People understood the difference between male and female before Nettie Stevens, but it was because of her work in genetics that we understand how that difference comes about. Born in 1861, Nettie was an exceptionally bright student, and, after teaching for a few years, received her BA and MA degrees from Stanford University. She chose to continue her studies at Bryn Mawr, and in 1903, received her Ph.D. She was granted an assistantship at Bryn Mawr, allowing her to continue her research and teach classes (though she wanted a pure research position and was eventually granted one, she died of breast cancer before she could take up the position).
Although her early research focused on marine life, her later research at Bryn Mawr centered around chromosomal behavior. It was this research into mealworms and other insects that led to her identifying the Y chromosome, as well as the realization that females have two large sex chromosomes (the X is larger than the Y chromosome). These discoveries allowed her to deduce that sex is based on the presence or absence of a Y chromosome. Although these discoveries were expanded and built upon by future scientists, without Nettie and her insects, the fields of embryology and cytogenetics would not have advanced as quickly as they did.