"Welcome, sulfur dioxide, hello, carbon monoxide," a woman sang from the 1968 countercultural Broadway hit, "Hair," at a rally in Philadelphia that day. Across the country, activists donned gas masks or spread out in grassy parks to hear speeches about overpopulation, smog and dirty rivers.
"It was brand new on the scene. We were basically using a new vocabulary," recalled Denis Hayes, who was the 25-year-old national coordinator for that first Earth Day. "So it was all fresh.
"In 1969, most Americans couldn't even define the word environment," Hayes said. "By the end of 1970, a huge fraction of them thought of themselves as environmentalists."
The movement capitalized on the experience and passion of activists who had organized anti-war, civil rights and feminist rallies in the 1960s. Today, the environmental cause is far more sophisticated, with thousands of environmental lawyers and advocates with advanced degrees and corporations rushing to advertise "green" products.
"But some of that passion that we had in 1970 has faded," Hayes said.
The original Earth Day was the brainchild of the late Sen. Gaylord Nelson, D-Wis., who called for a nationwide teach-in on the environment in a speech in Seattle in September 1969. His daughter, Tia Nelson, said he decided to launch it after a major oil spill in California, and wrote the speech on airplane napkins.
Forty years ago Thursday, the youth-driven movement sparked participation of about 2,000 college campuses and 10,000 elementary and high schools. Congress adjourned so members could give speeches, tens of thousands of people filled Fifth Avenue in New York City -- which was closed to traffic -- and millions took part across the country in activities like trash removal and bicycle rides.
Many people used the word "ecology" to describe the cause -- "a shorthand way to say we need to think more holistically," said Adam Rome, an environmental historian at Penn State who is writing a book on the first Earth Day.
"A lot of people were beginning to question our affluence, the huge environmental costs of the way we lived, and technological progress," he said.
"Ecology" went out of fashion later because it had a "a hippie-ish, countercultural" feel, Rome said, as the movement worked to cultivate an image of professionalism and legal expertise.
Although politicians took part in the first Earth Day, organizers stiff-armed the Nixon administration. Hayes declined a White House invitation for a meeting a few weeks before the event, and President Richard Nixon himself did not participate in any Earth Day activities. By contrast, the Obama administration is doing five days of events to mark the 40th anniversary. Russell Train, who was the first chairman of the newly created White House Council on Environmental Quality in 1970, told a TV interviewer at the time that Earth Day organizers were anxious to "make it their own thing" and not have the government take it over.
"And we've been anxious to not give the impression that we're trying to take anything away from them either -- it is their thing, and that's all to the good," said Train, who later went on to serve as Environmental Protection Agency administrator.
Train, now chairman emeritus of the World Wildlife Fund, said in an interview this week that Nixon considered Earth Day "a bit of an irrelevance."
"I don't think the environment came very naturally to Richard Nixon as a high priority," Train recalled. "But he very quickly latched on to it as an important thing for the administration to work on," in part because of political considerations.
In fact, Nixon had devoted a good chunk of his State of the Union address in January 1970 to the environment, saying, "Through our years of past carelessness we incurred a debt to nature, and now that debt is being called." The EPA was created later that year.
There was also a chasm between organizers and corporate America.
"In that first Earth Day, companies were not supportive of the cause," Tia Nelson remembers. Now corporations including Wells Fargo, UPS and Procter & Gamble sponsor Earth Day events.
Despite the differences, there are some striking similarities to today's debate -- such as dire predictions about the planet's future.
New York Mayor John Lindsey told a crowd on the first Earth Day that behind words like ecology, environment and pollution is a simple question: "Do we want to live or die?"
And Hayes, with a flop of hair dangling over his forehead and a deadly serious look on his face, told an audience, "Tens of thousands of people will soon die in Los Angeles in a thermal inversion that's probably now inevitable."
Hayes says today that he regrets using the word inevitable, adding that the environmental movement sometimes encounters a "self-undoing hypothesis" -- warnings that cause corrective actions that keep the warnings from coming true.
Some discredited the Earth Day cause, as some do today. The Daughters of the American Revolution passed a resolution calling the issue "distorted and exaggerated by emotional declarations and by intensive propaganda." One delegate called the environmental movement "one of the subversive element's last steps."
In Georgia, Comptroller General James L. Bentley warned that Earth Day might be a Communist plot -- because it fell on Russian revolutionary Vladimir Lenin's 100th birthday.