An estimated 20 million Americans marked the first Earth Day, April 22, 1970. Born during a period of political and social awakening, the event was heralded with rallies, teach-ins, community and park cleanups, and activities staged by students from grade school to grad school.

Today, the event is international and solidly mainstream as witnessed by the many companies launching corporate green initiatives in honor of Earth Day -- and sponsoring tree plantings, recycling drives and educational programs for their employees, their communities, their clients and potential customers.

To mark the 40th anniversary of Earth Day, GreenBiz.com asked thought-leaders in business and at NGOs to share their recollections of the first Earth Day and Earth Day 1990. That's when, as GreenBiz.com Executive Editor Joel Makower writes in "Strategies for the Green Economy," "the world (or at least some of it) awakened to the significant environmental challenges we faced" and some of the larger consumer product companies were just "dipping their corporate toes into the green waters."

Some of the people who responded to our query have vivid memories of both Earth Day milestones; others, just one. And one person, author and corporate and environmental strategist Bruce Piasecki, recalled both as well as providing his vision of Earth Day 2050.

Denis Hayes, who was a Harvard Law School student when he served as the national coordinator of the first Earth Day and is international chair of Earth Day 2010, writes about his reminiscences and the need for strong climate policy in a post shared with GreenBiz yesterday. His call for the next phase of environmental action appears tomorrow.

Here are other remembrances of Earth Days past:

Glenn Prickett, Chief External Affairs Officer, The Nature Conservancy

The big contrast between 1990 Earth Day and today is that on the one hand we saw then more of a true political demonstration and movement than we will probably see this year. I don’t want to understate what will happen [today]. The issue was fresh in 1990, a reawakening after 20 years, there was a freshness and innocence around it; big energy around climate change -- we called it global warming then -- rainforest, etc.

What’s different today is that we’ve now finally gotten the issue established, not only in the political space, but in the business space more importantly. We’ve got CEOs who think strategically about the environment and how they can use their [companies' leverage] to achieve environmental benefits that are also business benefits. The frustrating piece of it is that we’ve taken 20 years -- if not 40 -- to get there, but we seem to be ready as a society to confront these problems.

Jeffrey Hollender, Chief Inspired Protagonist and Co-Founder, Seventh Generation

In 1970, I remember it was a beautiful day in Central Park. I was 16, and I felt great. Lots of countercultural revolution was in the air and an environmental paradigm shift seemed as possible as everything else seemed in those heady days. It was just another bit of consciousness-raising. In 1990, I was so consumed with marketing the company to take advantage of the big 20th anniversary wave of environmental fervor that it’s all a blur.

Tensie Whelan, President, Rainforest Alliance

I was an elementary student [in 1970] and my school organized a great cleanup in Greenwich Village to commemorate the day. The city -- which was particularly dirty, back then -- was really buzzing with excitement. There was a general feeling of possibility and jubilation -- and a realization that it was our responsibility to keep our planet healthy.

[In 1990] I was working for the National Audubon Society and already deeply involved in the environmental movement. We hosted a booth to engage and educate people in honor of Earth Day. I recall a lot of really positive energy and excitement -- both at our booth and around the city.

Bob Langert,  Vice President, Corporate Social Responsibility, McDonald’s Corporation

I remember the first Earth Day festivities in 1970.  As a youngster growing up on the south side of Chicago, I also identified with the need for it.

After all, we couldn’t swim in Lake Michigan because too many dead fish were washing up on the beaches. Smokestacks spat out pollution that painted the skies gray and black. The environmental issues were personal because you could see and smell it, and it came from industry and manufacturing sectors that were the lifeblood of the economy.

Peter Seligmann, Co-Founder, Chairman and CEO, Conservation International

[In 1970, he was a] sophomore at Rutgers University studying wildlife ecology, sitting on lawn with hundreds of other celebrants, listening to speeches about the earth, her fragility and the need to reduce waste, prevent greenhouse effect and protect rainforests -- unfortunately, the lawn was littered with bottles and cups when the events ended.

Next Page: Renny Perdue, Alison Taylor of Siemens and Wayne Balta of IBM