Over time, however, scientific opposition to Gaia waned. In his early writing, Dr. Lovelock occasionally granted Gaia too much agency, which encouraged the misperception that the living Earth was yearning for some optimal state. But the essence of his hypothesis — the idea that life transforms and in many cases regulates the planet — proved prescient and profoundly true. We and all living creatures are not just inhabitants of Earth, we are Earth — an outgrowth of its physical structure and an engine of its global cycles. Although some scientists still recoil at the mention of Gaia, these truths have become part of mainstream science.

“It’s definitely time to revisit Gaia,” said Adam Frank, an astrophysicist at the University of Rochester. Some scientists even agree that the planet is in a very meaningful sense alive. “Life is not something that happened on Earth, but something that happened to Earth,” said David Grinspoon, an astrobiologist at the Planetary Science Institute. “There is this feedback between the living and nonliving parts of the planet that make the planet very different from what it would otherwise be.” As Dr. Margulis wrote , “Earth, in the biological sense, has a body sustained by complex physiological processes. Life is a planetary-level phenomenon and Earth’s surface has been alive for at least 3,000 million years.”

Those who bristle at the notion of a living planet will argue that Earth cannot be alive because it does not eat, reproduce or evolve. Yet science has never established a precise and universally accepted definition of life, only a long list of its qualities. Like many living creatures, Earth has a highly organized structure, a membrane and daily rhythms; it consumes, stores and transforms energy; and if asteroid-hitching microbes or space-faring humans colonize other worlds, who is to say that planets are not capable of procreation?

If Earth breathes, sweats and quakes — if it births zillions of organisms that ceaselessly devour, transfigure and replenish its air, water and rock — and if those creatures and their physical environments evolve in tandem, then why shouldn’t we think of our planet as alive?

Humans are the brain — the consciousness — of the planet. We are Earth made aware of itself. Viewed this way, our ecological responsibility could not be clearer. By fuming greenhouse gases, we have not simply changed the climate; we have critically wounded a global life form and severely disrupted its biological rhythms. No other member of this living assembly has our privileged perspective. No one else can see the sinews and vessels of our planetary body. Only we can choose to help keep Earth alive.

Gaia’s legacy can help us fulfill this responsibility. We can learn to recognize and amplify the planet’s innate climate-stabilizing processes. Earth has its own methods for storing carbon: A complex chain of chemical reactions involving plants, plankton and shellfish can lock atmospheric carbon in limestone. In addition to reducing carbon emissions, many Earth system scientists think we should study how to augment this natural sequestration and related processes.

In recent years the Amazon rain forest has endured unusually intense and frequent droughts, which some scientists have linked to deforestation and forest fires. It would be easy to compartmentalize these ecological shifts as local tragedies, but that detachment is an illusion.