on Earth. Complexity theorists may be groping toward a truly univer-
sal understanding of the nature of life.
Like Gaia theory, complexity theory is poised on the shifting border
between science and natural philosophy. It cannot be used to make
many specific, testable predictions. But, like Gaia, it is an idea that can
guide and recontextualize some of our efforts to study the general phe-
nomenon of life.
The tendency of matter, under certain conditions, to self-organize
suggests a new picture of evolution. Traditional Darwinian theory has
regarded evolution as a “blind watchmaker” where natural selection
between random mutations leads to all innovation and adaptation. But
complexity theory suggests that evolution may also refine and exploit
the nascent emergent properties of matter. Natural selection may be
helped along by some spontaneous pattern-forming habits built deep
into this universe.
We can begin to study the organizing principles of life that are not
necessarily based on any local chemical system. We can simulate evolu-
tion in a computer and begin to mathematically address one of our
most vexing questions: Is Earth life a fluke—an improbable, singular
occurrence—or does it represent the local flowering of a capacity for
life inherent everywhere in our universe? Some of us believe that com-
plexity is hinting at the latter.
All the significant developments in our cosmic story can be seen as
leaps to new levels of complexity. If the universe tends toward self-
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organization, and the epitome of self-organization is life, then rather than
some accidental occurrence here on an unusual ball of rock, life may be
implicit in the laws of nature, a stage of organization that this universe
goes through on its journey from atoms to minds. Complexity theory is
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the connection between the wildness of the universe and the simplicity of
math. Instead of a quest for familiar Earth-like conditions, the search for
suitable homes for life elsewhere becomes a hunt for places where self-
organization is likely to flourish and complexity can emerge. Thus we
grope toward a less parochial science of life’s universals.
Complexity theory also suggests a new take on an old question, long
a staple of science fiction and speculative science: When we do find
aliens, or they find us, what will they look like? By revealing many
forms of Earth life to be governed by deep geometrical rules of self-
organization in nature, complexity suggests a universal geometry of life
that should transcend worlds.
Many of the recurrent shapes found in terrestrial creatures are differ-
ent versions of those endlessly branching and self-repeating structures
known as fractals. Simple computer programs designed to simulate nat-
ural processes of self-organization also generate fractal shapes. The
fractal shapes of living creatures on Earth may be governed by univer-
sal principles of pattern formation.
Some computer-generated synthetic fractals have the look of life
reduced to its elements, as if Noah’s Ark had been caught in a colossal
whirlpool, smashing all the beasts of Earth into pieces and recombining
them at random, with tree branches blending into antlers, then veins, an
insect eye, a sea-horse tail, fish bones, palm leaves, and so on ad infini-
tum. But there is nothing Earth-bound about the mathematical origin of
these shapes; they are just as likely to hint at alien architecture.
If life is fundamentally a process of self-organization, then the fractal
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shapes of life will be found in other biospheres as well. There is no way
to predict precisely what aliens will look like, but the fractal geometry
of life gives us reason to believe that when they do finally land on the
White House lawn, whatever walks or slithers down the gangplank
may look strangely familiar.
L I V I N G W O R L D S
Are there other Gaias? I doubt it. Gaia/Earth is a unique individual.
Given its complex, contingent history, it’s unlikely that Gaia has an
identical twin. The question we could ask, though, is “Are there other
worlds that are alive in the same sense that Gaia is?” I call this hypo-
thetical class of Gaia-like entities “living worlds.”
How can we combine the insights from Gaia and complexity theory
to refine our schemes for finding other living worlds? Complexity the-
ory can, I think, help us become more sophisticated about which
worlds are likely to come alive. Gaia theory can make us more sophisti-
cated about the ways in which living worlds can evolve.
According to the Gaia hypothesis, complex feedbacks at all scales
have long ago made it impossible to draw a clean line between planet
and life. Earth is like a coral reef, where life’s environment is built of its
ancestors’ remains. In the Gaian view, habitability and inhabitation
are one and the same, a self-catalyzing property of a living planet.
Extrapolated to the wider universe, this suggests that life is not just
something that sometimes happens on a planet given certain condi-
tions. Rather, life is something that happens to a planet. A living world is more than just a planet with water, carbon, energy, and luck. Life is
what a planet becomes.*
In this view, conventional notions of habitability (solar distance, size,
water) are still relevant to the question of where life can get started. But
long-term habitability may depend more on the establishment of a
robust and resourceful global living organism than on having a lucky
planet and a lucky star.†
*The idea that a planet may be alive, not just a home for life but a living thing itself, has long been an exotic, enticing theme in science fiction. The most famous example is Stanislaw Lem’s Solaris, in which a planetwide ocean is found to be an intelligent but inscrutable living entity.
†Substitute “highly stable system of biogeochemical feedbacks” for “global living organism” here if you find it less offensive.
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The question may not be “Why did life evolve here, given certain
conditions?” but “How and why did Earth evolve into a living world?”
The difference is subtle, but can affect our interpretations of causality
in planetary history and suggest features and qualities to pay particular
attention to when exploring new planets. This might contribute in a
concrete way to our efforts to build the right telescopes and spectrome-
ters and to make the right observations to identify living worlds. It is
certainly true (as Lovelock noted thirty years ago) that life has left its
brand on Earth—and not in any subtle way. Earth’s atmosphere is so
unlike that of any imaginable nonliving world that it would not whis-
per but scream “Life!” to any alien with a spectrometer and a clue.
Complexity theory suggests that life is
most likely to emerge in con-
ditions of continuous flow of energy and nutrients, as long as some
suitable chemical architecture exists. Carbon in water is a good one.
Are there others? Once some kind of living organisms form, riding the
chemical flows of a young planet, they will use whatever fluid medium
there is (air, water, bkji pqek, whatever) both to extract raw materials
and to carry away their wastes. This extracting and dumping will
change the composition of these fluids, so organisms in one part of the
planet will be altering the environments of organisms elsewhere.
Organisms around the planet will evolve to take advantage of each
other’s wastes. These flows become mutualistic exchanges, then global
biogeochemical cycles that bind life to the planet. Once life starts, it is
only a matter of time before it develops, in concert with its evolving
planet, into a global system of interacting and self-regulating cycles.
These cycles will help to maintain the steady flows of energy and mod-
erate environmental conditions needed to facilitate complexity. Within
such a rich planetary environment, Darwinian evolution is free to work
its magic, all the while getting hints of innovation from the sponta-
neous formation of ordered structures.
Once global cycles are well established, then a living world is born:
the first globalization movement. Each kind of organism starts off in its
own little microenvironment and lasts up to the “living world” stage
only if luck keeps its surroundings reasonably stable. After that, organ-
isms do not need to be so lucky. Once the global cycles start up, then
they all evolve within a larger system that is largely made up of other
evolving organisms and all that they leave behind. This interdepen-
dence tends to moderate environmental changes: if things change too
much (too hot, too cold, too acidic, too salty, etc.), something will start
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to die off, or multiply, and counter these changes. The net effect is to
evolve a global regulatory system. Life is no longer completely subject
to the whims of external environmental change. A globally intercon-
nected biosphere—a living world—begins to make its own luck.
What kind of world is needed to maintain such a system for billions
of years? Our present efforts to define conditions for life elsewhere are
focused on the presence of water. It’s worth noting, however, the addi-
tional qualities that set Earth apart from most apparently lifeless
worlds. One of those qualities, which may be just as vital to a thriving
biosphere as water and organic carbon, is our high, steady level of
internally driven geological activity.
Earth is always remaking itself, its insides spewing out onto the sur-
face, while other surface areas are sucked back down into the furnace.
Like a restless poker player endlessly drawing new cards, Earth’s
dynamic atmosphere is continually trading in its molecules for new
ones from the planet’s interior. All of this changeability is directly
or indirectly attributable to Earth’s size-dependent internal thermal
evolution.
Indirectly, Earth’s size is also responsible for our climate and weather.
Atmospheric changes are driven by the sun, but Earth retains a thick,
active, erosive atmosphere and a filled-to-the-brim hydrosphere—
unlike, say, the dry, wispy atmosphere of Mars—because our planet is
large enough to hang on to its air and water.
Restless tectonics helps maintain active cycling of chemicals between
the different “reservoirs” of the planet (oceans, crust, atmosphere, inte-
rior). One crucial ingredient of Earth that I think is underrated as a
qualifier for life is this great changeability and all the fertile flows of
energy and nutrients that it sustains. As a necessary condition for a liv-
ing world, we should consider vigorous, continuous geological activity,
lasting over cosmological timescales, a candidate right up there with
water and organic molecules.*
Size matters. There is a critical mass above which planets stay active
over the lifetime of their star and below which they die out more
quickly. This suggests a lower limit for the size of a living world—
somewhere between the sizes of Mars and Earth.
*Notice that our remaining hopes for life on Mars are focused on potential hot springs and similar locales where, we infer, remnants of its lost internal heat may still be driving some sort of flow near the surface.
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V I T A L S
How can a “living worlds” view of life’s role in planetary evolution
help us in our search strategies? Our current efforts and plans for
studying extrasolar planets are largely focused on looking for “other
Earths”—places with environments, histories, and solar systems as sim-
ilar as possible to home. You can’t blame us for being curious to learn
how such alter-Earths have evolved, and how Earth-like they really are
in the details. But the search for life might wisely be conducted more
broadly. Soon we will start learning what the atmospheres of other
rocky planets are made of. So it’s an excellent time to be thinking about
what we should look for, and how to interpret what we find. Gaia says
look for disequilibrium. An excess of oxygen, especially in the presence
of a radically unoxidized gas like methane, would be one example.
Others would be a huge excess of methane with a little bit of oxygen,
or a large amount of sulfur dioxide (SO2) mixed in with hydrogen sul-
fide (H2S) or any other hydrogen-containing molecule. I could go on,
but I’ll spare you. It may be the disequilibrium of Earth, not the oxygen
or water, that is the universal life indicator. As we plan for the first
spectral observations of the atmospheres of roughly Earth-size rocky
worlds, we should keep Gaia in mind.
We are talking about comparative astrobiology. At present, this is
like comparative planetology was before the space age, theorizing with
abandon inside a knowledge vacuum. The idea of disequilibrium, of
anomalous composition of planetary atmospheres, as an indicator of
life is a good one. The idea is testable in principle and may soon be in
practice. We need to study many living worlds and their atmospheres.
Then we’ll be doing science. For now, even if we buy the disequilibrium
life test (I do), there is another important question that we can’t yet
answer: How much disequilibrium?
Disequilibrium isn’t an either/or thing. It’s a matter of degree. No
atmosphere is perfectly in equilibrium. Nonbiological sources of dise-
quilibrium are everywhere in the universe. Some that we know of on
nearby planets include gases cooked up by lightning flashes or by solar
ultraviolet radiation or squirted out of volcanoes. Note that each of
these requires an energy source. Each blows temporary puffs of non-
equilibrium gases into an atmosphere, so finding slight disequilibrium
would only tell us that something is going on there, but not necessarily life. We don’t know enough about the varieties of non
biological atmo-
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spheres to be certain what an anomalous signature is. Yet, if Earth is at
all typical, life will be marked by extreme deviations from equilibrium.
Now, let’s think about how complexity theory might help us in defin-
ing criteria for living worlds. Self-organizing emergent behavior hap-
pens within a flow of energy and/or matter. The steadier the flow, the
better. As discussed, puny worlds won’t make good candidates, except
perhaps for moons in orbit around giant planets, getting tidal heat.
Beyond this size criterion, what observable effects go along with vig-
orous and continued geological activity? For one thing, living worlds
are not likely to preserve the signs of ancient bombardment. Planets
with active geology will have erased all but their most recent impact
craters, so living worlds will not be heavily cratered. If we develop
ways to remotely sense the texture of cratered surfaces, we could use
this as a probable indicator of a dead world.
We may not be able to make a definitive test for life with remote
observations. Yet, we can use such observations to tell us which places
are worth investigating up close. Then we’ll need to send interstellar
probes. Only by sending imaging devices and other sensors will we
learn for sure if any place is inhabited.* Even if the results will not
come back until long after you and I are dead and gone, we should
launch them anyway. We need to send ships to the most promising
worlds so that our great-grandkids can find out if they’re really living.
We can best explore astrobiologically by roaming widely and keeping
a sharp eye out for anomalous order of any kind. This could include
strange, nonequilibrium mixtures of gases (or, conversely, too much
equilibrium in places where other known processes are creating dise-
quilibrium!), strange mechanical shapes and assemblages, or rhythmic
environmental changes without any obvious cause. Such anomalous
order will indicate either an interesting nonbiological process that we
need to learn about† or that we have at last found new life.
I’ve been talking about the search for life signs on worlds orbiting
other stars. But how do we apply this “living worlds” perspective to the
other planets in our own solar system? Our current exploration strat-
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