The legacy of environmental thought in the decades prior to the first Earth Day gave birth to the event in 1970. A body of environmental literature emerged in the United States which traced its roots to the colonial and post-Revolutionary War periods. The writings of Henry David Thoreau, John Muir, and George Perkins in the latter half of the 19th and in the 20th century stimulated and created a philosophy and ethic for the environment and concerns for nature and the wilderness.
The "birth" of the contemporary environmental movement began with the 1949 publication of Aldo Leopold's SAND COUNTRY ALMANAC, considered by many to be one of the most important books on conservation. This environmental classic was preceded by the evolution of a contemporary wilderness ethic that began in 1935 with the publication of the first issue of the magazine, The Living Wilderness by the Wilderness Society. The Society's first director, Howard Zahniser, drafted the first version of a wilderness bill in 1955. The bill was introduced in the U.S. Senate by Hubert Humphrey (D-Minnesota) in 1956 and signed into law as the Wilderness Act by President Lyndon Johnson in 1964.
The celebratory event known as "Earth Day," created in 1969 and 1970, found its initial inspiration in the 1950s and 1960s, decades marked by tremendous social and cultural awareness, times of activism and change. One cultural concept around which millions of people began to rally was the environment.