tiny sliver of time did we become able to tell this story.” This will
always seem a striking fact whether, on the planet in question, intelli-
gence arose after 100 million, 1 billion, or 10 billion years. The first
ones to discover cosmic time will always say, “The universe is so old,
and we are so young!”
Yet, it’s not “all relative.” There is an absolute clock of sorts against
which we can measure the arrival of intelligence on Earth and ask,
“Fast or slow?” We can measure our progress against the lifetime of the
Sun. If intelligence is always slow to develop, compared to the life
expectancy of the star powering its biosphere, then intelligence could be a rare thing. No matter what planet you live on, your days in the sun
are fleeting. It could be that most biospheres do not awaken to con-
sciousness before the lights dim and the show is over. In this context, it
is somewhat disconcerting that it took roughly half the expected life of
the Sun (5 billion years out of an expected 10 billion of sunshine) for us
to get to this point. If it had taken only twice as long, it would never
have happened. Therefore, it is sometimes said, we are damn lucky to
be here at all and may be the only ones around for thousands of light-
years. Do the prospects for consciousness in our universe come down to
a race between stellar evolution and the biological evolution of intelli-
gence?
Some evolutionary biologists have argued that intelligence in the uni-
verse must be rare because there are no other examples on Earth. We
are it. Dolphins and chimps notwithstanding, no one else has our cul-
tural, communicative, and technological abilities. Among all the mil-
lions of species that have come and gone, and those that are around
today, intelligence has evolved only once. If it was very useful or likely,
then many species would have it. Therefore, most planets will not
develop intelligence.
Am I the only one who finds this argument incredibly lame? The fact
is, we haven’t been here long. We speak these opinions as if we were the
omniscient observers, looking over the whole stretch of Earth history
So What?
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and drawing final, sweeping conclusions. We discuss evolution as if it
were a done deal, for which we were providing the wrap-up commen-
tary, rather than an ongoing, unfolding process that we are bound up
in. We are more like the first sunflower shooting up in a patch of thou-
sands, opening our petals, and confidently declaring, “There are no
other sunflowers and therefore sunflowers are extremely unlikely.” The
first intelligence will always be the only intelligence. Only if it is really
not too swift will it conclude from this that it represents something that
must be highly improbable.
R A R E E A R T H ?
How unusual is the evolution of advanced life? Two books published in
the last few years show the full range of answers that we can logically
reach: Rare Earth by Peter Ward and Donald Brownlee, and
Probability One by Amir Aczel. Each is written by credible scientists
using rigorous arguments backed by mathematical equations. Yet, like
the Epicureans and Aristotelians of ancient Greece, these thinkers reach
opposite conclusions, as can be seen when we juxtapose the subtitles of
the two works: “Why Complex Life Is Uncommon in the Universe”
(Rare Earth) and “Why There Must Be Intelligent Life in the Universe”
(Probability One). Each is written as a polemic, putting opposite spin
on the same set of observations to make antithetical points.
The focus of Rare Earth, as the title implies, is the many ways in
which Earth is an unusual planet. These, it is argued, have uniquely
qualified our planet to develop complex life. According to this view, a
great many ducks had to line up in a most fortuitous way for the evolu-
tion that led up to us. If just one of many factors had been different,
our planet would, at best, be a world of microbial slime. Since the odds
are against such a harmonic convergence, most or all other planets
must lack complex life.
Look at the myriad ways in which Earth is exactly suited for us: It is
the right distance from the Sun, neither too hot nor too cold. It is just
the right size to hang on to its atmosphere and oceans and retain
enough internal heat to drive plate tectonics. Earth has enough water to
maintain life, but not so much that the continents are entirely sub-
merged. We are blessed with a perfect atmosphere to support the kind
of life that has evolved here.
Several random historical events seem to have worked strongly in
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our favor. We were fortunate to acquire our large Moon in an early
freak accident. The Moon helps stabilize our orbital tilt, keeping cli-
mate steady.
Not only is Earth exceptionally lucky (or rare) in all these ways, but
we also grew up in a decent neighborhood. The Sun is the right kind of
star for us—stable, long-lived—a good provider. Our solar system has
orderly, stable circular orbits so Earth can count on avoiding nasty,
chaotic orbital shifts that would doom us. Our good neighbors have
pitched in to help us: Mars may have seeded Earth with life if we
couldn’t make it on our own. Jupiter, the big bouncer of the planetary
club, guards the inner solar system. It is just the right size and distance
from us to toss out most threatening asteroids and comets with its
mighty gravitational pull. We even orbit a star that is ideally located
within the galactic disk. Closer to the center we’d be threatened by
radiation from other stars, especially dying, exploding ones. Closer to
the edge, stars may not have enough heavy elements to make planets
and living creatures.
Reading through this (nonexhaustive) list, it is easy to get the impres-
sion that only on a planet just like ours, in a planetary system just like
ours, orbiting a star just like ours, located at just our position in a
galaxy like ours, could such a wondrous thing as us come to be. There
is something almost biblical about the way in which Earth seems to be
so uniquely and improbably suited for our comfort.*
But, so what? Planets, like people, are all pretty strange when you
really get to know ’em. I’m sure any slightly thoughtful creatures on
Titan believe that a methane atmosphere is essential for complex life.
The Plutonians are convinced that no life is possible above –200°F.
I do believe that Earth is a phenomenally rare and special place. Bless
its round little head. But I don’t agree with the conclusions the authors
of Rare Earth draw from this fact. They make several crucial errors.
One is the failure to fully recognize the role of life in creating the
Earth’s unusual character. The weirdness of Earth is at least as much a
product of life as it is a precondition for life. Rare Earth ignores Gaia—
by which, in this context, I mean all of the myriad, blatant, subtle, and
interwoven ways in which life has made Earth what it is today. Is life a
*In fact, Rare Earth has been accused of having a hidden creationist agenda (in David Darling’s book Life Everywhere).
So What?
145
backseat while Earth drives us through the universe? Hell, no. Life is a
major player in Earth’s story. What seem, after the fact, to be lucky
breaks in the creation of a planet perfect for us are actually a testament
to the resourcefulness with which life has exploited geological develop-
ments and shaped Earth to its needs.
This misunderstanding of the role of life in manifesting Earth’s idyllic
state is just one problem with the “Rare Earth Hypothesis.” The
biggest error is the conclusion that “this is the best of all possible
worlds.” Why should we think that conditions on our planet are opti-
mized for development of complex life? This leads me to propose the
Rare Wookie Hypothesis. There is no other cat just like our Wookie.*
An incredible set of circumstances had to occur for Wookie to become
the best of all possible cats. A chance encounter between his parents in
a Curtis Park alley, a roll of the genetic dice that endowed him with
thick, dark gray fur and a tail dipped in white, a narrow escape from
the dog that mauled his sister, a rescue, the party where I met him.
What if the dog had gotten him instead? What if I had not gone to the
party? What if Tory had never found him hiding in the Encyclopaedia
Britannica? The chances of all this happening ever again are vanish-
ingly small. Clearly we and Wookie are extremely lucky to have each
other. This leads inescapably to the Rare Wookie Hypothesis: there are
no other cats—at least none as perfect as ours.
The fallacy here is thinking there is only one way that complex life
could have happened and only one set of circumstances that could facil-
itate it. Heartbreaking as it is to think so, if we didn’t have Wookie, we
might have some other fine kitten. If Earth had come out differently, it
might still be a pretty decent planet. Hell, it might even be better. The
Rare Earth thesis implicitly assumes that our planet is perfect—that any
deviations from our evolutionary path would have had negative conse-
quences for life. But we do not know that this is the best of all worlds.
We don’t even know if it’s a particularly good one. Sure it seems great
to us, but we’ve evolved for billions of years to take advantage of all of
Earth’s quirks and idiosyncrasies. Looking back at the many random
events that led to a world where we feel perfectly at home, of course we
say, “How fortunate for us.” But we are the way we are because we’ve
rolled with Earth’s punches. Life and Earth have shaped each other.
*Wookie’s hip-hop name used to be Fluff Daddy, but he recently changed it to Flea Diddy.
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If saguaro cacti were intelligent enough to theorize about life else-
where, but not smart enough to think about observational bias, they
would conclude that most of Earth is uninhabitable since most of it is
not desert. (But they would be comforted to see that more of it is
becoming habitable all the time.)
While we’re searching for other Earths, we should also be asking
what a more habitable planet would look like. More habitable than
Earth? Dare I say it? Won’t lightning strike me down? What could pos-
sibly make a planet better than our perfect Earth?
People are always giving the Earth a hard time about its slow devel-
opment: “Look at that stupid world!” they say. “They didn’t go multi-
cellular until their star was half used up. No wonder they’re still in the
primitive, factional, warring phase.”
But, picture a planet that is similar to the Earth in many ways except
that it formed originally with much less iron in the mix (perhaps orbit-
ing one of those stars farther out in the galaxy having less of the heavy
elements with which to make planets). Remember that on Earth iron
hogged all the oxygen for the longest time and kept it out of the atmo-
sphere, stunting our growth for billions of years. On this iron-depleted
world, all other things being equal, oxygen would build up much faster
in the air. If that’s what complex life needs, on that planet they’d have
been finishing up grad school while we were still microbes in diapers.
Another variable is the way planetary size affects atmospheric evolu-
tion. We do know that all planets are constantly losing hydrogen. The
more hydrogen departs, the more oxygen is running free, not locked up
in H2O. Over long periods, this hydrogenous exodus helps planets
become oxidized. Now, the smaller a planet is, the faster it loses hydro-
gen. So, other things being equal, smaller Earth-like worlds might
become oxidized sooner. If high atmospheric oxygen content is the key
to animal complexity, then Earth may not be the fastest out of the gate
by a long shot.*
You could come up with countless other possible improvements, I’m
sure. The examples I’ve just given assume a biosphere with essentially
the same needs as ours, but an improved ability to meet them on time
and on budget. What about life that thrives on very different condi-
*But, if a planet is too small, it might lose all of its atmosphere to space. That’s what happened to the Moon and Mercury. Mars lost most of its atmosphere early on, rusted over, seized up, and, I suspect, dropped dead.
So What?
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tions? Remember the extremophiles. Evolution is clever, or can pass
itself off as clever through sheer persistence. Life is adaptable and
resourceful. Those scary regions nearer the center of the galaxy are rich
in alternative energy supplies. Who knows what grows there?
I’m not complaining about Earth. This is my favorite world by far.
I’m perfectly comfortable here and for the most part I’m glad things
have worked out the way they have. I’m just saying let’s think twice
before assuming that this is the only or best world that some kind of
cogitatin’ creatures could call home.
We’re just getting to the point where we can start to make decent
predictions about the likely evolutionary paths of other worlds. This is
an area of active research, funded by NASA’s exobiology and astrobiol-
ogy programs, where meaningful new results should be forthcoming. Is
Earth a late bloomer, a precocious planet, or typical? We don’t know
yet, but stay tuned.
In the next few chapters I’ll look in more detail at what we actually
know about other planets in our own solar system and the rest of the
universe. As we survey the lives of planets, think about what a planet
really needs for life to arrive and thrive. Which qualities of Earth are
simply local oddities to which life has adapted? Surely fate has dealt the
Earth some of the cards a living planet truly needs. But surely also the
wonderful adaptability of life, and its ability to shape its own environ-
ments, combined with our lack of imagination and perspective, could
make a random draw seem like a rare winning han
d.
I’ve had discussions with Peter Ward and Don Brownlee, the authors
of Rare Earth, and heard them speak. I don’t think they are as certain
about their pessimistic conclusions as you might think from reading
their book.* They do provide a useful antidote to the wishful thinking
about extraterrestrial life that dominates current thought. Many books
reflect the current mainstream scientific belief in “the extraterrestrial
life hypothesis”—a confident belief in what used to be called a “plural-
ity of inhabited worlds.” In Probability 1 Amir Aczel reaches this con-
clusion using arguments based on probability theory. The bottom line,
he believes, is that other intelligent life must be out there. There are
simply so many stars that it doesn’t matter if planets like Earth or
*My trickster wife has altered the dust jacket of my copy to say “Medium-Rare Earth,”
which may actually reflect the authors’ views more accurately.
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places with complex life are highly unusual. Astronomers have been
making this case for centuries.
We won’t find another biosphere exactly like Earth’s, just as there’ll
never be another cat like our Wookie. But there are lots of cool cats
and, if the logic holds, many places with complex life and intelligence.
As Aczel writes, “If you give something enough of a chance to happen,
it eventually will.”
T E A A N D S Y M P A T H Y
What kind of universe do we live in? My sympathies lie more with the
logic of plenitude than rarity. As the poster in Agent Mulder’s office
says, “I want to believe.” However, given what we actually know about
the universe, it is entirely possible that there is no other life. Even the
Rare Earthers believe that simple, microbial life is probably spread
throughout the universe. It’s the jump to complex life that they feel may
be unique to Earth. Given Earth’s history, where life stayed simple for
so long, we must at least accept this possibility.
For me though, that’s not enough. I want company. A universe thor-
oughly infested with bacteria but devoid of thought is not one in which
we can make any acquaintances. If microbes are widely spread among
worlds, but more complex life is nowhere else to be found, then the
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