mined, they were deserts. He developed an elaborate interpretation of
the current Martian condition, in which the canals had been built by a
race of intelligent Martians vastly superior to humans in intellect and
technical capabilities. Mars, once much like the Earth, had lost its
oceans. The Martians, trying to preserve life on their dying world, had
constructed the canals to carry spring melt from the polar caps to culti-
vated fields on the rest of the planet.
Lowell publicized this theory in a series of papers in scientific and
popular journals, in a well-attended lecture tour, and in his book Mars,
published in December 1895. Some critics noted the suspicious similar-
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39
ity of Lowell’s Martian observations to his pre-observational views of
cosmic evolution. Indeed, on the eve of his departure for Flagstaff he
had proclaimed to the Boston Scientific Society, “Investigation into the
condition of life on other worlds, including last but not least, their hab-
itability by beings like or unlike man . . . is not the chimerical search
some may suppose. On the contrary, there is strong reason to believe
we are on the eve of a pretty definite discovery in the matter.” Other
skeptics showed that a planetwide irrigation system fed by the tiny
polar caps would not work.
But the public ate it up. Such was the power of Lowell’s will and
imagination, and the skill of his oratory, that he took the whole world
with him on an elaborate, decades-long Martian fantasy ride. His
books were best-sellers, and his sensational lectures were standing-
room-only affairs, with frenzied throngs spilling onto the street. At the
beginning of the twentieth century, Lowell’s advanced, canal-building
Martians were all the rage on Earth.
The scientific community was sharply divided over the issue. The
furious disagreements, lasting for decades, mostly centered on the ques-
tion of whether the canals were artificial or “natural.” Most accepted
that they existed. Numerous careful and renowned observers the world
over also saw the canals.
At the time, that seemed like a powerful independent verification,
but now it stands as a warning of the traps we can set for ourselves
when we push science too far. When we overinterpret sketchy data at
the limits of current abilities, the gaps in our data may be filled by our
desires, by the power of suggestion, and by the undeniable force of con-
sensus in forming opinions.
Finally, in the 1920s, the debate ebbed. Improved telescopes and
photographic techniques showed that the question of interpretation
was moot. The canals were not the invention of advanced Martians try-
ing valiantly to save their planet from global change. They were created
by a turbulent atmosphere, an active imagination, a charismatic indi-
vidual, and a pervasive will to believe. They are simply not there.
L O W E L L ’ S L E G A C Y
Lowell went to his grave in 1916 firmly convinced of the reality of his
Martian civilization. Today, he is remembered as the man who, by force
of personality, led the world on a wild-goose chase that ultimately
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proved to be one of the most embarrassing episodes in the history of
science. It’s unfortunate, because his Martian obsession is only a highly
visible dark patch in a legacy with many bright spots. Lowell was a suc-
cessful popularizer of astronomy who established an observatory of
lasting value. His efforts to derive comprehensive theories of planetary
evolution paved the way for modern planetary science. Indeed, though
I find myself loath to say it (speaking as a comparative planetologist),
he may have been the first comparative planetologist.
Lowell’s turn-of-the-century views of planetary evolution now seem an
interesting mix of nineteenth- and twentieth-century ideas. His descrip-
tion of Mars was influenced by Kant’s nebular hypothesis in which the
planets formed sequentially with distance from the Sun, so that the farther
ones were older and those closer in were younger. Mars was ancient,
dried-out, and over-the-hill, a vision of Earth’s sad future, and Venus was
like a youthful, prehistoric Earth. The red planet had evolved much fur-
ther than Earth both geologically and biologically, losing its water as
Earth would at some time in the distant future. Martian evolution had
progressed eons further, producing much more advanced sentient life.
But advanced age was only one reason Lowell offered for the differ-
ences between Mars and Earth. He proposed a second explanation that
has proved more durable. His statement that Mars, “being smaller,
aged more fast than the Earth,” anticipated one of the results of mod-
ern comparative planetology. We have now established that the planets
formed all at the same time, not one at a time in sequence with distance
from the Sun. But we have also become increasingly aware of how a
planet’s size influences physical evolution. A small (compared to Earth)
planet like Mars has less gravitational hold on its atmosphere and
oceans, and less internal heat to sustain prolonged, replenishing geolog-
ical activity. Mars did lose most of its water early on and has since suf-
fered billions of years of geological dormancy. Thus its biological
potential seems largely buried in the distant past. Lowell’s portrait of
Mars as a world dry and old before its time, too small to hold its air
and water, is mirrored in our modern views.
Lowell boosted interest in extraterrestrial life to an all-time high among
both the scientific community and the public at large. This surge in aware-
ness would outlive Lowell and his theories by many decades, impacting
the pluralist debate throughout the twentieth century. The picture of Mars
as a reasonably Earth-like place with a moderate climate, water running
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41
on the surface, canals, and living creatures implanted itself permanently in
the popular psyche.
Science fiction writers conjured freakish images of Martian life,
which mingled with the science in the public imagination. The War of
the Worlds, H. G. Wells’s 1897 tale of interplanetary invasion, was
written in response to the 1894 “discovery” of a civilization on Mars.
It was Lowell’s dried-out, dying Mars from which the superior, malevo-
lent Martians launched their attack, seeking Earth’s greener pastures.
Since then a never-ending stream of fictional aliens have invaded, trying
to steal our water, our food, our women, our men, our cattle, and our
precious bodily fluids.
Lowell’s Mars persisted in fiction long after his theories were ban-
ished from science. Water-filled canals are found in Ray Bradbury’s The
Martian Chronicles (1950), where the alien invaders are humans from
Earth, in Arthur C. Clarke’s The Sands of Mars (1952), and in Robert
Heinlein’s Stranger in a Strange Land (1961). I mu
st admit that, having
absorbed all of these stories in my science-fiction drenched youth, I find
it a bit hard to give up entirely on Lowell’s canals—even though high-
resolution spacecraft imaging has thoroughly destroyed any rational
hope of finding them. For the public, the canals never entirely went
away. They just gradually morphed into the dried-up river channels
revealed by the Mariner 9 spacecraft in 1971.
Among scientists, however, when solid evidence inevitably caught
up with Lowell’s Mars—revealing a desiccated, frozen, desert world
with a thin, unbreathable atmosphere, and no canals or other signs of
civilization—the backlash was extreme. The pluralist cause was dis-
credited so severely that it has only recently recovered.
Scientific belief in alien life has always rested on some combination
of observational evidence and other, nonscientific reasons to believe,
which, inasmuch as they are conscious, may be classed as metaphysical
and, inasmuch as they are not, psychological. The Lowell affair left plu-
ralism with its pants down, exposing with embarrassing clarity the
roles that wishful thinking and herd mentality can play in scientific
claims about life elsewhere.
Belief in intelligent extraterrestrial life suffered a precipitous fall from
grace in the post-Lowell decades. A more general belief in life on
Mars, however, persisted. The new view, held by virtually all leading
astronomers in the 1920s and 1930s, was that although Mars had no
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intelligent life, its surface was covered by vegetation that caused the
margins of the broad dark regions to shift with the seasons.
In 1928, a symposium published in the New York Times Magazine
headlined “Eminent Astronomers Give Their Reasons for Belief That Life
Exists on the Great Red Planet” quoted many prominent astronomers,
including Harlow Shapley at Harvard and Henry Norris Russell at
Princeton, endorsing the likelihood of vegetable life and the possibility of
primitive animal life on Mars. Astronomers held this view until close-up
spacecraft pictures supplied a reality check in the 1960s.
Advances in spectroscopy gradually drove home the reality of
Mars—an incredibly cold and dry world with a thin atmosphere devoid
of oxygen. Yet, other measurements episodically resurrected hopes for
life. As recently as 1957, Harvard astronomer William Sinton reported
in Science magazine that he had found the spectroscopic signature of
chlorophyll, and “this evidence, together with the strong evidence given
by the seasonal changes, makes it seem extremely likely that plant life
exists on Mars.” Sinton made these observations from a telescope at
Lowell Observatory.
C O N V E R G E N T O R C O N T I N G E N T ? :
B I O L O G I C A L P E S S I M I S M
Twentieth-century advances in biology, chemistry, and astronomy so
drastically redrew the map on which the pluralism debate was held that
it was given a new name. The phrases plurality of worlds and pluralism, which smacked of seventeenth-century natural philosophy, disappeared, and the more scientific-sounding extraterrestrial life debate
came into vogue.
Setting the tone for modern biology’s involvement in this debate was
none other than Alfred R. Wallace, Darwin’s codiscoverer of the theory
of evolution by natural selection. Wallace, provoked by the popular-
ity of Lowell’s Martian theories, studied the probability of intelligent
life evolving on other planets. In his 1903 book, Man’s Place in the
Universe, he published his conclusion: intelligent life is unique to Earth.
This declaration set up an intellectual conflict between biology
and astronomy over the question of extraterrestrial life. Astronomers,
wowed by the sheer number of stars and planets where life could possi-
bly evolve, were generally the optimists. Biologists, impressed by the
arduous, and perhaps unique evolutionary journey of life on Earth,
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were the pessimists.* Twentieth-century astronomers were getting used
to the idea of cosmic evolution and developing a picture of stellar and
galactic life cycles in which one phase followed inevitably from the
next. They often assumed that life would spring forth naturally on
other planets at a certain phase of their development, as it apparently
had here.
Biologists, struggling unsuccessfully to find a theory of life’s origins
on Earth, concluded that such a development was difficult and unlikely.
They were more inclined to believe that life was unique to Earth.
Arguments in favor of extraterrestrial intelligence have primarily been
based on physics and astronomy (with a dash of metaphysics and wish-
ful thinking). Arguments against have largely been based on biology
(with a pinch of Earth-centered parochialism).
Wallace drew attention to the role of contingency in biological evolu-
tion. Since evolution of intelligence here required an absurd number of
lucky breaks, he argued, the probability against its ever occurring again
trumps even the unfathomably large number of places in the universe
where it might happen. Plenitude is defeated by the unlikely, contingent
path of evolution. Ever since, many biologists have echoed this, asking,
“What are the chances?”
This biological stance is best refuted not by astronomy but by another
biological argument. Wallace’s pessimistic reckoning of intelligence’s
prospects takes no account of convergent evolution. Evolution is remark-
ably good at finding solutions to the problems of survival in diverse envi-
ronments, and the same evolutionary invention often occurs separately in
very different species.
The eye of the octopus is a common example. It is remarkably simi-
lar in structure and function to the human eye, but we did not evolve
from octopi, nor they from us. Nor do we and octopi share an ancestor
that had eyes. Evolution independently found the same design for a
visual organ that helps you get around whether you’ve got eight legs
and suckers or two legs and toes. Similar examples abound in nature.
Another frequently cited one is the similar streamlined shape of marine
mammals, such as dolphins, and large fishes. These creatures are unre-
lated but evolution found them the same solution for swimming swiftly
through Earth’s seas.
*Yes, the terminology I use reveals a bias that life elsewhere is a good thing. Isn’t it?
A Wobbly Ladder to the Stars
45
Good designs that drastically increase the chances of survival often
evolve separately more than once, and perhaps with some inevitability,
among the diverse species of a biosphere. For this reason, it does not
seem unreasonable that animal species evolving on another planet
would have organs we would recognize as eyes. Can we say the same
about brains? That’s the big question.
The possibility that co
nvergent evolution applies to the development
of intelligence completely changes the probability arguments. If intelli-
gence confers a huge survival advantage, then perhaps it will inevitably
arise on other planets. Whether intelligence is such a trait is still a mat-
ter of significant debate.
P A N S P E R M I A : H E R E , T H E R E , A N D E V E R Y W H E R E
Where does life come from? We now regard this question as an essen-
tial component of the scientific quest for extraterrestrial life. Prior to
the twentieth century, however, it was only marginally a part of the
debate, regarded as more of a philosophical question we could not
hope to address scientifically. Forgetting the rest of the universe for a
moment, the ultimate source of life on Earth remains a deep mystery. It
is a mystery we need to solve if we hope to know something about life’s
distribution throughout the cosmos. Observing nature, you will quickly
find the age-old, obvious answer: “Life comes from other life.” And
like a three-year-old responding to every answer with another question,
we must ask, “But where did it come from originally?”
The explanation that sufficed for millennia—divine creation—ceased
to satisfy. Neither Galileo nor Darwin had questioned this solution to
life’s ultimate riddle. But their followers, impressed with the ability of
natural philosophy to resolve nature’s greatest puzzles, began a system-
atic search for answers. Discounting literal interpretations of Genesis,
which nineteenth- and twentieth-century scientists were increasingly
willing to do, there are really only two possibilities. Life either some-
how was brought to this planet from elsewhere— panspermia—or it
arose here by natural processes from nonliving substances— sponta-
neous generation.
The French chemist Louis Pasteur (immortalized on every carton of
pasteurized milk) sought throughout his life to find experimental evi-
dence for the prevailing view that life somehow arose from inanimate
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materials. He failed and, in doing so, believed he had proven the oppo-
site. His experiments demonstrated that the seemingly “spontaneous”
growths observed in spoiled food were actually introduced from the
outside. (This also led directly to the food sterilization technique that
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