The male and female sexes are ancient, having emerged on the evolutionary landscape more than a billion years ago. This is much older than humans, older than most plant and animal species, older than most marine life, and even older than the brain itself.
More than just keeping an individual alive, as food and water do, the two sexes keep entire species alive by producing more genetically unique individuals through sexual reproduction: the mixing of genomes and the fusion of sex cells called gametes. The evolution of male and female sexes is a major reason why the diversity of plant and animal species exists. It’s why we humans exist. It’s why you exist.
More than 99.9 percent of animal species that have developed since the emergence of male and female sexes reproduce sexually. And 95 percent of those animal species—including humans—have male and female sexes in separate individuals, whereby organisms are either male or female for their entire lives.
It may be surprising, but the two sexes follow a universal biological definition that applies to all species with male and female systems: the male sex is the phenotype (or structure) that produces the smaller gametes (i.e., sperm), while the female sex is the phenotype (or structure) that produces the larger gametes (eggs). The sperm are numerous and fast, contributing half the genetic material of the parent, but no resources for the survival of the fertilized egg (zygote). The eggs are relatively few, and very slow, contributing half the genetic material of the parent and all the resources for the zygote’s survival. Combine these two different gamete types together and a genetically unique individual is formed.
The technical term for this system is known as anisogamy (from the Greek aniso, meaning unequal; and gámos, meaning marriage), which involves the fusion of two gametes with different size and form. It is so efficient for reproduction and producing genetic diversity that it has evolved independently in nearly all lineages of multi-cellular organisms.
Biologists consider the evolution of the two sexes mathematically inevitable, because this system maximizes the efficiency of sexual reproduction—providing resources to the offspring through investment in large, nutrient-heavy eggs, while also maximizing gamete fusion through the production of many sperm that can quickly find an egg. This efficiency explains why the same system has evolved independently so many times.
Hellbound and Down 