Evolutionary Selection of Testosterone Copyright 2004, James Michael Howard, Fayetteville, Arkansas, U.S.A. It is my hypothesis that mammals evolved because of increases in dehydroepiandrosterone (DHEA) (Hormones in Mammalian Evolution, Rivista di Biologia / Biology Forum 2001; 94: 177-184.) I think increased testosterone in mammalia produced the primates (Rivista di Biologia / Biology Forum 2002; 95: 319-326) which culminated, as a result of further increases in testosterone, with humans (Androgens in Human Evolution, Rivista di Biologia / Biology Forum 2001; 94: 345-362.). I suggest estradiol and testosterone direct the use of DHEA. That is, target tissues of estradiol and testosterone are affected by these two hormones to increase absorption of DHEA. My principal hypothesis is that DHEA optimizes replication and transcription of DNA. Therefore, DHEA affects growth and development and later, maintenance, of all tissues. I suggest testosterone increases the use of DHEA more than estradiol, therefore, testosterone produces more robust growth. Men produce more testosterone than women; men grow bigger testosterone determines growth and development of tissues more robustly than estradiol. This robust growth produced the characteristics called "male" from the same primordial structures that produce "female." Heat increases testosterone formation. (I anticipate your reaction here, please wait for the full explanation.) In a study of the effects of exercise-induced increases in body heat, it was determined that plasma testosterone increases 33% while sperm counts were not affected (Med Sci Sports Exerc 1984; 16: 51-5). I suggest the accident that started this selection was connected with the loss of part of one X chromosome. This may have produced individuals that increased testosterone at the expense of steroids down stream. This would have been maximized in mammalia due to constant body temperature. This would have produced "male" offspring of high testosterone. In my work with testosterone in evolution, it has become clear to me that excess testosterone is detrimental. For example, increased testosterone reduces the immune response and wound healing; very negative characteristics. Individuals of high testosterone are at a selective disadvantage. Therefore, the second "accident" of evolution, external gonads, would be selective in reducing excessive testosterone levels. I am aware that undescended testes produce sterility, that is, poor sperm counts. It is my hypothesis that excessive testosterone reduces sperm count. This characteristic is currently being studied. That is, testosterone is currently being considered as a male contraceptive because excess testosterone decreases sperm count. Early in evolution, those individuals without external gonads would be sterile, I suggest as a result of their increased testosterone resulting from increased body heat. I suggest evolution selected those individuals whose gonads were external because of the increased growth and development which, eventually produced sperm in greater quantities, among other characteristics, and whose testosterone was not so detrimental. I suggest increases in testosterone produced the characteristics of maleness as well as those of primates and, eventually, humans.