Age: 49
Occupation: Ob-Gyn researcher, Harvard Medical School
Eight years isn't long in the world of science, and that's how long it took Tilly to overturn the long-held belief that women are born with all the oocytes they will ever have and cannot generate new eggs. Working with ovarian tissue from young patients undergoing sex-change operations, he showed that human ovaries possess egg stem cells that could allow women to continually make new oocytes. That rewrites the book on female reproduction and could lead to improved treatments for infertility.