Dr. Nettie Stevens, an American biologist, was one of the first scientists to establish a strong link between a chromosome and the sex of an organism, and put to rest the long-standing debate on whether sex is determined by heredity or embryonic environment. This and many other achievements of Dr. Stevens is not unexpected considering that Stevens showed signs of academic excellence from early life. She was one of the very few women to graduate from her school and was always near the top of her class and frequently on the roll of honor. She was bright enough to finish a four-year teacher’s course in two years.
Though interested in research, Stevens started her career as a teacher in different schools to save enough money and get a college degree. At the age of 35 she moved to Stanford University, California to pursue her B.A. and M.A. in biology. At Stanford, Stevens studied physiology and histology during semesters and spent her summer vacations doing research at a marine biological station. Here she investigated the life cycle, morphology, taxonomy, conjugation, and regeneration of unicellular ciliates and the two new species of ciliates, Licnophora macfarlandi and Boveri subcylindrica that she discovered.
During her Ph.D. at the Bryn Mawr College, Pennsylvania, Stevens continued studying these organisms and the development of sperms and eggs, and cell division in sea urchins and worms. She then turned to cytology and regenerative processes that led her to study the differentiation in embryos and the study of chromosomes. Her comprehensive work of various ciliate species was compiled in her master’s thesis titled Studies on Ciliate infusoria and doctoral thesis titled Further studies on the Ciliate infusoria, Licnophora and Boveria. By the time Stevens completed her Ph.D. she had already published nine papers.
Following her Ph.D., Stevens began working on the chromosomal determination of sex which later turned out to be her greatest contribution to science. Using germ cells of different insects, she discovered a difference between some chromosomes of the two sexes. In her studies with the mealworm Tenebrio she identified a small chromosome, today known as the “Y” chromosome, and deduced that an egg fertilized by a sperm containing this chromosome becomes a male whereas the egg fertilized with the larger “X” chromosome becomes a female. She continued her research on various insects, discovering supernumerary chromosomes in certain insects and paired state of chromosomes in flies and mosquitoes. During the same time another scientist, Edmond Wilson came to the same conclusion after performing independent studies.
However, like many other women scientists, Stevens also fell victim to the “Matilda effect”, and her work was attributed to her male colleagues. Although Stevens and Wilson both worked on chromosomal sex determination, Wilson is acknowledged more for this discovery. Furthermore, Thomas Hunt Morgan, who had once argued against Wilson and Stevens’s interpretations about these cytological discoveries was credited for the discovery of sex chromosomes and awarded the Nobel prize in 1993.
At the age of 50, and only 9 years after completing her Ph.D., Stevens succumbed to breast cancer on May 4, 1912. She never got married or had any children. In her short career span, she managed to publish nearly 40 papers. Following her death Morgan wrote an obituary for the journal Science and mentioned that Stevens had a share in the discovery of the importance of chromosomes. He also described some detailed implications of her work. He also mentioned in a letter of recommendation “Of the graduate students that I have had during the last twelve years I have had no-one that was as capable and independent in research as Miss Stevens.”