Can a planet be alive? Lynn Margulis, a giant of late 20th-century biology, who had an incandescent intellect that veered toward the unorthodox, thought so. She and chemist James Lovelock together theorized that life must be a planet-altering phenomenon and the distinction between the “living” and “nonliving” parts of Earth is not as clear-cut as we think.
Many members of the scientific community derided their theory, called the Gaia hypothesis, as pseudoscience, and questioned their scientific integrity. But now Margulis and Lovelock may have their revenge. Recent scientific discoveries are giving us reason to take this hypothesis more seriously. At its core is an insight about the relationship between planets and life that has changed our understanding of both, and is shaping how we look for life on other worlds.
Studying Earth’s global biosphere together, Margulis and Lovelock realized that it has some of the properties of a life form. It seems to display “homeostasis,” or self‐regulation. Many of Earth’s life‐sustaining qualities exhibit remarkable stability. The temperature range of the climate; the oxygen content of the atmosphere; the pH, chemistry, and salinity of the ocean—all these are biologically mediated. All have, for hundreds of millions of years, stayed within a range where life can thrive. Lovelock and Margulis surmised that the totality of life is interacting with its environments in ways that regulate these global qualities. They recognized that Earth is, in a sense, a living organism. Lovelock named this creature Gaia.
Life and Earth have been co-evolving in a continuing dance.
Margulis and Lovelock showed that the Darwinian picture of biological evolution is incomplete. Darwin identified the mechanism by which life adapts due to changes in the environment, and thus allowed us to see that all life on Earth is a continuum, a proliferation, a genetic diaspora from a common root. In the Darwinian view, Earth was essentially a stage with a series of changing backdrops to which life had to adjust. Yet, what or who was changing the sets? Margulis and Lovelock proposed that the drama of life does not unfold on the stage of a dead Earth, but that, rather, the stage itself is animated, part of a larger living entity, Gaia, composed of the biosphere together with the “nonliving” components that shape, respond to, and cycle through the biota of Earth.
Yes, life adapts to environmental change, shaping itself through natural selection. Yet life also pushes back and changes the environment, alters the planet. This is now as obvious as the air you are breathing, which has been oxygenated by life. So evolution is not a series of adaptations to inanimate events, but a system of feedbacks, an exchange. Life has not simply molded itself to the shifting contours of a dynamic Earth. Rather, life and Earth have shaped each other as they’ve co-evolved. When you start looking at the planet in this way, then you see coral reefs, limestone cliffs, deltas, bogs, and islands of bat guano as parts of this larger animated entity. You realize that the entire skin of Earth, and its depths as well, are indeed alive.
The acceptance of the Gaia hypothesis was, and remains, slow, halting, and incomplete. There are several reasons for this. One is just the usual inertia, the standard conservative reluctance to accept new ways of thinking. Yet Gaia was also accused of being vague and shifting. Some complained that the “Gaians” had failed to present an original, well‐defined, testable scientific proposition. How can you evaluate, oppose, or embrace an idea that is not clearly stated, or that seems to mean different things to different people? There was certainly some truth to this. Gaia has been stated many different ways. Also, it didn’t help that Margulis and Lovelock were more than willing to mix science with philosophy and poetry, and they didn’t mind controversy; in fact, I’d say they enjoyed and courted it.
The truth is, despite its widespread moniker, Gaia is not really a hypothesis. It’s a perspective, an approach from within which to pursue the science of life on a planet, a living planet, which is not the same as a planet with life on it—that’s really the point, simple but profound. Because life is not a minor afterthought on an already functioning Earth, but an integral part of the planet’s evolution and behavior. Over the last few decades, the Gaians have pretty much won the battle. The opposition never actually surrendered or admitted defeat, but mainstream earth science has dropped its disciplinary shields and joined forces with chemistry, climatology, theoretical biology, and several other “‐ologies” and renamed itself “earth system science.”
The Gaia approach, prompted by the space-age comparison of Earth with its apparently lifeless neighbors, has led to a deepening realization of how thoroughly altered our planet is by its inhabitants. When we compare the life story of Earth to that of its siblings, we see that very early on in its development, as soon as the sterilizing impact rain subsided so that life could get a toehold, Earth started down a different path. Ever since that juncture, life and Earth have been co-evolving in a continuing dance.
As we’ve studied Earth with space-age tools, seen her whole from a distance, drilled the depths of the ocean floor, and, with the magic glasses of multispectral imaging, mapped the global biogeochemical cycles of elements, nutrients, and energy, we’ve learned that life’s influence is more profound and pervasive than we ever suspected.
All this oxygen we take for granted is the byproduct of life intervening in our planet’s geochemical cycles: harvesting solar energy to split water molecules, keeping the hydrogen atoms and reacting them with CO2 to make organic food and body parts, but spitting the oxygen back out. In Earth’s upper atmosphere some of this oxygen, under the influence of ultraviolet light, is transformed into ozone, O3, which shields Earth’s surface from deadly ultraviolet, making the land surface habitable. When it appeared, this shield allowed life to leave the ocean and the continents to become green with forests. That’s right: It was life that rendered the once deadly continents habitable for life.
The more we look through a Gaian lens, the more we see that nearly every aspect of our planet has been biologically distorted beyond recognition. Earth’s rocks contain more than 4,000 different minerals (the crystalline molecules that make up rocks). This is a much more varied smorgasbord of mineral types than we have seen on any other world. Geochemists studying the mineral history of Earth have concluded that by far the majority of these would not exist without the presence of life on our planet.
So, on Earth’s life‐altered surface, the very rocks themselves are biological byproducts. A big leap in this mineral diversity occurred after life oxygenated Earth’s atmosphere, leading to a plethora of new oxidized minerals that sprinkled colorful rocks throughout Earth’s sediments. Observed on a distant planet, such vast and varied mineral diversity could be a sign of a living world, so this is a potential biosignature (or Gaiasignature) we can add to the more commonly cited Lovelock criterion of searching for atmospheric gases that have been knocked out of equilibrium by life. In fact, minerals and life seem to have fed off each other going all the way back to the beginning. Evidence has increased that minerals were vital catalysts and physical substrates for the origin of life on Earth.
Is it really a huge leap, then, to regard the mineral surface of Earth as part of a global living system, part of the body of Gaia?
What about plate tectonics and the dynamics of Earth’s deep interior? At first glance this seems like a giant mechanical system—a heat engine—that does not depend upon biology, but rather (lucky for life), supports it. Also, although we’re probably still largely ignorant about the deeply buried parts of Earth’s biosphere, it’s unlikely there are any living organisms deeper than a couple of miles down in the crust, where it gets too hot for organic molecules.
Yet, just as we’ve found that life’s sway has extended into the upper atmosphere, creating the ozone layer that allowed the biosphere to envelop the continents, more and more we see that life has also influenced these deeper subterranean realms. Over its long life, Gaia has altered not just the skin but also the guts of Earth, pulling carbon from the mantle and piling it on the surface in sedimentary rocks, and sequestering massive amounts of nitrogen from the air into ammonia stored inside the crystals of mantle rocks.