Impact of evolution on human thought
For a long period of time humans could not answer the question of what makes a man, a man, and a women, a women. As was recently found main difference is not just in presence of genitals. Probably one will ask why? The answer is that genitals themselves are indirect products of another matter. The name of this matter is testosterone.
"Testosterone is a hormone that stimulates sexual development in male human beings. It belongs to a family of hormones called androgens. Primarily the testicles, a part of male sex glands, produce androgens. The ovaries in females and the adrenal glands in both sexes also yield small amounts of testosterone and other androgens (The world book encyclopedia)."
In its earliest crucial functions, in developing embryo, testosterone play the key role in telling the cells of the genetically male embryo to develop as a male. It is amazing, but all human embryo begins as females for the first few weeks of an embryo's life, a small group of cells have potential to develop either as ovaries or testes. About one million of genes that are needed to direct the development of a human being, one single gene (the "SRY" gene), which is carried on the Y chromosome, is responsible for determining the sex of the embryo. If the embryo's cells contain the "SRY" gene, the embryo will develop testes, which at some point and for a limited period of time early in its development produce and release a big amount of testosterone. It turn, testosterone then signal the cells of other parts of the embryo to develop as a male.
As shown in the study by a Stanford research group namely testosterone is responsible for formation of genitals. An experiment was held on newborn female rats. They were injected with testosterone. Surprisingly, but the female rats developed male genitals, and female genitals began disappearing. Later "converted" female rats started revealing purely male behavior, and they were fully aware of usage of their "new" male genitals. The same experiment was performed with male newborn rats. It is amazing, but their penises withered and later completely disappeared. This experiment is not ethical to perform on humans, but random facts suggest that all most the same can be done with humans. For instance, in lesbian couple, butches (female that represents male) use testosterone injections to develop male features such as deep voice, facial hair and muscles ("The He Hormone," Andrew Syllivan)
In girls, the adrenal glands begin to produce testosterone earlier than in boys. That's is how and why girls mature earlier, as a rule, than boys. It was testosterone that stimulated the growth of girls pubic hair and underarm hair (there are testosterone receptors in the skin of the pubic area and the skin of the underarm that are genetically programmed to react to testosterone by producing hair). And testosterone stimulated girls skin to produce more oil, contributing to the acne of girls early teenage years, but also to the healthy glow of the skin and the shine of the hair.
The medical book Reproductive Endocrinology by Drs. Samuel Yen and Rovert Jaffe, states; "Testosterone and other androgens have some biological activity o virtually every tissue in the body." Among the most important functions listed are "anabolic actions, such as stimulation of linear body growth, nitrogen retention, and muscular development." This statement means that testosterone works to keep the cells of the body functioning efficiently, making the best use of nourishment of growth and maintenance, and particularly contributing to the health of bones and muscles.
Having knowledge of the way testosterone function in the body, artificial testosterone injections can be used for variety of purposes. One of the purposes of artificial use of testosterone is to stimulate sex drive. The balance of this hormone is responsible for initiating and maintaining the production of sperm from early puberty throughout adulthood in male body. During childhood and adolescence, these hormones are responsible for:
Growth of genitals
Depth of pitch of the voice, increase muscle mass, and growth of bones.
Appearance of the body and facial hair.
Overproduction of testosterone caused by testicular, adrenal, or pituitary tumors in the young male may result in precocious puberty. Overproduction of testosterone in females, caused by ovarian and adrenal tumors, can result in muscessation of the menstrual cycle and excessive growth of body hair. Level of testosterone are low before puberty; they begin to increase at the onset of puberty and continue to increase during adulthood. Production begins to diminish at about age of 40, eventually dropping to about one-fifth of the peak level by age of 80 ("Hormone of desire," Susan Rako). Scientifically established men's testosterone, level is at least ten times higher than any of the women. According to the last available date, even male population accounts for less than 50 percent of the world's population, major part of violence is perpetrated by men.
Currently many males use testosterone shots or gel for the purpose of body-building. Testosterone shots trigger imbetterment of appetite and muscles growth. Besides, testosterone can have effects on the function of several brain areas. One additional surprising effect of testosterone is that it can not only affect the function of the brain but also its development and anatomy. Study in laboratory of Roger Gorskis at the University of California, Los Angeles, has revealed that the sex difference in the brain anatomy appears to be due to the presence of male sex hormones at a certain stage of brain development in male rats. Later work has shown a similar effect of testosterone at other brain areas: for example, the spinal nerve cells that control the muscles of the genital area are more numerous in male rats (cross sections through the hypothalamus of a male and female rats (male, (thinner and longer)).
Below is a human life example how hormone level can influence human health, and behavior brought by John K. Young in his book "Hormones; molecular messenger."
"Bridget was divorcing Bob, her husband of tem years, so she was not terribly surprised when she started to feel anxious and upset upon awakening in the morning. She grew worried, however, when these anxiety attacks became more frequent and stronger, coming upon her at all times of the day. She went to see a psychiatrist. The doctor also assumed the problem was stemming from her divorce proceedings and prescribed an antidepressant. After two weeks of more anxiety and no relief coming from the dedication, Bridget felt like committing herself to a institution. Her nervousness was now even making her hair fall out; her skin began to feel thin and bruised. She was not eating properly either, but was surprised to see she had lost over twenty pounds.
One night Bridget's best friend Virginia invited her to a dinner party. Over the meal, someone mentioned how traumatic divorce was; Bridget laughingly said that it was not making her thin and anxious, it was also thinning out her skin and hair. One of the other guests, a young woman, asked her to elaborate, and Bridget shyly told her the symptoms she had been suffering from. The young woman then introduced herself as a doctor. She asked Bridget to come down to her office for a few tests. Bridget did and these tests revealed what the doctor had first suspected; that Bridget was not suffering from a psychiatric disorder, but rater a hormonal one. Her thyroid gland was overworking, giving her the symptoms of thinning hair, thin skin, excessive weight loss, and anxiety. Afte4r treatment all of Bridgets' symptoms disappeared."
The example described above suggests that level of testosterone influence not only physical development of the body, but also is responsible for emotions and behavior. One researcher, curios about the fact that only male canaries sing during mating season, gave a testosterone shot to female canaries. The result was amazing, the female canaries "burst into song." The same experiment was performed on zebra finches. Dr. Christina Wang's study reveals that men with low testosterone level are irritable and aggressive than those with high-normal level. "When their testosterone level was increased during hormone-replacement therapy, their anger diminished and their sense of well-being increased."
Robert M. Sapolsy, in his book "The trouble with testosterone," takes opposite opinion side to scientists who think that increasing testosterone level leads to changes in behavior (such as becoming more aggressive). He holds an opinion that changes in surrounding environment trigger change in the level of testosterone.
"Okay, suppose you note a correlation between levels of aggression and levels of testosterone among these normal males. This could be because (a) testosterone elevates aggression; (b) aggression elevates testosterone secretion; (c) neither cause the other. There is a huge bias to assume opinion (a), while (b) is the answer. Study after study has shown that when you examine testosterone levels when males are first placed together in the social group testosterone levels predict nothing about who is going to be aggressive. The subsequent behavioral differences drive the hormonal changes, rather the other way around ("Trouble with Testosterone," pp. 152)."