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. [...]
Now, 40 years after Viking landed on Mars, we’ve learned that planets are common, including those similar in size to Earth and at the right distance from their stars to allow oceans of liquid water. Also, Lovelock’s radical idea to pay attention to the atmosphere and look for drastic departures from the expected mixture of gases now forms the cornerstone of our life‐detection strategies. Gaian thinking has crept into our ideas about evolution and the habitability of exoplanets, revising notions of the “habitable zone.” We’re realizing that it is not enough to determine basic physical properties of a planet, its size and distance from a star, in order to determine its habitability. Life itself, once it gets started, can make or keep a planet habitable. Perhaps, in some instances, life can also destroy the habitability of a planet, as it almost did on Earth during the Great Oxygenation Event (sometimes called the oxygen catastrophe) of 2.1 billion years ago. As my colleague Colin Goldblatt, a sharp young climate modeler from the University of Victoria, once said, “The defining characteristic of Earth is planetary scale life. Earth teaches us that habitability and inhabitance are inseparable.”
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