How often do new biospheres get started on planets? How likely is life,
once started, to develop intelligence? What might intelligence become?
Okay, three questions, then. We’ll deal with the third one later.
Can we really say anything in answer to these, based on writing and
reviewing our biosphere’s autobiography? We’ve put together a reason-
ably credible story for the formation of our solar system, for the birth
and evolution of the Earth and its biosphere. The hard part is figuring
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out how universal this story might be. We study it intensely, trying to
glean whatever meaning we can from the clues at hand. But there is a
danger in this close reading—a temptation to see more significance in
the details than may actually be there. We think we’re deciphering an
instruction manual for making habitable worlds, but maybe we’re just
learning the colorful story of one eccentric place. On questions of
extraterrestrial life, we make the most of the evidence we have, because
we can’t stand not knowing. But we may just be reading tea leaves.
So, what lessons can we take away from our autobiography that
might help us gain perspective on the biographies of other distant bio-
spheres? Can we find any messages with significance beyond this orbit?
First of all, there is evolution itself. Any place where sufficiently flex-
ible and variable structures (organic or otherwise) are given the time
and freedom to play at combination and recombination, natural selec-
tion will grow beyond a simple contest of stability and longevity to self-
replication, making the jump to hereditary memory. Logic dictates that
any chemical system that can do it, will. Once you have self-replication
that is imperfect (so there will be variation), then some varieties will be
better at reproducing themselves, and evolution must occur. On this
level, evolution is not a radical proposition. The interesting question
then becomes, how far can it get in different environments?
Life on Earth is all made of cells. Will life elsewhere have “cells”?
Yes, I think so, because the separation of interior from exterior envi-
ronment seems like a certain requirement. Some sort of minimal orga-
nizational unit with an outer boundary separating organisms from their
surroundings seems called for. Once such units exist on any planet, then
competition and Darwinian evolution will lead to refinement of the cel-
lular machinery.
Every survival trick in the book, and then some, will be tried sooner
or later. Some of these “cells,” or cell-like thingies, will do better by
teaming up. Symbiosis will lead to the formation of larger, more sophis-
ticated assemblages. The resulting compound structures will have sub-
units optimized for specific functions. To maintain the individuality of
the collective, they may need something like a nucleus, which stores the
genetic material and handles reproduction. Nothing says they have to
look like our cells or be made out of the same stuff, but these overall
structural principles seem like universals.
What else happened here that seems essential? Photosynthesis. We
jumped one crucial hurdle when our little green brethren learned,
So What?
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before it was too late, to feed off the Sun. We do not know if this was a
lucky break, or if the ability to manufacture food from sunlight is
something that always evolves easily in the right conditions.
Photosynthesis seems somehow ingrained in the idea of a living uni-
verse. I mean, what’s the first thing that comes to mind when you think
of the universe?
Quick, what is it?
Stars, right?
We live in a universe of stars. They’re the most energetic, stable
power sources in the cosmos, and for life not to use them would be like
a hungry pack of dogs ignoring a pile of juicy T-bones. Starlight is there
for the taking. Photosynthesis is probably not the only way to power a
biosphere. Even on Earth, some species ride the chemical surf of
hydrothermal vents or otherwise draw power from the planet itself.
However, even these creatures are not completely “off the grid.” All
reside in a solar-driven biosphere. Over the long haul, all live off the
Sun in various indirect ways.
To support life without photosynthesis, a planet would need some
long-term source of chemical energy either from geothermal sources (or
whatever-planet-you’re-from thermal sources) or other kinds of radia-
tion from space. There are many types of energy pouring through
space, most of which we make no use of. What we call light, radiation
in the visible range, is a tiny portion of all available electromagnetic
radiation, and not the part that carries the most energy. More energetic
radiation can kill. But that’s just us. I’ll bet life has evolved elsewhere
that can use ultraviolet light for serious power, rather than cowering
from it in fear as we do. High-energy cosmic rays could be another
awesome source. We think of these things as lethal, and we are grateful
for the sky that shelters us from them. Someplace else, creatures may
have learned to embrace these huge sources of energy, though they
would have to be made out of different stuff than we. Complex organic
molecules are ripped to shreds by these hard radiations.
Even nuclear energy may be exploited by some kind of biology else-
where. One good trick is to find ways to indirectly use energy sources
that would burn you if you got too close. Life on Earth keeps its nuclear
fusion reactor at a safe distance, 93 million miles away. Similarly, other
energy flows that we think of as dangerous may be used indirectly in
other environments by “clever” (i.e., well-adapted) organisms.
The most obvious alternative to solar power in our solar system is
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tidal power in a tidal system. Some moons of Jupiter (and of other
giant, outer planets) are left cold by the Sun but are heated by tides.
Jupiter’s fearsome gravity raises tides that make the Bay of Fundy look
like a birdbath, tides so strong they squish and warm the insides of the
grateful moons. Creatures living in these moons would pray to Jupiter
first, and the Sun would be a lesser god. Solar energy might never occur
to them, and the idea of life on a tidally challenged world such as Earth
could seem laughable.
Although we don’t exactly know what life is, I think we can safely
say that some long-term, stable energy source will always be required.
Life needs energy, and early on it will have to successfully plug into a
star or some other live wire if it is to survive over planetary timescales.
What other general principles can we abstract from the particulars of
our own story? It seems likely that the nature of life will always be to
multiply exponentially. Therefore, it stands to reason that if life sur-
vives for long, it will become a planetary entity, a biosphere, exchang-
ing matter and energy with its environment on a global scale. Sooner
or later, life will always cause major chemical transformations on its
planet, and these changes will create new challenges to survival. At
these junctures, life will either die out or learn to love the new reality.
The best example here is oxygen. We are all extremophiles living in an
oxygenated world that is hostile to organic life. But that which doesn’t
kill us only makes us stronger. Oxygen could have done us in, but
instead we learned to breathe. We evolved the capacity to use the highly
reactive nature of oxygen to charge our chemical batteries like never
before.
One critter’s poison is another one’s fuel. Life is opportunistic and
resourceful, and it’s a fine line between a dangerous toxin and a bounti-
ful energy source. Life, through adaptation, can cross this line. For this
reason, I think we have to be extremely cautious in declaring planetary
environments with an excess of “dangerous” energy or “caustic” chem-
icals to be uninhabitable. The very property that make these conditions
dangerous to us, the powerful ability to induce chemical reactions,
could turn them into a gravy train for some clever thing.
The only places that are surely uninhabitable are those where
absolutely nothing is happening. I’m not talking about Pauls Valley,
Oklahoma. I mean places where really nothing is going on, where there
is no chemistry, no flow of matter or energy, where everything is just sit-
ting there at equilibrium, at rest. Such places are safely declared dead.
So What?
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S O M E T H I N G O L D , S O M E T H I N G N E W
What about the recent discoveries of extremophiles living on Earth?
How do they really change the equation for ET life?
Recently in a used-book store a few blocks from the Natural History
Museum in Manhattan, I found, tucked away on a high, dusty shelf
accessible only with a rickety ladder, an original edition of Biography of
the Earth written in 1941 by George Gamow, the Nobel Prize–winning
physicist and prolific author. The price printed on the cover is thirty-
five cents, but it cost me two bucks (seventeen cents in 1941 dollars).
As I read it, the brittle pages have been disintegrating in my hands,
leaving a trail of little yellow crumbs wherever I go.
Gamow begins a section entitled “Conditions for Life on the
Planets” as follows (I need to hurry before the page crumbles com-
pletely):
When we discuss the possibility of life on other planets, we come to
a delicate point, for we do not actually know what life is or what
forms of life different from those on the Earth are possible. Life in
any form is doubtless totally impossible at the temperature of molten
rock, or at absolute zero, at which all materials become quite rigid,
but these are extremely wide limits. If we restrict ourselves to the
ordinary forms of life found on the Earth, we can narrow these lim-
its roughly to the temperature range in which water, the most essen-
tial constituent of organic structure, remains liquid. Some bacteria,
of course, can stand boiling water with impunity for a time, while
polar bears and Eskimos live in regions of perpetual frost.
These 1941 ideas about the limits of life would not seem out of place
in a modern astrobiology journal. We still recognize, as Gamow did in
the forties (and others suggested in the seventeenth century), liquid
water to be the magic elixir for our kind of life. In some ways, the mes-
sage of extremophiles is “no surprises.” All life we know, no matter
how freaky in other respects, is still based on organic molecules dis-
solved in water, and we all use the same basic cellular machinery.
Extremophiles haven’t fundamentally changed the way we think
about strategies to look for life, but they’ve bolstered the optimism
with which we search. Right now anywhere with liquid water is consid-
ered a possible habitat, and this guides our quest.
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What we have learned from the extremophiles is that life is much
more adept than we ever thought at adapting to use different chemical
sources of energy. The microbial world, in particular, has found ways
to gain energy by trading electrons between a surprising variety of mol-
ecules that trickle through the recesses and backwaters of our world.
We’ve found creatures exploiting the energy released in reactions
between different combinations of hydrogen, methane, iron, sulfur,
manganese, and nitrogen. The big surprise in the world of the
extremophiles is really this wide range of chemical fuels.
Each one of these power sources required evolutionary innovations.
That this has occurred many times on Earth probably tells us some-
thing profound about life. Once started, life can make huge leaps, fun-
damentally changing its energy-extracting metabolism while conserving
its systems of heredity and self-construction. Like modular homes that
can be run on AC electricity or go off the grid and use wind, solar,
hydro, or geothermal, living cells on Earth have learned to run on an
astonishingly diverse range of chemical sources. If need be, we can
become whole new kinds of beasts feasting on energy sources poison-
ous to our ancestors or distant relatives.
Extremophiles teach us that on a planet with rampant disequilib-
rium,* life will find multiple chemical systems to feed off. It must be
“easy” from an evolutionary perspective to switch to new chemical
food sources. This tells us that once life gets started on a planet with
deep and long-lived energy sources, it can survive drastic environmental
changes.
Extremophiles, like the rest of us, are carbon-based. They all use
DNA and proteins. The lack of other chemical architectures even in a
biosphere that has found diverse energy pathways probably means
something. It could mean that carbon is the only way to live in this uni-
verse—at least on a planet like Earth. Or, it could just mean that once a
certain basic architecture takes hold on a planet, others don’t stand a
chance.†
These are the kinds of clues that we sift through in search of general
principles of cosmic biology. Yet, no matter how extreme some organ-
isms may seem to us, we have studied only one form of life. It is only
*One with active chemical flows, where different chemicals coexist that will spontaneously react when mixed.
†The Microsoft effect again.
So What?
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through exploration that we will learn anything about ET life—
whether it is we or they who are the successful explorers.
W H O W A N T S T O K N O W ?
As it has on Earth, life elsewhere is likely to mimic the overall trend of
Cosmic Evolution, with aggregates of smaller bodies coming together
to form successively larger and more complex objects and organisms.
Multicellularity seems obvious and inevitable. So, we are rankled that
cells took billions of years to make this step on Earth. Life seems to
have passed through some kind of bottleneck before it could procee
d
from microbial forms to larger, more complex organisms with highly
differentiated bodies and breakable hearts. Can we draw sweeping con-
clusions from this historical observation? Is this good evidence of a uni-
versal obstruction on the road to complex life, or could it be just an
extended rut that life accidentally stumbled into here that has no bear-
ing on the probability of complex life beyond Earth? It has often been
conjectured that this history might mean that planets with simple life
are common in the universe, but planets with more complex life are
quite rare.
For those of us who worry about the odds of complex life existing
elsewhere, that such complexity came so late in the game is one of the
most widely interpreted facts, perhaps the most overinterpreted fact, of
Earth history. Many learned opinions regarding the prevalence of com-
plex and intelligent life in the universe rest largely on interpretations of
this timing. “If complex life was a likely or inevitable product of bio-
logical evolution, it would have happened much sooner in Earth’s evo-
lution. Therefore it must be exceedingly rare in the universe and per-
haps exists only on Earth.” The great conviction with which this
opinion is often expressed might be justified if we understood why this
step took so long.
The final biographical fact that demands an interpretation is this: the
development of intelligence happened so late in the game. Even though
complex, multicellular animals have been around for 600 million years,
we “got smart” only in the last few million years—just a nanotick of
the cosmic clock. SETI theorists debate whether this fact is profound or
mundane. Is intelligence, because it took so long to arrive here, an
unlikely adaptation that might never occur on many other worlds with
life?
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When we ask ourselves this question, the first reply we should con-
sider is “Who wants to know?” We have to watch carefully for any bias
that may creep in because of our unusual place in history. Immediately
after any major evolutionary shift, the shift will seem very recent. This
statement is only saved from being a meaningless tautology by the
word seem. The first autobiography written by any planet will always
say, “Intelligence did not evolve until very recently—only in the last
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