Primate sexuality is based on estrus. The vagina is high on the rear for easy access by males, and the tissue in its vicinity often changes shape and color to indicate ovulation. Males use large canines and brute force to discourage other males and control females until they come into estrus, or for influencing females during their estrus. Females are attracted to strong males for their ability to protect them and their progeny. Genes for strength and aggressiveness equate with progeny viability.

The dopamine release from eating high-fat, high-energy foods is the reward and motivation to eat more. The dopamine release from orgasm is the reward for the effort of fornication and fertilization. The related muscle contractions facilitate ejaculation and sperm transport to the ovaries.

Over 4 million years ago a lemur like female primate living in a forested area with thick brush discovered she could trade sexual intercourse for food even when she was not in heat. Consequently, she did not have to forage for food, and expose herself and her progeny to danger during pregnancy or after birth, and her progeny were well-nourished. She had more and more viable progeny than other females, so the prostitution behavior spread with the procreation of her genes.

The males that brought the most food had more opportunities to procreate. Carrying food in hands and arms as well as mouth increased the copulation rate, so the males that could scurry the fastest on their hind legs over brush spread that ability with procreation. Despite having a long tail and grasping feet for tree climbing, female as well as male progeny came to walk on two feet.

Estrus and large male canines decreased in genetic value, and gradually disappeared.

Homo thusly differentiated from the rest of their primate family as "Ardi" (ardipithecus).

Where and when plant and animal food is distant or sparse, males were barely able to satisfy the sustenance requirements of one female and her progeny in addition to themselves, so males concentrated their efforts on one receptive female, who gave sexual preference to that one male as a reward for his performance relative to other males, which motivated monogamous pair bonding.

In the quest for more food for more procreation, innovative males developed strategy, and invented hunting and carcass division tools. Females invented gathering and food preparation tools. Intellect became more genetically important. Innovation requires imagination, which at least temporarily requires a subjective reality. Unfortunately, subjective reality can also lead to self-delusion. See Realities.

Brains enlarged. Pelvises enlarged to accommodate larger fetal heads to the point of skeletal inefficiency, then more immature births were required to accommodate larger brains, and females were burdened with longer rearing durations, which extended the male support requirement, and the need for monogamy.

Pairs with the higher dopamine, vasopressin and oxytocin release during orgasm stayed together longer, because sexual intercourse was addictive, and bonding was associated with pleasure. Their procreation success further selected for the orgasmic release of larger amounts of the neurotransmitters and hormones that motivate monogamy.

Females with a more forward vagina were able to make it less vulnerable to unrewarded sexual intercourse, so they were able to elicit more food in exchange for sexual intercourse, and thereby increase the likelihood of procreation success.  Consequently, the homo vagina migrated forward. Males with longer penises that could reach and deeply penetrate the forward vagina from the rear or front procreated more successfully. Consequently, human males have the largest penis in proportion to body size of any primate.

Human Sexual Relations

The heritage of exchanging material things for sexual intercourse explains why female humans who achieve many more orgasms and more intense orgasms than the males who facilitate that pleasure still insist on some form of compensation during the relationship for the sexual intercourse to continue. This is true even when pregnancy can be prevented with contraception or is impossible with older or otherwise infertile females, because exchanging sexual intercourse for things is genetic in human females.

Unfortunately, there is no politically correct English term for this behavior. The term "prostitution" is strictly defined as exchanging money for virginal, oral or other methods of stimulation that facilitate male orgasm. This fine distinction between accepting dinner, jewelery or flowers in exchange for male orgasm versus accepting money in exchange for male orgasm is more to differentiate monogamous sexual relations from sexual relations that are felt to threaten monogamy. One is admired while the other is denigrated.

While the amount of compensation corresponds to the method of stimulation in both cases, ejaculation into the mouth of a mate is sometimes valued more highly than doing so with a prostitute, because the female derives pleasure from copulation as compensation, but derives no such pleasure when performing fellatio. It is presumed that a prostitute derives no pleasure from any form of sexual intercourse.

Although it would seem that procreation is entirely driven by dopamine rewards, there is an intellectual component, because both sexes orgasm faster and more intensely when they suspect fertilization is likely. How far back in human evolution this cause and effect awareness extends is a matter of conjecture.

There is no visual cue for intellect on the male body. While intellect may manifest in conversation, only visual examples of the material wealth resulting from intellect like clothing, cars, jewelery, neighborhood and job can be used to initially attract females. Hence, intellect in young males who have yet to acquire wealth is of little value. Hence, young females will mate on the basis of physical strength and stamina, while older females may mate more on the basis of intellect as it is materially manifested with the increasing age of males. If sustenance, shelter and security as well as progeny genetic viability cannot be had in one male, females will merry the best provider while secretly procreating with the best genetic match. See Sex and Sexuality.

Material wealth as an indicator of intellect can be distorted by males who use force or its threat or fraud to amass wealth for exhibition to females. See Voluntary Society - Conditioning.

Sexual Differentiation by Role

Chimpanzees have sexually differentiated into hunters (male chimp monkey hunts) in order to trade meat for sex, while females sexually differentiated into gathers of edible plants. Hunters visualize in three-dimensions and quietly communicate with body movement more than gatherers. Gatherers have a finer sense of touch to nurture offspring in the dark, and verbalize more than hunters. Similarly, human males can read maps and represent three-dimensional objects and assemblies in two dimensions better than females, and females use more words and can engage more females in simultaneous conversations than males.