As the chimpanzee made small talk with biologist Raymond Ditmars, Standard Oil heir Laurance Rockefeller sipped tea with fellow primates and allies in conservation on a sunny afternoon at the Bronx Zoo in 1941. The five men—zoo directors or board members of the New York Zoological Society—had gathered for this formal picnic to celebrate the zoo's pioneering redesign, which traded cages for naturalistic habitats. The zoo, opened in 1899, was founded on a conservation mandate, and its first director, William Hornaday, was directly responsible for saving the American bison from extinction.

Rockefeller devoted much of his life (and many of his millions) to environmental causes. He donated hundreds of thousands of acres, from Vermont to the Virgin Islands, for parks. And he helped found the Rockefeller Brothers Fund, which handed out almost $11 million in grants last year for environmental and sustainability-related work.

Seated to Rockefeller's left is the man he credited as his inspiration, Fairfield Osborn, an ecologist and president of the zoological society. In 1948, Osborn wrote Our Plundered Planet—the Inconvenient Truth of its time—one of the first popular texts to warn that Mother Earth was under siege. His work helped spark the environmental movement that is so vocal today. While Rockefeller's impact can be measured in acreage and grants, Osborn's most enduring legacy is the event taking center stage this month: Earth Day, the world's largest secular holiday. Following the example set by Rockefeller and Osborn, a billion people are expected to participate in the event this year by hugging a tree, or even a chimp.