since the time of the Greeks, science has tried to make sense of the universe and of our place in it. More than two millennia ago, a Greek named Thales of Miletus, credited by many as the founder of Western philosophy, was among the first to leave a record of his musings about the place of Earth in the cosmos. Thales thought that the cosmos was an organic, living thing, and in that he may not have been far wrong if bacteria or bacteria-like organisms are as common as we believe them to be in the Universe. Thales's student Anaxi-mander was among the first to place Earth at the center of the cosmos, postulating that Earth was a floating cylinder with a series of large wheels with holes in them rotating around it. The Pythagoreans tried to break from this central-Earth motif, proposing that Earth moved in space and was not the center of the Universe. But Earth's centrality was restored by members of Plato's school and became exalted by the students of Aristotle. Eudoxus placed Earth at the center of 27 concentric spheres, each of which rotated around it. Soon two schools of thought competed: the "sun-centered" model of Aristarchus and the Earth-centered model of Ptolemy. The latter held sway through the Middle Ages.
During the Middle Ages, Earth was not only regarded as the center of the Universe but was again believed to be flat. St. Thomas Aquinas made Earth a sphere again but codified its place as the center of the Universe. It was
Nicholas Copernicus who finally shattered the notion of an Earth-centered Universe and put the sun at the center of all orbits. But even with this great leap forward, the sun remained at the center of the Universe as well, according to Copernicus in his revolutionary book of 1514, Commentariolus.
Copernicus forever destroyed the myth that our Earth lay at the center of the Universe, with the sun and all other planets and stars revolving around us; his work eventually led to the concept of a "Plurality of Worlds"—the idea that our planet is but one among many. This has now been described as the "Principle of Mediocrity," also known as the Copernican Principle. Yet an even greater blow came with the invention of the telescope. There is still debate about who built the first optical telescope, although Dutch optician Johannes Lippershey obtained the first official license for construction of a telescope in 1608. The device was an immediate sensation, and by 1609 this revolutionary new instrument found its way into the hands of Galileo, who built his own soon after hearing of the concept. Before Galileo, telescopes had been used to assess the terrestrial world (and for various military applications), but Galileo pointed his into the heavens and forever changed our understanding of the cosmos.
Galileo quickly surmised that there are far more stars in the sky than anyone had guessed. He discovered that the Milky Way is made up of uncountable individual stars. He observed the Moon, discovered satellites revolving around Jupiter (and in so doing showed that our Earth could just as conceivably orbit the sun). Earth's central place in the Universe, the fervent belief of Aristotle, was now observationally shown to be wrong. Copernicus had dealt with theory; Galileo and his telescopes dealt with reality. Galileo's message, published in his booklet Siderius nuncius, or "Messenger from the Stars," was about the truth told by the stars: that Earth is but one of many cosmic objects. To illustrate his point, he noted the presence of faint patches of light just visible to the unaided eye—objects called nebulae. Even with his primitive and tiny telescopes, Galileo could see these curious objects far better than anyone before. He thought them to be great masses of stars, made indistinct by their very distance.
The decentralization of Earth continued in relentless fashion. In 1755 Immanuel Kant theorized that a rotating gas cloud would flatten into a disk as it contracted under its own gravity. Kant was familiar with the numerous nebulae of the night skies, the faint glowing patches of luminosity scattered through the heavens. All the early astronomers knew of the faint cloud in the constellation of Andromeda. He knew these objects to be one of many distant groups of stars he called "island Universes." But Kant didn't stop there: He theorized that the sun, Earth, and other planets might have formed in this swirling mass of gas. This concept was taken a step further by Pierre-Simon de Liplike, who speculated in detail about how planetary systems might form from nebular origins. He invoked a dynamical mechanism for the formation of stars and their planets. Earth and the solar system became one of many such systems all formed in the same way.
But how far away were these island Universes? Was there only a single galaxy in the Universe, of which our star was part, or were there many? This debate was not resolved until the early twentieth century, a time when gigantic new telescopes were being constructed and outer space was being probed as never before. The conflict came to a head on April 26, 1920, when Harlow Shapley from the Mount Wilson observatory in California and Heber Curtis from the Allegheny Observatory in Pittsburgh met before the members of the National Academy of Sciences, a clash that came to be known as the Great Debate. The debate ended inconclusively, because it was not yet possible to assess the distance of the nebulae. That soon changed, however, thanks to the efforts of astronomer Edwin Hubble. Using a newly constructed, 100-inch reflecting telescope, Hubble was able to make observations that proved conclusively that the island nebulae were not associated with our Milky Way but were far-distant objects. Even the closest, the Andromeda galaxy, was found to be at least 2 million light-years from Earth and similar in shape to our Milky Way galaxy. The debate was over. The Milky Way is one of a vast number of scattered and widely separated galaxies floating in space. We became even more trivialized—now our galaxy was but one of many.
Two millennia of astronomers and philosophers removed Earth from the center of the Universe and placed us orbiting a sun that is but one of hundreds of billions in a galaxy itself but one among billions in the Universe. And it was not only astronomers who changed the world view. Einstein showed that there is no preferred observer in the Universe, and quantum mechanics told us that chance is king. Charles Darwin and his powerful theory of evolution demoted humans from the crown of creation to a rather new species on an already animal-rich planet, the chance offspring of larger-scale evolutionary and ecological forces. Nothing special. And yet . . .
The great danger to our thesis (that Earth is rare because of its animal life, the factors and history necessary to arrive at this point as a teeming, animal- and plant-rich planet being highly improbable) is that it is a product of our lack of imagination. We assume in this book that animal life will be somehow Earth-like. We take the perhaps jingoistic stance that Earth-life is every-life, that lessons from Earth are not only guides but also rules. We assume that DNA is the only way, rather than only one way. Perhaps complex life—which we in this book have defined as animals (and higher plants as well)—is as widely distributed as bacterial life and as variable in its makeup. Perhaps Earth is not rare after all but is simply one variant in a nearly infinite assemblage of planets with life. Yet we do not believe this, for there is so much evidence and inference—as we have tried to show in the preceding pages—that such is not the case.
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