To clearly understand the present one must look into the past and try to understand why things are shaped the way they are. First let's take a look at the two-sex system. Deborah Blum, in her book Sex on the Brain asks, "Do we really need two sexes? It is worth asking only because sex is so fundamental to life on earth, to who we are now. For species to evolve and prosper self-productive sex should be adequate. But in nature and biology, everything that is living belongs to one sex or the other. Two sexes are a part of life, and frankly nothing can be done about it, so why not concentrate on how to live with it?
In the beginning of time, the earth was just a partially cooled rock flung off from a distant star. Scientists believe that the oxygen rich environment that we live in now was the direct by-product of plant life. The plants breathe in carbon dioxide and give out oxygen, which we essentially need to survive. The planet conditions were harsh, many species of single-celled organisms survived only because they somehow developed a system of transferring genes. If one genetic combination was vulnerable to the environment, then another was less so. If two organisms, vulnerable ones, could mix their genes, each stood a chance of developing better defenses.
It was the union of two different genetic packages, the theoretical beginning of sex. Since life does tend to be efficient whenever possible, two "parents turned out to be adequate, as opposed to a reproduction system involving 14 different partners. So there we have it: biology's rule, as far as reproduction, is that there must be two. It is the truth for roaches as well as people, for fish as well as cats. Anyone with the energy can have countless sexual partners, but reproduction requires only one.
After careful delegating the dual-sex system of biology to the multiplication of single-celled organisms for better survivabi