Call it Mediocrity!

An insignificant pebble in the immensity of space. But its varied and complex environment, inhabited by more than 10 million different species, is unique. The importance of the Earth in the Judeo-Christian tradition

BY MARCO BERSANELLI

The progressive generalization of the Copernican principle has caused the conviction to take root and spread that the Earth is in no way a noteworthy or special point in the universe. We have known for some centuries that the Earth is only one of the planets belonging to the star called the “Sun.” Ever since then, our planet has become ever smaller and more marginal in the growing vastness of the known universe. For some years now, we have even had direct evidence of the existence of planets around other stars (“extra-solar” planets), whose discovery confirms for us that the “planet” phenomenon is not exclusive to our solar system. “The Earth is a very common planet, and it is in a very ordinary part of space”–this is a typical statement of the so-called “principle of the mediocrity of the earth,” a modern extrapolation of the Copernican paradigm. Many think, on the basis of this “mediocrity principle,” that also the evolution of various living forms, all the way to the existence of beings with a high degree of complexity, is a “normal” and widespread phenomenon in the universe. In any case, it is clear that our little Earth does not hold an especially prominent position in the immensity of the cosmos. Thus the emphasis with which Judeo-Christian tradition underlines the importance of the Earth in Creation risks appearing somewhat naive and embarrassing. The modern eye can certainly not miss seeing a certain disproportion when we speak of the creation “of heaven and earth,” which puts the immense vastness of the firmament on the same plane with a minuscule speck lost somewhere in the whole picture.

Only on Earth
But is Earth really just any place? Let’s look around. The solar system includes nine planets (plus a tenth that did not succeed in forming) and about fifty satellites, some quite large, plus a very great number of comets and other smaller bodies. None of these heavenly bodies has an environment whose richness can be even remotely compared to Earth’s, despite the fact that planets like Mars and Venus are close to the same size as the Earth and their distance from the Sun is not too different. Mars in particular seems to offer a promising habitat for hosting some form of life, a possibility that is still open despite the numerous failures thus far. In the distant past, there was water on Mars, and some trace of it probably remains. It could very well be that some form of elementary life, similar to certain germs discovered in the depths of the Earth, might be found under Martian soil. There is hope of finding some micro-organisms also on some large satellites like Europa or Titan (in the orbits of Jupiter and Saturn, respectively). But it is clear by now that life is a rare phenomenon in the solar system, and certainly a unique characteristic of Earth is the presence of evolved organisms like plants and animals (with one exception: the Moon’s surface has known a form of developed life, twenty years ago when it was visited by a little group of Earthlings!).
On Earth, instead, we have a breathtaking panorama in front of us. Here, there is an incredible number and variety of living beings, capable of exploiting every inch of the environment, from minuscule bacteria to the largest and most complex beings–in all, more than 10 million different species! What makes Earth so special? How can it be so generous toward life? Only recently have scientists begun to have sufficient tools to approach these questions systematically.Various space missions planned for the first quarter of this century include among their aims the search for planets similar to ours, beyond the solar system. But already, surprisingly, a picture emerges in which our planet appears to be a splendid natural rarity.

Special conditions
First of all, the Earth enjoys astronomical conditions that are very special ones. The Sun and Moon, the two heavenly bodies deified by every ancient civilization, which accompanied the first steps of human awareness and imagination, have also shaped the earth’s environment and decisively contributed to the appearance and maintenance of life on Earth. We are in a very neat, practically circular orbit at an optimal distance from the Sun; a variation of 5% would be lethal. The Sun is a “normal” star, but this does not mean that just any other star would suit us equally well. Its size is neither too big (in which case its life would be too short to accompany biological evolution), nor too small (the Earth would end up synchronizing its daily rotation with its revolution around the Sun, destroying its temperate climate and the changing of the seasons). Only 2% of all stars are this ideal size. The Sun is at a good “safe distance” from the center of the Galaxy, where most of the stars are crowded, and where large doses of ionizing radiation (X and gamma rays) are produced that are capable of destroying or inhibiting the delicate rise of life.
Perhaps less evident, but just as crucial and even more surprising, is the Moon’s participation in the habitability of Earth. It has been demonstrated that, due to the Moon’s gravitational field, the inclination of the earth’s axis has remained more or less constant for more than 3 billion years, ensuring the necessary climatic stability during the enormous period necessary for life to flourish. This is not the case with the other planets; the Earth-Moon system is an anomaly, in which the size of the satellite can be compared to the size of its planet. The French astrophysicist Jacques Laskar, who made the basic calculations which led to this conclusion, has said, “These results show that Earth’s situation is highly peculiar…. We owe the stability of our climate on Earth to an exceptional fact: the presence of the Moon.” Many pieces of evidence suggest that the Moon originated 4.5 billion years ago from a huge, fortuitous collision of the Earth, still in the phase of formation, with a body the size of Mars. The astrobiologist James Kasting, of Penn University commented, “The stability of the earth’s climate largely depends on the existence of the Moon…. If collisions capable of generating objects like the Earth-Moon system are rare, then other habitable planets could be equally rare.”
But we owe a great deal also to other entities in the solar system, such as Jupiter, the good giant: its intense gravitational field, located at the proper distance from the Sun, acts for us as a guardian against the dangerous asteroids that, in its absence, would bombard the earth much more frequently and intensely, provoking mass extinction to the point of totally eradicating every form of life. Not to mention the comets: not only do they attract our wondering gaze to the sky every so often, but probably in a long-ago past they even transported huge quantities of water to the earth, indispensable for the maintenance and evolution of life. When we dive into the sea in the summer, we can recall that a large part of that water was brought there by an ancient comet shower.

“Evolutive” cataclysms
The history of Earth is not linear and monotonous, and even less so is that of the living world that has found a home on it. There have been about 15 episodes of mass extinction on our planet in the last 500 million years, five of which eliminated more than half of the living species. Recently, geologists have pointed out that twice in the past (2.5 billion years ago and 700 million years ago), the Earth went through periods of global freezes (“Snowball Earth”); for reasons yet to be deciphered, the entire Earth, from the poles to the equator, was covered with ice. It must have been a dramatic challenge. Most species were extinguished, and life was probably on the verge of disappearing altogether. But various scholars, among them Joseph Kirschvink of Cal Tech and Paul Hoffman of Harvard, are convinced that precisely these two events were crucial for two fundamentally important evolutionary leaps: the first was the appearance of eukaryotes, the second was the rapid diversification of biological types known as the “Cambrian explosion.” As Hoffman has noted, “probably without the ‘Snowball Earth’ events, there would not be animals or higher plants today.” Other episodes of mass extinction occurred because of the impact of large meteorites and comets. In particular, the dinosaurs were suddenly exterminated 65 million years ago; without this terrifying event, there would most likely not have been an age of mammals, which had until that moment played a very small role on the world stage.
The sophisticated and particular geological structure of the Earth, in turn, has decisive consequences for the planet’s ability to host and sustain life. As Ward and Brownlee have emphasized, the plate tectonics which characterizes our planet (the only case in the solar system) establishes a veritable thermostat based on the capacity to indirectly regulate the quantity of carbon dioxide present in the atmosphere. This, together with the gravitational pull of the Moon, are the main reasons for the maintenance of the earth’s temperature in an interval in which water has been able to exist in the liquid state almost uninterruptedly for more than 3 billion years. Furthermore, plate tectonics is responsible for the emergence of the continents, without which animal life would have remained confined to the oceans, precluding the more advanced steps in its evolution. The Earth is also equipped with an ingenious protective weapon crucial for our survival: its magnetic field. The cosmic rays, charged particles coming from outer space at speeds close to that of light, would have a destructive effect if they were not deviated by the earth’s strong magnetic field. This is another anomaly of planet Earth, due to the particular structure of its partially liquid iron core. On Mars, for instance, the magnetic field is practically non-existent.

Heaven and earth
If the Earth at first glance is an insignificant pebble, upon closer observation it appears as an environment of unheard richness and complexity, forged by a dramatic succession of events and maintained thanks to the confluence of unusual astronomic and geophysical circumstances. The Earth hosts an endless variety of living beings. Among these are elementary organisms, such as certain germs (called Extremophiles) capable of resisting extreme environments, at very high pressure and temperatures or on the ocean floor. Considering their formation (an event which in itself is anything but taken for granted!), it seems possible that analogous micro-organisms could maintain themselves even in much “rougher” atmospheres than earth’s, and in the future we could perhaps discover their presence also on some other planet or satellite in the solar system. But the Earth is inhabited also by an infinity of incredibly more complex and highly developed animals, like eagles and bears, sharks and antelopes, which require all the specialization and delicate balance of an environment like the Earth’s in order to exist. They have traversed a very long evolutionary history to become what they are, a history lasting more than a quarter of the age of the universe, inseparable from the particular properties of Earth’s physical atmosphere and its slow process of change over time. They have needed the Moon’s discreet assistance and Jupiter’s guardianship, continental drift and massive extinctions, Earth’s anomalous magnetic field and tremendous Ice Ages.
Despite the “mediocrity principle,” our planet appears today more than ever to be an authentic jewel of creation. “Where were you when I laid the earth’s foundations?,” God asked Job. “Who decided its dimensions, do you know? Or who stretched the measuring line across it?” (Job 38). The ancient Biblical emphasis on the Earth seems a little less ingenuous than before. And while the possibility that life may exist elsewhere as well remains an open and fascinating one, the sure–and even more fascinating–fact is that life is. Life certainly exists in oneplace, with the providential confluence of an infinite number of different and independent circumstances. The first sturdy foundations for the possibility of the emergence of life date to the origin of the universe itself and its structure on a large scale, but it is through the subtleties of a small planet that the development of life has been carried to its greatest heights. “Heaven and Earth”: the cosmos in its mighty greatness and the very delicate properties of Earth work together to host our life; together they form the “earth” out of which we human beings were shaped. We utterly exceptional beings, we who not only nourish ourselves, grow, and die like all other living beings, but are here consciously amazed and marvelling at the home that has been prepared for us and at our very prodigious existence.