mind heavily in favor of space
  or time appropriately. 
  All of these well-timed functions serve to ensure
  the greatest chance for survival of the young, and the greatest chance for survival of the
  species as a whole. 
  Before society, men were men and women were women,
  for any other arrangement, deviation, or lack of bias led to a lessened chance for
  survival. When society grew naturally as a fractal reflection of an organisms natural
  organization, it was based primarily on a structure, as opposed to a flexible, dynamic
  system. This is in line with the immediate survival needs of the society. 
  Such a structure would best survive if spatial
  thinkers were motivated to employ spatial skills and temporal thinkers to employ temporal
  skills. And, since the functions of child-bearing and territory-taming had fallen into
  line with the mental bias of the two species of organism, it was quite natural that those
  would be the jobs incorporated into the society's structure. 
  To motivate each species to function according to
  its capabilities at the greatest efficiency, rewards and punishments evolved that were
  appropriate to each kind of mind. The rewards functioned as exciters and the punishments
  as inhibitors. 
  This worked fine for several thousands of years.
  The initial heavy bias created an inverse bell-curve that was so spread to the two sides
  (space and time) that it was almost truly binary. The world was inhabited by spatial
  thinkers who were more or less alike, except for experience, and temporal thinkers, who
  were equally cut from a single cloth. But, as society became so successful at its task
  that it moved farther and farther away from immediate survival needs, those natural
  variations in the bias toward space or time had a better chance for survival. The trough
  in the bell-curve began to fill in. Same sex partnerships began to find ecological niches
  in society where they could prosper. Some women were born with a bias still temporal, but
  more toward the spatial, and some men born vice versa. 
  In the present day and age, there are more
  individuals in the trough than ever before. Some are even born to the bias opposite to
  what their physical sex would indicate. Many of these are unhappy when puberty hits
  because the meatball is stewing in the wrong juice, so to speak. Even those that are born
  to the bias consistent with their physical sex may be so close to the center that the
  rewards and punishments offered by society are no longer appropriate and fail to function
  as intended. Spatial women tend to become feminists, because as spatial thinkers, they are
  more interested in territory. Temporal men tend to become cross-dressers, for as temporal
  thinkers, they are more interested in environment. 
  But, this is all part of the natural evolution of a
  society. The rigid nature of the roles provided is essential during survival times, but
  hard to change as the society moves more toward information and relationship than physical
  organization. Structures cannot really be changed, but must be dismantled and reassembled
  in a new form. An information society requires a more flexible form, more like a Rubik's
  cube, where it always maintains its integrity as a cube and corners always remain corners,
  but the arrangements in which it might be manipulated can bring an astronomical number of
  variations to bear. 
  And this is supported by our bodies as women are
  growing in height as much as an inch per generation, puberty ages are dropping
  consistently, and the average testosterone level in men is dropping as much as twenty-five
  percent per generation. All of these things lead us through an evolutionary period in our
  society in which the spatially/temporally balanced individual is more suited to the new
  tasks of our times than the highly biased men and women of old. 
  So, men and women: they aren't what they used to
  be. But that is as right for our time as being almost binary was for theirs.