By Taylor Marvin
Enough policy, here’s something completely different:
How common is intelligent life in the universe?
This question reduces to four possibilities: that all forms of life are rare in the universe and Earth’s life is the result of extraordinary circumstances, that simple life is commonly found but the complex life we are most familiar with is very rare, that both simple and complex life are common, and finally that intelligent life is found throughout the universe.
It’s important to remember that this question and the entire field of exobiology is extremely speculative by definition. Many of the field’s core ideas and claims can’t be falsified or even observed. For the most part, the lens through which we view life is very limited. The only examples of life that we can observe are those found on Earth, which all share a common ancestor and are fundamentally similar. All of Earth’s species share a system of coding genetic information in DNA molecules and similar metabolisms. We don’t know if this is the only possible structure of life or even if this is the only type that evolved on Earth; it’s entirely possible that another alternate biochemistry arose on the early Earth and disappeared without a trace. What is true is if a form of extraterrestrial life is based on the same class of biochemistry found here it would share its broad environmental requirements with the life we’re familiar with. This is interesting- because we can investigate how common Earth-like environmental conditions are in the galaxy it’s possible to make educated guesses about the frequency of extraterrestrial life chemically similar to that found here. For life dissimilar to that of Earth its impossible to say. It’s easy to imagine some exotic form of life radically different than anything we have ever encountered- interesting to be sure, but this is ultimately speculation without any real scientific backing. Any discussion of the frequency of extraterrestrial intelligence is limited to life broadly similar to what we are familiar with here on Earth because the environmental requirements of hypothetical exotic forms of life are too far beyond our comprehension even to guess at. For this reason all estimates of intelligence in the universe are inherently conservative.
The first possibility is that life of any form is very rare in our universe and the Earth is exceptional. It’s entirely possible that life of any kind simply doesn’t exist beyond Earth. Many solar systems and even galaxies are fundamentally incompatible with life. Any form of life we can understand requires a terrestrial surface, and older stars and galaxies are so metal-poor that they don’t support this type of body. Additionally, large regions of our galaxy appear too hostile for life to exist. The outer rim of the galaxy is too poor in heavy elements to allow terrestrial planets to form, and the inner regions seem to be too saturated with dangerous radiation to support life. Additionally, because stars are so close together in the galactic core planet sterilizing stellar disasters like supernovas would be much more common, imperiling any life that did evolve there.
The view that all life is extremely rare is also supported by the uncertainty surrounding the origins of life on Earth. Life is an extremely complicated chemical process and scientist are unsure of the exact conditions necessary for its inception. Certainly an element of chance is present; there could be millions of worlds with the conditions that would permit the inception of life but where it never actually begins. Unfortunately our uncertainty about just how life begins is so great that questions about how frequently any type of life arises remain pure speculation.
One other possibility that would indicate a lack of life in the wider universe would be that while the conditions that allow life to begin are common they don’t last. In our solar system both Mars and Venus had climates similar to Earth in their early histories but changing solar and atmospheric conditions slowly transformed Venus into a burning hell and Mars into a dry cold desert. If this is a common path of planetary evolution many early biospheres could become more and more inhospitable, slowly suffocating their inhabitants and greatly reducing the total number of life-bearing planets in the galaxy.
Let’s consider the second possibility: perhaps simple life is common in the universe but complex multicellular life is not. After all, simple life like bacteria and archaea have much less strict environmental requirements than complex life like animals and plants. On Earth, simple organisms are found in all extreme environments. Microscopic extremophiles thrive in very acidic or basic environments or extreme heat or cold. Simple life is also found deep underground or at the hostile ocean bottom, using chemical sources for energy in the absence of the sun. It’s very hard to kill bacteria- it’s likely that some varieties of Earth’s microorganisms could survive on Mars, and bacteria have been demonstrated to be able to survive in the vacuum of space. These types of simple life could the survive hard radiation or drastic environmental changes that are the most common survival challenges in the universe. In fact, most varieties of simple life seem capable of enduring asteroid impacts, changes in solar energy, and nearby gamma-ray bursts, all of which are the greatest threats to nascent life’s long term survival.
For complex life, environmental requirements are much more stringent. Plant and animal-like life broadly similar to Earth’s would require a planet with a thick atmosphere and fairly constant environmental conditions. This rules out most of the galaxy; the inner core with its dense concentrations of stars suffers from high radiation and gravitation perturbations that increase the danger from asteroid impacts while the outer regions lack the heavier elements necessary for terrestrial planets or moons. In fact entire types of galaxies, notably globular clusters, are unfit from terrestrial-like life. Even worse, these conditions are not constant. Stars move through their galaxies in long orbits, and if a potentially life-supporting solar system’s movement brought it too close to an inhospitable galactic region its life would be extinguished. Also, the need for a constant climate excludes many types of stars; any star with a variable output or giant stars with short life spans would be incompatible with the emergence of advanced life. While terrestrial rocky planets seem to be common in our galaxy the specific conditions complex life requires would exclude many extraterrestrial planets. A life-bearing rocky planet cannot be too small: below a certain size a planet cannot gravitationally retain a significant atmosphere and will quickly exhaust its core’s heat necessary for the plate tectonics that stimulate evolutionary development. A life-bearing planet cannot be too close or too far away from its star- because all forms of biochemistry as we know it require a liquid solvent a planet’s climate must allow either liquid water or another exotic solvent, like ammonium.
While it is possible to imagine many exotic life forms able to thrive in environmental conditions very different from Earth’s it is clear that there are many more environments in the universe suitable for simple rather than complex life.
Just because the conditions that would permit the development of complex life are common does not imply that the life itself is. Complexity, like intelligence, is an evolutionary adaption and is not guaranteed to ever arise even on a fertile planet. Life first arose on Earth about four billion years ago, while animals only in the last 600 million years. It is entirely possible that a planet teeming with simple life would never experience the conditions or chance that sparked the emergence of more complex, familiar life.
A third possibility to our question is that, despite formidable environmental challenges, multicellular, complex life is common in the universe. This view is most supported by the size of the galaxy: the Milky Way contains hundreds of billions of stars, a sizable percentage of which host rocky planets. Supporters of this idea argue that it’s wrong to believe that the Earth is the only planet out of hundreds of millions to develop complex life. Furthermore, the specific environmental conditions found on Earth aren’t as rare as commonly thought. Mars certainly could have supported some form of life early in its history and other bodies in our solar system like the moons Titan and Europa offer tantalizing clues to their potential habitability. Astronomer have also discovered at least one extrasolar planet, Gliese 581 g, with surface temperatures that could support liquid water, a prerequisite for Earth-like biochemistry. In spite of its specific environmental requirements and vulnerability to varied environmental threats complex life could be common, an assertion that is ultimately best supported by the sheer size of the universe.
Just because multicellular life is common in the universe does not mean intelligence is. On Earth animal life existed for 600 million before the emergence of intelligent humans. What is important to understand is that the history of life is not a grand structured procession towards intelligence; rather, human intelligence is an evolutionary adaption that arose in response to specific environmental pressures. It foolish to expect an extraterrestrial world, after 600 million years of complex life, to suddenly see the emergence of consciousness. We have a poor understanding of exactly what conditions favored the beginning of advanced intelligence among early humans- it’s entirely possible that an extraterrestrial equivalent of the great apes could face different environmental pressures and would never evolve the extraordinary cognitive abilities that define us. Intelligence is only a tool for survival and if opportunity and environmental conditions don’t select for it advanced cognition won’t emerge.
However, there are plausible theories that support the notion that if complex life is common in the universe intelligence could be as well. One of the strongest arguments to support this idea is the history of human development here on Earth. On the surface, human intelligence seems excessive; while toolmaking and complex communications certainly had valuable evolutionary utility it is harder to imagine what environmental pressures would select for, say, the human preferences for visual art or music. Moreover, our intelligence comes at a high cost. Humans’ long vulnerable childhoods and need for large amounts of parental care is directly due to our intelligence: human babies must be born relatively premature so that infants’ large brain-cases can pass through women’s hip structure. Even then, human childbirth is uniquely dangerous; it’s rare in the animal world for so many mothers to die is such a fundamental activity. What then could have led to the emergence of humans’ fantastic intelligence in spite of its cost?
One of the most convincing theories, the ecological dominance-social competition model, holds that early humans grew so dominant over their environment that competition from other humans in a social group, not outside environmental pressures, became the single greatest determinant of reproductive success. In this environment it was the humans with greater intelligence and accompanying social skills needed to navigate increasingly complex group dynamics, not the strongest or most aggressive, that spread their genes the farthest. This led to intense reproductive competition that favored cognition and social abilities far beyond any pressure from the natural environment. Complex social structures, language, and the rest of otherwise superfluous human cognition all evolved due to social selection, rather than the pressures of a specific natural environment. What’s most interesting about this theory is that it appears to apply universally- any extraterrestrial social animal that achieved dominance over its natural habitat would experience the same slow shift from environmentally to socially driven evolutionary pressures and possibly subsequent runaway cognitive development.
There is another possibility. Perhaps intelligence is rare in the universe not because it rarely arises but because it is short lived. Most species survive for only a few million years before becoming extinct, and the natural world holds many hazards intelligent life is not immune too. Changing climates, asteroid impacts or a nearby supernova could all randomly end a extraterrestrial intelligence in its infancy. This threat is closer than most of us think; on Earth natural disasters have almost drove humans to extinction. 75,000 years ago a huge volcano on the Indonesian island of Sumatra erupted, lowering global temperatures and possibly reducing the human population to as few as 1,000 reproducing pairs. Such a close call wouldn’t be unique to humans and there is no reason to suspect another intelligent race would be so lucky.
More likely is the threat posed by intelligent species to themselves. Humans are the only species in the history of the world to have the power to terminate their own existence. Intelligence brings a high level of control of the natural environment- this presents a long-term danger. In the last century our civilization has developed technologies that have the potential to end much of life on Earth, and these same capabilities would be available to any sufficiently advanced extraterrestrial civilization. Acquiring potentially destructive technologies like nuclear weapons or bioengineering is a natural byproduct of scientific development and would probably be quite common to all intelligent species. It could be that civilizations as a rule don’t survive long after developing this armageddon capability; after all, humans have only managed to survive 60 years of nuclear armaments with several close calls, and we still have plenty of time to destroy ourselves. However, there is reason to hope: the technologies to develop world-ending weapons and advanced space travel are linked. It seems that most civilizations that survive their first few centuries of nuclear power would be able to spread their species over nearby worlds and greatly reduce their vulnerability to one species-ending disaster.
Of course, any attempt to answer the question of extraterrestrial intelligence is ultimately just and interesting exercise- informative and based in science, but ultimately no better than pure speculation. We simply don’t know enough about the universe or life itself to have any idea of what’s possible. However, this is one of the most fundamental questions people can ask, and one that has been a subject of human curiosity for millennia. From angels to modern science fiction the possibility of alien intelligence is universally fascinating. There are worse questions we can wonder about.
Note: This post borrows heavily from the excellent book Rare Earth. Other sources include:
Alexander, R. D. How did humans evolve? Reflections on the uniquely unique species. Museum of Zoology (Special Publication No. 1). Ann Arbor, MI: The University of Michigan, 1990.
Ambrose, Stanley H. “Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans.” Journal of Human Evolution Volume 34, Issue 6. June 1998, 623-651.