By the question I have posed in this essay’s title, I do not mean merely to ask why Life exists on the Earth at this particular moment in time, but why, once it began, has Life existed continuously since its inception, a period of at least 3 billion years, according to the most recent geological evidence. Considering all the many different things that could have gone wrong and put an end to this biological terrestrial eruption, which at first was only a very tentative foray, this is truly an incredible, astonishing, extraordinary, and highly improbable occurrence. Because most people have had no personal experience whatsoever of the conditions that exist outside our Earth’s protective atmosphere and magnetic field, they have no idea just how hostile and inhospitable to life are the physical conditions that are found in the rest of the solar system, the Milky Way Galaxy, and the Universe.
Although we bandy around words like “million” and “billion” regularly, the fact is that we simply cannot conceive of how long a period of time a billion years represents. From the time of the life of Jesus or the ancient Greeks to the present, which is most of the period of recorded human history, is a period of roughly 2,000 years, a length of time that already greatly exceeds the personal experience of any individual human being. In order to get to a billion years, we would have to multiply this figure by 500,000: in other words, one billion years is equivalent to 500,000 periods of 2,000 years repeated one after another after another.
The phrase, “Familiarity breeds contempt,” would be more accurate if it were restated as “Familiarity breeds complaisance or indifference.” We fail to see the miraculous or wonderful in the things that we observe regularly all around us, for the simple reason that what is commonplace ceases to evoke feelings of awe and wonder in most of us. The fossil record and other scientific evidence, together with the fact that so many different life forms continue to exist presently, lead many people to believe that the survival of Life on Earth for billions and billions of years was inevitable. But there are numerous reasons that suggest that such an outcome was extremely unlikely, if not impossible, solely by the concatenation of natural, meaning completely random, processes.
To give just one example of how unlikely is the continual survival of Life on Earth for billions of years, we need only consider the very great difference that exists between the surface temperature of the lit and the unlit sides of the Moon. The Moon, being the Earth’s satellite, is sometimes a little closer to, and sometimes a little farther away from, the Sun than the Earth; but on average, it is the same distance from the Sun as the Earth, meaning that, on average, a given surface area of the moon will receive the same amount of light and heat as the same surface area of the Earth that is at a comparable latitude and orientation towards the Sun.
There are several important differences between the Earth and the Moon: first, the Moon has no atmosphere, no invisible layer of gases that regulate its surface temperature and protect it from ultraviolet radiation and the regular impact of meteorites, and no electromagnetic field that protects its surface from harmful cosmic phenomena like violent solar flares, which spew large amounts of harmful radiation into space; and second, in relation to the Sun, the Moon only rotates roughly once every 30 days. The second of these two features means that a point on the Moon will remain lit for a period of roughly 15 days, followed by 15 days of complete darkness. If the Earth were subjected to such extreme heating and cooling differentials, then the temperature differences between night and day would be many times greater than they actually are, a change that would quickly eliminate the vast majority of larger life forms on the Earth, both animal and vegetal, and perhaps make any form of life impossible.
We must remember that the figures for the Moon’s temperatures are only estimates, since there are no lunar stations that record the temperature on a regular basis; but they do give an idea of the vast differences in temperature that exist between the Moon’s “day” and “night”. Moreover, there will be significant variations depending on where the temperature is measured, just as there are on the Earth, and at what time during the Moon’s much longer day or night it is measured, since there are differences in the Earth’s temperature, depending on whether it is measured at dawn, noon, or in the evening. According to one estimate, the average daytime temperature at the Moon’s equator is 106°C, while the average night-time temperature is – 183°C. Such a large temperature change of almost 300°C at regular intervals of roughly 15 days would be enough to kill the vast majority of life forms that exist on the Earth. As this discussion shows, another of the important features needed to support life, at least the kinds of life that exist on the Earth, is a fairly rapid rate of planetary rotation, for even a doubling of the Earth’s day and night would cause havoc, both by increasing diurnal temperature differentials, thus making the days hotter and the nights colder, and also by increasing the severity of winds and storms, since these are due to differences in air pressure and energy distribution.
One of the doomsday scenarios that have been advanced by scientists during the debate about climate change is that increased carbon dioxide and methane in the atmosphere could lead to a runaway greenhouse effect. As global temperatures increase, this will cause the polar ice caps to melt, which will diminish the amount of ice covering parts of the Earth, which will increase the amount of radiation that is absorbed by the Earth’s land and water, which will increase its temperature, causing even more ice to melt, and so on. There have been times in the Earth’s past when its temperature was as high or even higher than it is presently; and yet, it was somehow prevented from going even higher, so that it began to kill off many forms of life that existed at the time. Similarly, in the past, we know that there have been periods when much of the Earth’s surface was covered by ice. But in this case, as more and more of the land and water were covered by ice, this would mean that an increasing amount of the Sun’s radiation would have been reflected back into space, which would have lowered the Earth’s temperature even more, which would have caused more ice to form, until the Earth was covered entirely by ice. And yet, during each Ice Age, this expected result did not happen. In other words, once the Earth started to heat up or cool down, the expected result would have been for it to continue in that direction and become a hellish inferno, as exists on Venus, or a cold and barren wasteland like Mars, our two nearest planetary neighbours. But instead of these possible outcomes, the Earth has maintained a nurturing and hospitable environment so that the extremely delicate entities known as life can continue to exist and thrive on or near its surface.
These kinds of observations have led some people to adopt what is called the Gaia hypothesis. This theory, formulated by James Lovelock, holds that the Earth is a living organism and maintains its environment in homeostasis, which is a fancy word for equilibrium, so that Life can continue to exist.
The fossil record supports the notion that the Earth’s surface has been continuously regulated since the earliest widespread appearance of microbial life. The Gaia hypothesis that the temperature and reactive gas composition of the Earth’s atmosphere are actively regulated by the biota was developed by Lovelock while he was working for NASA on ways to detect life on Mars. He found that gases which when tested in simple chemical systems react quickly, easily, and completely to make stable compounds coexist in our atmosphere. These gases seem to remain aloof in apparent disregard for the laws of standard equilibrium chemistry. Lovelock found the chemistry of the Earth’s atmosphere so persistently bizarre that it could only be attributed to the collective properties of organisms, namely to the biota. Indeed, the biota, especially the microbiota, constantly produces prodigious amounts of these reactive gases. By looking for such unlikely gas mixtures in the atmospheres of other planets with spectroscopes mounted on telescopes, he could, he thought, detect alien biospheres without leaving the Earth. Turning his attention to Mars, Lovelock discovered it to be in a balance entirely understandable on the basis of physics and chemistry alone. He postulated the absence of life on Mars by observing the absence of the Gaian phenomenon.
Gaia thus may keep atmospheric nitrogen and oxygen, so important to life, from degenerating into nitrates and nitrogen oxides, into salts and laughing gas which could halt the entire system. If there were no constant, worldwide production of new oxygen by photosynthetic organisms, if there were no release of gaseous nitrogen by nitrate- and ammonia-breathing bacteria, an inert or poisonous atmosphere would rapidly develop. Under the reactive influence of scores of lightning bolts sparking the Earth’s atmosphere every minute, the Earth would be no more hospitable to life than is acid Venus.
In my opinion, those who believe in the Gaia hypothesis have made the mistake of taking an analogy too far. The belief that, since all living organisms maintain their bodies in equilibrium while they are alive, and the Earth exhibits many similar homeostatic processes, the Earth must also be a living organism is farfetched, to say the least.
There are many planets in the Universe – indeed, probably the vast majority of them – that don’t support, and never have supported, any life. This is true of all the other large astronomical objects in our own solar system apart from the Sun: none of the other planets or their more numerous satellites presently support, and perhaps have ever supported, any form of life during their very long existence. Since other planets are not alive, and since the Earth is a planet, it follows that the Earth is also not alive. To believe otherwise, as the adherents of the Gaia hypothesis claim, is no different from the Christian belief that Jesus was conceived immaculately, that is, that he alone of all human beings was not begotten by the usual process of sexual reproduction.
A human child that doesn’t know how food is made or grown, processed, prepared, and cooked, and has never been to a supermarket, will believe that food comes from the fridge or cupboard shelf of one’s kitchen, ready to eat – for example, that milk comes from a glass, plastic, or paper container, soup from a metal can, and rice or pasta from a sealed bag or cardboard box. Similarly, there are many human beings that naively believe that, once it started, the survival of Life on Earth was inevitable, and that all the many different things that we find here on the Earth for our survival, comfort, convenience, and development as a species all happened as a result of completely random processes. For just like the ignorant child, we too fail to see all the many different processes that are needed for Life to have survived for such a long time, while being able to develop into its astonishingly varied present forms.
Anyone who has taken care of a living organism, whether it is a plant or animal, knows that it requires regular care and attention in order to stay alive and flourishing. If we are not attentive, there are any number of different things that can happen to it and end its life prematurely. The same is true of all our many different non-living human creations, such as houses, books, cars, paintings, roads, and communication networks like the Internet: every single one of them requires regular care, protection, and maintenance in order to remain in their original or functional state, otherwise they will deteriorate, break down, and return to the terrestrial elements from which they were originally made.
The more one learns about Life on Earth and the conditions that are needed, first, for Life to exist at all, and, second, for it to continue to exist and thrive, as it was doing before our very brief period of terrestrial dominance, and even in spite of all our selfish and reckless activities that have eliminated and decimated a great many other life forms, one realizes that these outcomes are extremely unlikely, and therefore neither of them could have happened merely by chance, that is, as a result of purely natural or accidental processes. The likelihood that, after it first appeared, Life could have continued to exist on its own during an uninterrupted period of billions of years – a period of time that is far beyond our limited human ability to comprehend – without some kind of supernatural intervention in order to nurture, protect, and help it develop in particular ways, such as from relatively simple unicellular organisms to the more complex multi-cellular organisms that presently abound, is, in my opinion, zero.
If one begins, as most scientists do, with the assumption that everything in the Universe proceeds by natural, meaning non-divine, causes, then one will attempt to find these kinds of natural explanations for everything that happens or exists. But a strict adherence to this assumption, which we would do well to remember is nothing more than an unproven, and, moreover, an unprovable, assumption about the operations of the Universe, can beget a form of blindness, for it leads one to deny the evidence that is so often before our eyes: that there is indeed a divine Creator who created not only the Universe but also Life on Earth, and who intervenes periodically to enable that Life to continue to exist, thrive, and multiply, while directing or guiding its development.
Those Christians who believe in the literal truth of everything that is contained in the Bible fail to realize that the geological evidence for the Earth’s age – billions instead of mere thousands of years, which is the Earth’s age according to the Bible – together with the fact that Life has existed continuously for the greater part of that time, is the clearest evidence for God’s existence. That such an unlikely occurrence happened by chance, as the many irreligious scientists and their legion of credulous followers believe, is as much an article of faith as the belief that God has intervened periodically in order to preserve the conditions that are necessary for Life to continue to exist on the Earth, while allowing it to thrive, multiply, and diversify into a truly astonishing variety of different forms and sizes.
 Similarly, in human relationships, with habituation, the things that formerly excited, intrigued, and charmed us in another person cease to evoke those intense feelings of desire, passion, excitement, and longing which they once did, and which many people spend their whole lives constantly searching for, much like a drug addict. Furthermore, the pleasant feelings are sometimes replaced by unpleasant feelings such as anger, jealousy, dislike, disgust, regret, frustration, disappointment, and sorrow.
 A lunar month, which is slightly shorter than most terrestrial months, is the time it takes the Moon to go around the Earth and return to the same point in its orbit in relation to the Earth. However, to calculate how long it takes for the Moon to make one full rotation in relation to the Sun, since the Moon rotates on its axis at exactly the same rate at which it rotates around the Earth, we must also take into consideration the Earth’s orbit around the Sun.
 I am, of course, criticizing what is called the “Goldilocks” range of distances from a star – not too close and not too far away – that are believed by many credulous individuals to be orbital zones around a star where a solid planet like the Earth can potentially support life for long periods of time. Even if a planet is within this temperate zone, there are other factors, such as its relatively slow rate of rotation or its lack of a strong magnetic field or a sufficiently dense atmosphere, that would render the planet incapable of supporting life.
 A possible reason why atmospheric carbon dioxide levels were higher at some points in the Earth’s past may have been to compensate for the fact that the Earth was further away from the Sun. The assumption that the Earth has always been at the same distance from the Sun as it is presently is being challenged by some scientists who seek to discover its orbital trajectories in the past, when the solar system was not yet stable, and when the Earth’s motions were perturbed by other large objects in the solar system. See, for example, “The Madness of the Planets” by Corey S. Powell in The Best American Science and Nature Writing 2014. Edited by Deborah Blum. Houghton Mifflin Harcourt, Boston, 2014.
 Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors by Lynn Margulis and Dorion Sagan, p. 266. Touchstone Books, Simon & Schuster, New York, 1986.
 Ibid, p. 265.