In elementary school, "girls" tend to be physically & intellectually dominant, often literally taller & larger than "boys." "Men" are all johnnies-come-lately.
Stephen Jay Gould speaks of "a single ground plan" in human biology in "Male Nipples and Clitoral Ripples," _Bully for Brontosaurus: Reflections in Natural History_ (1991):
***** If you are committed -- as Erasmus [Darwin, grandfather of Charles] was, and as a distressingly common version of "pop," or "cardboard," Darwinism still is -- to a principle of pervasive utility for all parts of all creatures, then male nipples do raise an insoluble dilemma, hence (I assume) my voluminous correspondence [from the readers puzzled by the existence of male nipples]. But as with so many persistent puzzles, the resolution does not lie in more research within an established framework but rather in identifying the framework itself as a flawed view of life.
Suppose we begin from a different point of view, focusing on rules of growth and development. The external differences between male and female develop gradually from an early embryo so generalized that its sex cannot be easily determined. The clitoris and penis are one and same organ, identical in early form, but later enlarged in male fetuses through the action of testosterone. Similarly, the labia majora of women and the scrotal sacs of men are the same structure, indistinguishable in young embryos, but later enlarged, folded over, and fused along the midline in male fetuses.
I do not doubt that the large size and sensitivity of the female breast should count as an adaptation in mammals, but the smaller male version needs no adaptive explanation at all. *Males and females are not separate entities, shaped independently by natural selection. Both sexes are variants upon a single ground plan, elaborated in later embryology. Male mammals have nipples because females need them -- and the embryonic pathway to their development builds precursors in all mammalian fetuses, enlarging the breasts later in females but leaving them small (and without evident function) in males.* (emphasis mine) *****
Moreover, in "Hyena Myths and Realities," _Hen's Teeth and Horse's Toes_, Gould writes of a male course of development as a "modification" of the "female course of development...biologically intrinsic to all mammals":
***** Mammals share a common pattern for the embryology of sexual organs, and we may therefore use humans as an example. The early embryo is sexually indifferent and contains all precursors and structures necessary for the development of either male or female organs. After about the eighth week following conception, the gonads begin to differentiate as either ovaries or testes. The developing testes secrete androgens, which induce the development of male genitalia. *If androgens are absent, or present at low levels, female genitalia are formed.*
The internal and external genitalia develop in different ways. For internal genitalia, the early embryo contains precursors of both sexes: the Mullerian ducts (which form the Fallopian tubes and ovaries of females) and the Wolffian ducts (which form the vas deferens -- the ducts that carry sperm from the testes to the penis -- in males). In females, the Wolffian ducts degenerate and the Mullerian ducts differentiate; males develop by the opposite route.
The external genitalia follow a markedly different pattern. Individuals do not begin with two distinct sets of precursors and then lose one while strengthening the other. Rather, the different organs of male and female develop along diverging routes from the _same_ precursor. The male's penis is the same organ as the female's clitoris -- they form from the same tissues, are indistinguishable in the early embryo, and follow different pathways later. The male's scrotum is the same organ as the female's labia majora. The two lips simply grow longer, fold over, and fuse along the midline, forming the scrotal sac.
*The female course of development is, in a sense, biologically intrinsic to all mammals. It is the pattern that unfolds in the absence of any hormonal influence. The male route is a modification induced by secretion of androgens from the developing testes.* (emphasis mine) *****
Both from evolutionary and embryological points of view, we must proclaim: "One is not born a man, but rather becomes one" (to paraphrase Simone de Beauvoir).
evolveanglerfish,
Yoshie