Gaia Is Earth Alive

using a natural philosophy approach, perhaps we can study life's universals without simply projecting visions of our own kind out into the cosmos. Two controversial new fields of thought promise to help lift astrobiology beyond this conundrum of self-reference. These are the Gaia hypothesis and complexity theory.

The Gaia hypothesis, named after the Greek Earth goddess, was first proposed in the mid-1970s by James Lovelock, a British atmospheric scientist and inventor, and American microbiologist Lynn Margulis. Margulis is the mother of serial endosymbiosis theory, which states that major evolutionary innovations occur when more complex forms of life arise out of symbiotic collectives of smaller organisms (as described in chapter 8).

How far can we extend this principle? Gaia scientists regard Earth in its totality as one giant superorganism incorporating many parts of our planet that traditional science sees as nonliving.* The atmosphere and

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*Though many older, prescientific traditions have long perceived a living Earth.

oceans are the breath and blood, and the rain forests the lungs of this great global beast. Human culture is, perhaps, its awakening mind. Gaia suggests that life is seen as not merely incidental, but integral to the evolution and functioning of the planet. Gaia is a theory of biology, but also one of Earth history, geochemistry, and climate. The Gaian approach toward the Earth sciences is "geophysiology," studying Earth's functioning and health as a physician might approach a patient.

In 1988, as a graduate student, I attended the first major mainstream scientific conference on the Gaia hypothesis, sponsored by the American Geophysical Union in San Diego. It was fascinating to watch skeptical traditional scientists do battle with those from the new Gaian camp as they attempted to get Earth scientists to take their new approach seriously.* Science still doesn't quite know what to do with the Gaia hypothesis, because it isn't science-as-usual. Yet it is more than just a pretty metaphor. The Gaia hypothesis is guiding the way some biologists model life and its role on Earth, even as other (mostly older) biologists completely dismiss it.

Gaia actually began as an idea about exobiology. James Lovelock, consulting for NASA during the design of the Viking life-detection experiments, was thinking about how to look for life on Mars. He realized that the unusual atmosphere of Earth is by far its most distinctive sign of life. In considering the global properties of life that might be observable from another planet, he started noticing the many ways in which Earth's biosphere behaves like a giant living organism. In 1974, he and Margulis presented the Gaia concept in a paper called "Biological Modulation of the Earth's Atmosphere," published in ICARUS, International Journal of Solar System Studies.

The Gaia hypothesis has caused a quiet revolution among Earth scientists, many of whom are now realizing that life participates deeply in the physical evolution and functioning of Earth. A small school of scientists has fully embraced Gaia and dedicated their careers to it directly. A much larger group has been more guardedly receptive to the viewpoint, incorporating it into their work, or at least their worldview.

The Gaia perspective views evolutionary change as a creative interplay between biosphere and Earth—an intricate partner-dance between life and the changing planet in which neither seems to be leading. Life

*My friend Dorion Sagan suggested that we try to get everyone to gather one morning and run down the beach naked yelling, "The Earth is alive!" but this idea never caught on.

on Earth is no accidental collection of organisms lucky enough to find a hospitable planetary home. Rather, life has largely created the world we know. Life, we are learning, has altered many of Earth's basic physical properties, investing the air, the rocks, and the water with qualities they would not possess on a dead world. Thus, Gaia has important implications for the kind of relationships that biospheres can have with planets, and this should inform the way that we search for other inhabited worlds.

With a Gaian picture of evolution we see that some properties of the "nonliving" parts of Earth are actually encoded in the DNA of the world's organisms. Are other planets blessed with their own genomes?

It seems beautiful and true, but is it science? Some scientists complain that the hypothesis is more hype than thesis. They say there is no way to test or falsify it. Yet, the Gaia perspective has clearly led to some good science and to a new framework guiding some of the science we were already doing. In my view, it's right on the border of science and natural philosophy.

Gaia scientists have discussed the significance of Earth's unusual atmosphere, which is drastically out of equilibrium. Without the incessant, life-driven chemical cycles that permeate our world, the oxygen and methane would rapidly react, leaving only CO2 and water, producing a mix of gases that we would find unrecognizable and certainly unbreathable. The strange brew we breathe would never be found on a nonliving world.

Gaia proposes that the cumulative activity of all life on Earth acts to keep conditions here stable and comfortable for life. This happens by the evolution of numerous negative feedbacks in which the growth, death, or evolution of organisms creates environmental changes, which in turn affect the growth and evolution of other organisms. The net effect acts to pull the climate and various chemical balances back toward a certain moderate range if they begin to stray.

Short of comparing and contrasting numerous inhabited worlds, you can't do an experiment to test the idea as a whole, but you can look for active feedbacks on Earth that may be part of such a system. For example, some plankton act, collectively, as an air conditioner for the oceans. When the water gets warm, these guys get frisky and start multiplying. Their growth produces a chemical called dimethylsulfide (DMS), which diffuses up into the atmosphere. DMS is great for seeding clouds. As the amount of DMS rises, clouds build up over the ocean. The ocean surface cools off, which chills out the plankton orgy. The production of DMS then declines. As a result, it doesn't get too cloudy or too sunny for long, and ocean temperature remains in a moderate range favored by life.

The Gaians suggest that such mechanisms have been biologically regulating conditions on Earth for billions of years. This homeostatic self-regulation makes Gaia very much like a living organism, with the atmosphere and oceans behaving like circulatory and respiratory systems. However, obviously Gaia is not like any other organism we know in some important ways. For example, it has not reproduced, although you can't say we aren't trying.

How deeply ingrained is the biosphere in the physical functioning of our planet? We don't know. Gaian science endeavors to find out. It could go very deep indeed. Life clearly has hold of the atmosphere and oceans. Numerous cycles connect the atmosphere with the chemical state of Earth's interior rocks. Can life have actually assumed control of the plate tectonics that controls all terrestrial geology? If so, then the entire thermal evolution of the Earth is controlled by life.

Does it really go that deep? Or might Gaia be a spherical superorgan-ism riding around on a nonliving core? How could you define the boundary dividing creature and core? Perhaps by looking for a level, at some depth within the Earth, where things are exactly as they would have been if life had never come along. When it comes to the deep interior of the Earth, we don't yet know if Gaia is holding the reins or skillfully riding bareback.

The Gaia hypothesis reveals life to be a planetary-scale phenomenon with a cosmological life span. Gaia can help us identify those global qualities that distinguish planets having billions of years of life ingrained in their cyclic chemical activity from those orbs not blessed by this world-altering magic.

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