Fifty years ago, flames sprang from the oil-slicked surface of a Cleveland river. Smog choked Los Angeles. Pesticides silenced millions of insects and birds. Oil gushed from a busted well off California, swamping anything that lived in the ocean.

Then, on April 22, 1970, 10 percent of America took to the streets for the first Earth Day. It was an unprecedented demonstration on behalf of nature, a declaration that people could not thrive unless the planet did, too. “Things as we know them are falling apart,” one of the movement’s young organizers, Denis Hayes, told a crowd in Washington. “A whole society is realizing it must drastically change course.”

Government responded in stunningly short order. The Environmental Protection Agency was created, a suite of powerful laws were enacted to protect the air, water and endangered species. The release of toxic substances was controlled.

But half a century later, one of the problems that motivated the early Earth Day activists remains unsolved, said Hayes, now 75. We haven’t quit the fossil fuels scientists say are warming the atmosphere and harming the Earth. Humans use more resources than the planet produces. Society has not changed course.

In this moment of overlapping crises, activists say it’s all the more important to make good on the promises of 50 years ago. To avoid a future as painful as the present, people must learn to live more sustainably — respectful of the living things whose fates are linked to ours, aware that the laws of nature apply to us as well, experts say.

In other words, we have to become better Earthlings.