In 1969, Denis Hayes was 25 years old in grad school. After just one semester and a 15-minute meeting with Sen. Gaylord Nelson in Washington, D.C., Hayes traded in his academic books for a picket sign and became a central organizer of the nonprofit group Environmental Teach-In, Inc., which would found Earth Day in 1970.

After the first Earth Day, a momentous accomplishment in and of itself, Hayes and his colleagues continued their activism by campaigning against the “Dirty Dozen” of incumbent anti-environment congressmen. Ultimately defeating seven out of the 12 congressmen, the group’s advocacy was also integral to the passage of the Clean Air Act shortly after the 1970 midterm elections.