The vast expanses of interstellar space make the notion of contact with intelligent life seem rather bleak. Attempts to detect extraterrestrial civilizations have so far been in vain. The awe that such a possibility might engender and the concerns and fears that it might equally evince have fueled much of traditional thinking about the universe. But, in all our endeavours to bring that possibility into a daring reality, we have largely been limited not only by the vast stretches of space but also by the transient epochs into which our biological systems have been locked.
Might another astronomically remote civilization surpass our scientific progress to reach a level of technological posture far too high for our minds to conceive? There is no reason to presume that the processes of planetary formation that led to the evolution of the Earth and the processes of evolution that led to complex and eventually intelligent life would not be able to unfold elsewhere in the universe. Conditions favouring the presence of organic life are equally common elsewhere in the universe. However, quite conveniently, the Earth seems to have the conditions that are just right for complex and intelligent life to evolve. Does that mean that the Earth indeed just happened to be incredibly special to have been endowed with such a unique set of circumstances that ultimately favoured our kind of life, or are such circumstances bound to typically manifest elsewhere in the hundreds of billions of galaxies that our universe so abundantly boasts?
The location of the solar system within our galaxy itself is thought to be conducive for the development of life. The solar system happens to be located within the ring of our galaxy, 26,000 light years from the galactic core where abundances of heavy elements are satisfied for the emergence of life and where dangerous radiation from the centre is too far to pose a threat. The Earth happens to be in orbit around a G-class star in that star’s own habitable zone. The habitable zone is defined as the range of distances from a parent star where conditions for surface temperature and pressure are just right for water to flow as a liquid on the planet’s surface. In addition to inhabiting the habitable zone, the Earth also possesses a large moon which exerts a gravitational force that stabilises its obliquity, the tilt of its spin axis relative to the orbital plane, and which confers a necessary climatic stability that would otherwise not have been available. The Earth’s obliquity of 23.45 degrees is thought to be just right for a stable climate. An increase in the tilt angles would lead to frozen equatorial oceans and warmer poles as the poles get closer to the sun. The presence of the moon prevents any such chaotic seasonal and climatic changes from occurring such that variations in the obliquity of the ecliptic are only about 1 degree or so from 23.45.
The formation of the moon itself is thought to have been a chance event. In its later stages of formation, the Earth was indeed quite lucky that it had grown to a large enough size such that it could be hit by a colliding protoplanet. A Mars-sized planet is thought to have collided with the developing Earth, belting out debris that were then captured by Earth’s gravity to eventually coalesce into our moon. The object must have collided at a steep angle but not head-on in order to eject this disk of debris. The object must also have collided at a speed that was not too fast for it to vaporise nor too slow for it to accrete into the early developing Earth. Indeed, the collision is thought to have conferred orbital eccentricity to the Earth and also accorded some sort of spin angular momentum such that the speed of the Earth’s rotation became faster.
One possible life-conducive characteristic of the solar system also rests in the fact that it has the giant planet of Jupiter which, by virtue, of its enormous gravity is thought to deflect comets and asteroids heading towards the Earth.
This seemingly unique set of characteristics with which our planet abounds is thought to constitute what is known as the Rare Earth Hypothesis, which argues that Earth-like planets are exceedingly rare and are put together only by a particular sequence of improbable coincidences. This is contested by the mediocrity principle, the notion that there is nothing very unusual about the Earth and that the elements and organic molecules that life is made of are commonly present throughout the universe. In addition, the kind of star that our solar system orbits is commonly available in the universe and solar systems alike could similarly arise. Given sufficient time, the laws of nature will conspire to replicate such biological systems that we know of elsewhere. And, indeed, the unique set of features presented by the Rare Earth Hypothesis might not be necessarily unique or conducive to life after all. For instance, simulations have shown that Earth-moon systems could be fairly common throughout the universe, as much as 1 in 12 terrestrial planets boasting a moon such as our own (Elser et al., 2011). In addition, the idea that Jupiter safeguards the Earth from asteroid attacks has been challenged. Asteroids in the asteroid belt between Mars and Jupiter occasionally get knocked off into orbits headed towards the Earth. Indeed, there is no reason to presume that only these special properties are necessary for the development of intelligent life. There might be other properties, hitherto unheard of, that might be even more conducive to life as we discover other planets in the universe. In addition, the story of life on Earth itself has not necessarily been a story of success all the time. Indeed, more than 99 % of all species that have ever lived on this planet have gone extinct.
But, would the process of evolution be expected to unfold in a similar fashion elsewhere in the universe as it did on Earth? One particular notion about evolution is that it has been characterized by stochastic periods of rapid processes that are shaped by historical contingencies. Such historical contingencies lead to sudden bursts of evolution. According to paleontologist Stephen Jay Gould, evolution is replete with countless branches and various possible courses of direction and the multitude of organisms that exist today represent just a possible outcome of the plethora of others that could have otherwise existed. “Each step proceeds for a cause, but no finale can be specified at the start, and none would ever occur a second time in the same way, because any pathway proceeds through thousands of improbable stages.”, writes Gould. “Alter any event, ever so slightly and without apparent importance at the time, and evolution cascades into a radically different channel.”
So, how likely would it be that adaptive complexity and resulting self-consciousness emerge again, if we were to restart evolution? To Gould, the chances are quite minute, if not probably impossible. Gould harkens back to the Cambrian explosion, upon which major designs of all multicellular life are based off, to expound upon this. The Burgess Shale is a fossil bed in British Columbia, Canada that preserves the best record of Cambrian animal fossils, which are 500 million years old. Among these fossils, there exist 25 different anatomical basic plans. Only four survived to have modern descendants today. However, any of the 25 different basic plans could have equally led to distinct phyla, had they survived. “If the human mind is a product of only one such set, our origin is the product of massive historical contingency, and we would probably never arise again even if life’s tape could be replayed a thousand times….The history of multi-cellular life has been dominated by decimation of a large initial stock, quickly generated in the Cambrian explosion”, writes Gould.
However, the evolutionary record on the Earth has shown us that convergent patterns of evolution have independently arisen in isolated geographical locations that boast identical habitats, lending credence to the notion that similar evolutionary pressures will shape similar evolutionary patterns. Binocular vision has evolved a number of times independently in vertebrates e.g., in birds and mammals. Similarly, wings have convergently evolved in birds, bats, and flies. The fact that species responding to the same selective pressures lead to similar evolutionary outcomes regardless of history, suggest that evolution can in fact be predictable.
In an informal meeting in 1961, astronomer Frank Drake proposed an equation to estimate the number of intelligent civilizations in the our galaxy. The equation can be presented as: N = R * fp * ne * fl * fi * fc * L where R is the rate of star formation in the galaxy; fp is the fraction of those stars that would have planetary systems; ne is the number of planets, per solar system, where environmental conditions are conducive for life; fl is the fraction of those planets onto which life actually develop; fi is the fraction of those life bearing planets onto which intelligent life actually evolves; fc is the fraction of civilizations that develop radio communication technology, releasing signals that we can detect; and L is the average length of time such civilizations would spend releasing detectable signals into space. Much of the guesswork advanced had put estimates of N from as low as one to as much as one million communicating civilizations in the Milky Way.
Suppose indeed, that the processes of planetary formation that lead to the evolution of the Earth and the processes of evolution that led to complex and eventually intelligent life did also unfold elsewhere in this galaxy. If the evolution of intelligence would take 4.6 billion years old to unfold on this planet and the universe is 14 billion years old, that gives 9.5 billion years worth of time before the formation of the Earth. Civilizations that would take 4.5 billion years to form would have started to emerge about 5 billion years ago already. Those would be civilizations that are 5 billion years ahead of us. If we presuppose that in our vast galaxy of an estimate of 100 billion stars, one in a million stars would harbor a civilization, that would mean a 100,000 intelligent civilizations in our galaxy alone. Over a period of 5 billion years, that would have meant a civilization emerging every 50,000 years.
Our entire recorded history is no more than 5000 years old and assuming that such civilizations would be much more technologically advanced than we are, they would have indeed been able to develop advanced rocket technology (nuclear, solar sail propulsion etc) that could reach speeds of 10 % of the speed of light. Assuming they are successful and the average distance between stars in our galaxy is only five light years, it would take on average 50 years to travel from one star to another. In about 10 million years, they would expanded throughout the entire galaxy. Shown below is such a model of galactic colonization known as the “Coral Model”, a civilization starting at one star would travel to another, colonize it, and then after establishing their presence, send colonists to yet another star.
The fact that we do not see any evidence of intelligent civilizations when all our calculations would lead us to believe that we should constitutes what is known as the Fermi paradox. Where is everybody? Why has no one visited us?
Possible solutions to this paradox have been advanced. One is that civilizations are indeed extremely rare and a good case could made for the fact that we are the only civilization to have arisen in the Milky Way galaxy. Another consideration is that civilizations are actually common in our galaxy but no civilization has managed to colonize the entire galaxy due to the technological impossibility of interstellar travel or due to the possibility that advanced civilizations might have little to no interest in colonizing the galaxy and the tendency to explore might only be inherently human. Technological civilizations could have also simply self-destructed through warfare or natural catastrophe before being able to perpetuate themselves across the galaxy. A third consideration and perhaps one that is very intriguing is that advanced civilizations do exist but have deliberately chosen to conceal themselves from us.
The future of our own civilization lies in our hands. Whether we survive into the conceivable future or whether we self-destruct has profound implications on how any future civilizations would view the galaxy. If we are alone in this galaxy, our civilization is a remarkable rarity, one that warrants a most careful stewardship.
‘Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.” — Arthur C. Clarke
Bennett, Jeffrey, Bruce Jakosky, and Seth Shostak. Life in the Universe. San Francisco: Addison Wesley, 2003.
Elser S., Moore B., Stadel J., and Morishima R. 2011. Icarus. 214:357–365.
Stephen Jay Gould, Wonderful Life: the Burgess Shale and the Nature of History (New York: Norton, 1989).