Or Why there are two sexes?
Or why the sexes are different?
As you know, evolution shapes it all. We can refer to this non-existent ethereal concept as a person or object, because it works out things mathematically, and often we can 'predict' in biology with evolutionary theory. We can do some armchair theorizing also, with the help of TOE (Theory of evolution).
How sexes evolved
As you already know, at first there were asexual organisms and asexual reproduction. The progeny just resembled the parent, and the evolution and adaption to the environment was slow. When the gametes of two organisms exchange their genetic material, the offspring carries different combination of the species' genes, and thus, the population comprised individuals with different characters, which let the evolutionary force select the fitter ones. Thus when sexual reproduction appeared in the scene, it had advantage over the asexually reproducing ones, and they out-populated them soon. The advantage was worth the pain of evolving the complex mechanisms for this purpose, but as seen repeatedly, nature (or evolution) can go to any length, for a survival advantage.
Why sexes are different
The sexual reproduction only requires that the offsprings are produced by exchange of genetic material between different individuals. Not between two sexes.
Then why there are two sexes? Because there are different sex chromosomes? The sex chromosomes are different because, they are needed to produce the differences in male and female, not the other way.
In the beginning, indeed there was no different sexes. Any individual could exchange the genetic material with any other of the same species. (You can call all individuals were unisex or that all were true hermaphrodites. All chromosomes were same in all individuals. There were no sex chromosomes.
The advantage of different sexes was in the sea, where life was thriving then. The organisms used to move in the sea, either carried by the current or by swimming with own power. The chance of meeting another individual of same species, and then mating and passing one's genes to next generation before one's death was a big thing then. The organisms adopted all possible strategies to achieve this. One organism could increase it's chance mainly by two strategies. Either moving fast and far, or by becoming big and easily seen. So, different individuals of same species adopted these strategies, and both were mutually exclusive. To move fast, one has to be small, and to be easy target, one has to be big. There cant be intermediate organisms. So all species evolved their members into two distinct sets, one big and one small. They had to have distinct genetics, distinct genetic control mechanisms (steroid hormones control the expression of genes) etc. So, sexes evolved to evolve members of different characters, especially sizes. As the species evolved, and required different characters in different enviornments, the difference between sexes also kept varying, and adopting.
Why the sexes have different characters?
Our steroids make us different not only in sexual organs, but all over physically, on our muscles, shape, psyche etc. These differences are required for the differing needs of the different strategies of two sexes.
To this date, we have retained some of the original characters of sex differences, male gametes are small and fast and female ova are big and non-motile. Only that the original character has moved from the individual to it's gamete!
The individual itself also retains these original characters. The females are stationary, like to attract males, be the target. Males run around, eying any female at sight, and are involved in temporary relationships! We still carry the characters of those small insignificant creates in the sea!