Originally (in the beginning) all vertebrates look alike. Everyone is familiar with the tadpole-like images of early human fetal development. At the initial stages the embryo's rapid changes on the outside are matched by the rapid pace of changes on the inside. The DNA of each species calls the shots that will allow development along different paths. One of the major separations in fetal development is whether a fetus is destined to be an aquatic animal with gills or a lung-breather like us.
The pharynx, that part of the digestive tract that complains when we have strep throat or tonsillitis, is a shared feature of all animals with backbones. There must be some common inherited genetic code going way back, because the development of the pharynx gives hints of other animals during its fetal changes. Little pouches start to bulge out of the pharynx, and eventually they can rupture through to the surface. In aquatic animals, these "pharyngeal slits" persist to house gills. In air-breathers like us, these slits are only temporary, but their structures persist in other ways. For instance, the Eustachian Tube, that connection from the throat to the middle ear, is the remnant of the first of the pharyngeal slits. Rarely, when the second one doesn't close all the way, newborns can be seen to have an opening on their neck.
At one time, scholars in the field of Comparative Anatomy suggested that since the embryo of the human looks so much like the embryo of the lower animals and then goes on to develop into something altogether different (human), that the stages can tell the story of our evolution. This is not true, but it is easy to see how scientists made this misstep when beginning to study our development.
There are many changes that make us human--and very early on. But there is another forking of the road as well. As humans, we also differentiate into either male or female humans. Welcome to the concept of gender embryology. Too bad those early scientists weren't right, because they could have postulated the transformation of the embryo through the snake and snail and puppy dog tail stages to the male...and through the sugar and spice and everything nice stages to the female. Since truth is stranger than fiction, fasten your seatbelts, please.
All men are suppressed women!
Defining The Sexes: Baby Gender
But all developing fetuses do not become women thanks to a hormone called Muellerian Inhibition Substance. This hormone, produced by the testes, suppresses the fetus's natural tendency to go on as a female, holds the female structures back as mere wisps of tissue, while allowing the male structures to flourish. All one needs to have Muellerian Inhibition Substance is a good Y-chromosome, unique to males.
So this is why the mere flip of the coin is not a good metaphor. It doesn't just go this way or that. It tends to go the female way, and it can only go male if the female tendencies are actively suppressed with the right chemical. The lesson for all of the rib-counters out there is that it may be said that we all began as both men and women (embryonically), then we all began as women(biochemically), and then some of us go on as men.
Before the miracle of birth is the miracle of the fetus, the earliest form being a hermaphrodite being. If there's a Y-chromosome, then the testes will make Muellerian Inhibition Substance, the female will be suppressed, and the male will develop. Without a Y-chromosome, the testis cannot make Muellerian Inhibition Substance and the fetus goes on as a female.
So calling all men: As you yell for Honey to bring you that beer while tightly clasping the remote control, having left the seat up for the umpteenth time, use the commercial break at the two-minute warning to get in touch with your feminine side. It's there--and now all of you hardware-store-enjoying, stereo-component-rigging, underwear-on-the-floor guys know it. For every good man there's an equally good suppressed woman. It's in our designer genes.