Theo Colborn in 2012. Photo by TEDx MidAtlantic, used under Creative Commons licensing. Click the photo to watch her TED talk, an open letter to the President and First Lady about endocrine disruptors, in a new window.Years ago, before the world thought about “endocrine disruptors” as a broad category of chemicals that turned the reproductive lives of animals and possibly humans upside down, I was just learning about the receptors that were fooled by those chemicals. Receptors that combined with plastics and pesticides and plant products rather than the proper male and female hormones with which they'd evolved an excellent working relationship with over millions of years. Then Theo burst onto the scene. It was sometime around 1993 or '94. My adviser at the time had met her while testifying for Congress about these chemicals. “She's like the little old lady in tennis sneakers, knocking on doors and shaking up congress,” he'd said. That was twenty years ago—and Theo was a mere 67 or so years old. I mention her age because in that moment she became a role model. A woman who came to that particular science later in life. A second career that she approached with the energy of a twenty-year old and the confidence and wisdom of an experienced woman. It was a powerful combination that enabled her to change the way we think about how chemicals interact with life. How, in the most minute amounts they can work their way into a pregnant mother influencing the baby she is carrying—or into a father's sperm—and alter sexual development, brain development, and so many other systems dependent upon life's hormonal chemical messages. When I talk to students about toxic chemicals in the environment, I am often asked how I don't get totally depressed. I usually tell them that they—the students of today—make me hopeful. But I am also hopeful because of scientists like Theo. In her own words to me: Do not give up. You just might witness the hybridizing of toxicology and endocrinology with some useful policy toward cleaning up the mess my generation is passing on to you. So onward we go, but thanks to her efforts, with a far greater awareness about the interplay between the chemicals that make us what we are, and those that we make because of who we are. Editor's note: To learn more about Theo Colborn's work, visit The Endocrine Disruption Exchange or read an interview with her, the piece naming her a TIME Magazine Hero of the Environment, or a recent story about her work.