At the dawn of the 20th century, a groundbreaking discovery was made by biologist Nettie Maria Stevens, who unraveled one of humanity's oldest enigmas: why do men and women exist as distinct sexes? Her pioneering research at Bryn Mawr College revealed the existence of sex chromosomes, which determine our biological sex. This discovery was monumental, providing clarity to what had long been a deeply mysterious question.
Before Stevens' work, countless theories tried to explain gender determination. Aristotle, for instance, suggested that a man's body temperature during conception influenced whether a child would be male or female. In the 19th century, European societies speculated that parental nutrition played a key role, with wealthier families supposedly birthing more daughters. Even in the 18th century, there were bizarre claims that removing an ovary or testicle could influence the sex of offspring.
Stevens' research, however, provided irrefutable evidence. Born in Vermont in 1861, she didn't begin her formal scientific education until she was 35. Despite the late start, her academic journey quickly advanced. After earning both a bachelor's and a master's degree from Stanford University, she pursued a Ph.D. at Bryn Mawr, a rarity for women at the time.
Her breakthrough came through the study of Tenebrio molitor, commonly known as the mealworm. Using a microscope, she observed that female cells contained 20 large chromosomes, while males had 20 chromosomes, but one was notably smaller. This discrepancy, she concluded, was due to the sperm cells of the mealworms, which carried either a large or small chromosome. The presence of the smaller chromosome determined maleness, while the larger chromosome resulted in femaleness.
These findings not only validated Mendel's principles of genetics but also established a foundation for modern biology. Yet, despite her profound contributions, recognition eluded Stevens. Her contemporary, Edmund Beecher Wilson, published similar findings around the same time. However, Wilson's work involved species where females lacked a chromosome rather than males, making Stevens' XY chromosome model more applicable to human biology.
Wilson's earlier publication and higher academic stature overshadowed Stevens' work. Consequently, Wilson became widely credited as the discoverer of sex chromosomes, even though Stevens' findings were more comprehensive and accurate. This is a classic example of the Matilda Effect, where women's scientific achievements are often credited to their male peers.
Stevens passed away from breast cancer in 1912. The New York Times acknowledged her contributions, calling her "one of the few outstanding female scientists." Yet, her legacy remains undervalued compared to her male contemporaries. Today, she is remembered not only for her groundbreaking research but also as a symbol of the challenges faced by women in science.
Written by Brian Resnick
Translated by Wang Ke
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