If we cast off our bonds of subjectivity about Earth and the solar system, and try to view them from a truly "universal" perspective, we also begin to see aspects of Earth and its history in a new light. Earth has been orbiting a star with relatively constant energy output for billions of years. Although life may exist even on the harshest of planets and moons, animal life—such as that on Earth—not only needs much more benign conditions but also must have those conditions present and stable for great lengths of time. Animals as we know them require oxygen. Yet it took about 2 billion years for enough oxygen to be produced to allow all animals on Earth. Had our sun's energy output experienced too much variation during that long period of development
(or even afterward), there would have been little chance of animal life evolving on this planet. On worlds that orbit stars with less consistent energy output, the rise of animal life would be far chancier. It is difficult to conceive of animal life arising on planets orbiting variable stars, or even on planets orbiting stars in double or triple stellar systems, because of the increased chances of energy fluxes sterilizing the nascent life through sudden heat or cold. And even if complex life did evolve in such planetary systems, it might be difficult for it to survive for any appreciable time.
Our planet was also of suitable size, chemical composition, and distance from the sun to enable life to thrive. An animal-inhabited planet must be a suitable distance from the star it orbits, for this characteristic governs whether the planet can maintain water in a liquid state, surely a prerequisite for animal life as we know it. Most planets are either too close or too far from their respective stars to allow liquid water to exist on the surface, and although many such planets might harbor simple life, complex animal life equivalent to that on Earth cannot long exist without liquid water.
Another factor clearly implicated in the emergence and maintenance of higher life on Earth is our relatively low asteroid or comet impact rate. The collision of asteroids and comets with a planet can cause mass extinctions, as we have noted. What controls this impact rate? The amount of material left over in a planetary system after formation of the planets influences it: The more comets and asteroids there are in planet-crossing orbits, the higher the impact rate and the greater the chance of mass extinctions due to impact. Yet this may not be the only factor. The types of planets in a system might also affect the impact rate and thus play a large and unappreciated role in the evolution and maintenance of animals. For Earth, there is evidence that the giant planet Jupiter acted as a "comet and asteroid catcher," a gravity sink sweeping the solar system of cosmic garbage that might otherwise collide with Earth. It thus reduced the rate of mass extinction events and so may be a prime reason why higher life was able to form on this planet and then maintain itself. How common are Jupiter-sized planets?
In our solar system, Earth is the only planet (other than Pluto) with a moon of such appreciable size compared to the planet it orbits, and it is the only planet with plate tectonics, which causes continental drift. As we will try to show, both of these attributes may be crucial in the rise and persistence of animal life.
Perhaps even a planet's placement in a particular region of its home galaxy plays a major role. In the star-packed interiors of galaxies, the frequency of supernovae and stellar close encounters may be high enough to preclude the long and stable conditions apparently required for the development of animal life. The outer regions of galaxies may have too low a percentage of the heavy elements necessary to build rocky planets and to fuel the radioactive warmth of planetary interiors. The comet influx rate may even be affected by the nature of the galaxy we inhabit and by our solar system's position in that galaxy. Our sun and its planets move through the Milky Way galaxy, yet our motion is largely within the plane of the galaxy as a whole, and we undergo little movement through the spiral arms. Even the mass of a particular galaxy might affect the odds of complex life evolving, for galactic size correlates with its metal content. Some galaxies, then, might be far more amenable to life's origin and evolution than others. Our star—and our solar system—are anomalous in their high metal content. Perhaps our very galaxy is unusual.
Finally, it is likely that a planet's history, as well as its environmental conditions, plays a part in determining which planets will see life advance to animal stages. How many planets, otherwise perfectly positioned for a history replete with animal life, have been robbed of that potential by happenstance? An asteroid impacting the planet's surface with devastating and life-exterminating consequences. Or a nearby star exploding into a cataclysmic supernova. Or an ice age brought about by a random continental configuration that eliminates animal life through a chance mass extinction. Perhaps chance plays a huge role.
Ever since Polish astronomer Nicholas Copernicus plucked it from the center of the Universe and put it in orbit around the sun, Earth has been periodically trivialized. We have gone from the center of the Universe to a small planet orbiting a small, undistinguished star in an unremarkable region of the Milky Way galaxy—a view now formalized by the so-called Principle of Mediocrity, which holds that we are not the one planet with life but one of many. Various estimates for the number of other intelligent civilizations range from none to 10 trillion.
If it is found to be correct, however, the Rare Earth Hypothesis will reverse that decentering trend. What if the Earth, with its cargo of advanced animals, is virtually unique in this quadrant of the galaxy—the most diverse planet, say, in the nearest 10,000 light-years? What if it is utterly unique: the only planet with animals in this galaxy or even in the visible Universe, a bastion of animals amid a sea of microbe-infested worlds? If that is the case, how much greater the loss the Universe sustains for each species of animal or plant driven to extinction through the careless stewardship of Homo sapiens?
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