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.