Crowds of people celebrate Earth Day 1970 in a park.

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­The very first Earth Day, in April 1970, was a grassroots protest. It was a nationwide, coordinated effort of teach-ins, speeches and picket signs in true '60s fashion. In fact, many organizers of Earth Day 1970 had learned their craft in the antiwar movement, and they applied the lessons to a new foe: pollution and destruction of the environment.

By the late 1960s, the air, rivers and forests of North America were in serious disrepair as a result of massive industrial development. Rivers were catching fire, the sludge and debris dumped by factories igniting with the slightest spark. In a midsize city like Portland, Ore., people were breathing air that polluted their lungs at the same level as smoking two packs of cigarettes a day [source: Van Fleet]. Forests were being leveled at an alarming rate, pesticides were clogging farmland and beaches were littered with dangerous waste. By the time Ohio's Cuyahoga River combusted in 1968, the 10th time in less than a century, people were ready to move the small, grassroots "conservation" effort into the mainstream.

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­That's when Wisconsin Sen. Gaylord Nelson came up with the idea for Earth Day. He'd been working for years to get his peers in Congress to join him in the fight to clean up the environment, but few were interested. He even convinced President Kennedy to do a national speaking tour about environmental issues in 1963, but it failed to garner much attention. Then, in 1969, he announced a national day of environmental protest, scheduled for April 22, 1970. The press liked it. Major media outlets started running stories about pollution that drove the issue home for the general public.

As a result, that first Earth Day was a surprisingly huge success.

In this article, we'll find out what that first Earth Day was like, and see how it has evolved. The Earth Day we know now is quite different from Earth Day 1970, although the underlying sentiment is still the same.

It's no wonder Earth Day has changed over the years. The '60s and '70s were a time that can't be replicated.