The most primitive of multi-celled creatures may reproduce both by conjugation or fission. However those that produce by conjugation survive many more generations than others reproducing by fission simply because then genes that develop errors can be ignored and deleted. On the other hand organisms reproducing by fission cannot maintain the genetic integrity required to perpetuate more complex multi-celled creature. The most primitive of the conjugating organisms can play the role of a male or female according to convenience. With further evolution the role gets segregated into distinct male and female characteristics.
The need for gene repair is the primary reason sex evolved. A second reason for encouraging sex is that it leads to the development of societies and that is essential for evolution of different forms of life. For evolution it is necessary to infect large populations living in societies to affect a genetic change. A third reason is that in more complex life forms, nurturing the young requires assistance of both the father and mother. Complex societies also need variety that sexual reproduction produces rather than a series of identical clones that mere division by duplication would produce.
Asexual organisms, which cannot pair-up their chromosomes to edit them, accumulate mutational damage in a process of irreversible genetic decay biologists call Muller’s Ratchet, a progressive loss of genes that leads to eventual extinction. men and women differ by 1 percent of their genomes. Males, by contrast to females, have only one copy of this X chromosome, not two. The other chromosome of the pair in males is called the Y chromosome and is much smaller than the X. There are about 80 genes in the human Y chromosome. It is the key in determining sex of the child. Sex provides the opportunity for DNA strands of the male and female to be compared and therby edit errors. The Y chromosome on the other hand carries a duplicate set of genes that repair by a comparison within itself.