Earth Day will be celebrated by more than a billion people, making it the largest secular observance in the world. How did this singular day and its ideas become so engrained in society? In an ironic twist of fate, books — those strange, obsolete things made from the destruction of trees — have a lot to do with it.

You’ve learned about Rachel Carson’s book Silent Spring. The charmingly illustrated and eloquently worded volume against the insecticide DDT captured the imaginations of government officials and the public alike. However, a similar book even more terrifying was published during the feverish haze of 1968.

The Population Bomb was Stanford University professor Paul Ehrlich’s dire and impatient warning to mankind. Ehrlich painted an apocalyptic picture of the future: Too many people were being born and too many resources were drying up. The professor believed this was a fatal scenario for both the planet and humanity. He even went so far as to compare humanity to cancer.

“A cancer is an uncontrolled multiplication of cells. The population explosion is an uncontrolled multiplication of people. Treating only the symptoms of cancer may make the victim more comfortable at first, but eventually he dies, often horribly. A similar fate awaits a world with a population explosion, if only the symptoms are treated,” Ehrlich said.

Ehrlich had many actual solutions on how to combat the disease of the surplus population. He was a staunch supporter of families having no more than two children, so much so that he outlined in The Population Bomb how to attack the media for promoting large families. Paul Ehrlich also floated the idea of creating a federal Department of Population and Environment (DPE), arguing that one of the DPE’s main focuses would be encouraging more research on human sex determination to ensure first born children were males.

The Population Bomb proved so popular Paul Ehrlich was able to co-found an activist group named Zero Population Growth (ZPG). Its members were passionate about decreasing the population and expert at using sympathy to get their talking points across. The group still exists as the re-branded Population Connection, continuing to spread their morbid fantasies about who should be born into this world — and who should not.

If you’re one of the billion people celebrating Earth Day this week, just remember: One of the founding environmentalists in America thinks the best way for you to celebrate is to drop dead.