What could life on other planets be like? This page looks at the extremities of similarity and difference, from biochemical make-up to major substantial and macrostructural differences.
Given that life cannot exist on all planets and that it does on ours, without further information we have to conclude that life is more likely to exist on a planet similar to ours than on those that are dissimilar. Life requires stable conditions to enable reproduction and then evolution. It requires a stable and consistent mix of chemicals so that life can start. Without this stability, life can never evolve to be able to cope with instability.
Carbon is the most likely source of organic material on most planets due to the ease at which it is formed during planet formation and its readiness to form complex and predictable molecules. Given that carbon is likely, so are the rest of the necessary molecules such as hydrogen, oxygen and nitrogen. It can be noted that all the molecules of life are also the most abundant in the atmosphere in which we live.
Life is probably on stable planets with an atmosphere (or local areas of atmosphere) that contain these chemicals. Life will, just like on this planet, evolve to use the most efficient methods possible for its own reproduction. Regarding metabolism, as nearly all cells use similar biochemical methods that these reactions may be the most efficient way of fuelling carbon-based life form's cycles. Therefore it seems that similar metabolic paths will exist in alien life as in ours; similar sugars could be used. Similar excretion could exist, as will a similar food chain.
One can be confident in stating that life on other planets will utilize many of the same reactions as we do in our cells - both driven by the fact that those who are most efficient sooner or later dominate the population. There is no doubt that aliens would have efficient organs and organelles that carry out specific tasks, like our cells do.
At Cambridge University, Dr Simon Conway-Morris is one who supports the concept of Convergent Evolution, unlike evolutionists such as Stephen Gould.
“[Dr Morris] thinks that if evolution were replayed from the beginning, a lot of things would turn out the same [... an effect known as] Convergent evolution. [...] Darwin himself was intrigued by this phenomenon, in which different groups of organisms independently evolve similar solutions to similar problems. [...] His argument is that, given the nature of physics and chemistry, there may be only a limited number of ways in which things can work. Evolution will be channelled into these successful paths, and thus does have trends. Two of these, he thinks, are towards complexity and intelligence. [...] The nature of molecules themselves means that evolution is more likely to follow a path determined by their basic structure. Evolution is a mechanism, and it works within rules.”
The Economist (2009)1
The visible spectrum of the electromagnetic spectrum is the narrow part of it that we think of as being colourful but this only colourful because we have receptors that can perceive it. Our sun's radio frequency output peaks along the frequencies that our eyes have evolved to detect2. Although some animals see a wider range of colours3, we all do center on the same range of frequencies. Hence, it stands to reason that life that evolves on planets nearby a similar star to ours will perceive the same colours. They'll be able to read our writing and look at our art without the need for scientific equipment. If we look at our own graphical outputs with infra-red eyes, for example, everything looks so much different. We can predict much of an alien species behaviour simply from knowing the solar output of their nearest star.
Life could indeed by quite similar to that which we see here. It won't have evolved the same species or phenotypes but we could be certain of seeing 4-legged animals, birds, fish, etc, the same basic classes that we have on Earth, and even a similarity in cellular (or pseudo-cellular) structure.
The foremost evolutionary biologist, Prof. Richard Dawkins, explains that while on Earth DNA molecules are the unit of natural selection, in alien worlds this may not be so:
“The laws of physics are supposed to be true all over the accessible universe. Are there any principles of biology that are likely to have similar universal validity? When astronauts voyage to distant planets and look for life, they can expect to find creatures too strange and unearthly for us to imagine. But is there anything that must be true of all life, wherever it is found, and whatever the basis of its chemistry? If forms of life exist whose chemistry is based on silicon rather than carbon, or ammonia rather than water, [...] If a form of life is found that is not based on chemistry at all but on electronic reverberating circuits, will there still be any general principle that is true of all life? Obviously I do not know but, if I had to bet, I would put my money on one fundamental principle. This is the law that all life evolves by the differential survival of replicating entities. The gene, the DNA molecule, happens to be the replicating entity that prevails on our own planet. There may be others.”
DNA came to be the unit of self-replicating life because its constituent atoms happen to be frequent enough on Earth to facilitate reproduction. In other environments the chemical mix may well be different, meaning that the fundamental building-blocks of life could be different. So although the chemical mix might be the same and life that is broadly similar- made of amino acids and proteins, using DNA expressed through RNA - there is good chance that it'll be different. We start off by looking at life that is different in its specifics, before imagining intelligent life that is so utterly different we might not even recognize it.
Life could be surprisingly similar to that which we already know. But the potential differences are even more amazing. Major differences could arise from even small differences between the alien planet's solar system and ours.
“The initial gases, and the energy sources, are common throughout the Cosmos. Chemical reactions like those [duplicated by researchers in laboratory experiments] may be responsible for the organic matter in interstellar space and the amino acids found in meteorites. Some similar chemistry must have occurred on a billion other worlds in the Milky Way Galaxy.
But even if life on another planet has the same molecular chemistry as life here, there is no reason to expect it to resemble familiar organisms. Consider the enormous diversity of living things on Earth, all of which share the same planet and an identical molecular biology. [...] There may be some convergent evolution because there may be only one best solution to a certain environmental problem - something like two eyes, for example, for binocular vision at optical frequencies. But in general the random character of the evolutionary process should create extraterrestrial creatures very different from any that we know. [...]
Some people - science fiction writers and artists, for instance - have speculated on what other beings might be like. I am skeptical about most of those extraterrestrial visions. They seem to me to rely too much on forms of life we already know. [...] I do not think life anywhere else would look very much like a reptile, or an insect or a human - even with such minor cosmetic adjustments as green skin, pointy ears and antennae.”
Many of the specific and accidental ways in which life evolved on Earth may mean that any alien life is radically different. For example, ancient cellular life absorbed a bacterium, possibly a virus-like predator, and co-evolved with it. Over time, this became the mitochondria that fuel the energetic reactions of all cells. This has resulted in an entire cellular energy flow that is distinct to our planet - it is almost certain that any life elsewhere will not have had the same symbiosis develop. If this part of the cell is different, then the overall picture is also likely to be very different too, even if the underlying chemical reactions are technically the same to cellular life on alien planets.
If the Sun emits a different pattern of radiation, microbial and plant life may evolve to use completely different biochemical pathways to ours. Their fundamental biochemistry could be vastly different. A different sun implies different metabolisms, different consumption patterns, different amino acids. Proteins, amino acids and the fundamental basis of living life could be completely different. In a hundred science fiction films, few have ever really come up with life that is fundamentally different to ours. The possibilities are said to be largely beyond imagination.
Daniel Dennett points out that we do not consider chemicals to be alive. Even self-replicating crystals and clever nanobots are not "living"; we consider them to be completely mechanical. Yet we are made of these non-living, "mechanical" chemicals. Not only are we made of robots, and we still consider ourselves to be valid conscious life, but the evolution of all life is from such mindless robots. As life is based in biology, and biology in biochemistry, chemistry is based on the cold, non-living laws of physics.
“We are made of robots - or what comes to the same thing, we are each a collection of trillions of macromolecular machines. And all of these are ultimately descended from the original self-replicating macromolecules. So something made of robots can exhibit genuine consciousness, because you do if anything does.”
"Kinds of Minds" by Daniel C. Dennett (1996)6
It is easy to imagine that we are capable of building computers that have massive lifespans. It is also very possible that machines and robots outlive the species - and planet - that created them. A homeless, wondering robot or machine might be the sole survivor of an intelligent species. Impossible, you think? Well consider the Voyager satellite and space probe; our creation is now speeding through space far, far away. In how many thousands of years will it be discovered by another intelligent species? In a million years? Or if somehow it does not fall into a star, a billion year in the future, will a spacefaring race stumble across it, in an era when Human beings have long died out? What a melancholic representative such a robot would be.
Given the current pace of development, it is easy to imagine artificial life venturing out into the universe and leaving us fragile humans behind. It may even be more likely than human-first space exploration. If it is more likely for us, it may well be more likely for others, too. A robot handshake is likely to come many thousands of years before a biological one.
There is a huge range of life on Earth, from specialized multicellular beings that are alien and exotic to familiar 4-legged fuzzy pets, and from plant life to viruses that are not much more than chemical assembly-lines. The ecosystem that we are familiar with is clearly not the only possible design for life. Creatures living on Thermal vents at the bottom of the oceans and aphotic life deep in the oceans may as well be two zones of life on alien planets for all their differences to animal life on the surface. But that diversity exists right here on Earth.
Life in other places in the universe could easily be utterly different to ours. Our life is based on organic biochemistry: this is not known to be the only way that life can exist. Where any pattern can reproduce itself, life could evolve, in any medium. Life could exist inside complex echoes of sound waves, in the patterned reflections of radiation, or could form out of the complicated eddies of gravitational fields or in the movement of liquids. Life could be gaseous, liquid or solid, or in combinations that we find it hard to imagine. We could even find that such life exists on our planet and we simply don't see it. Planets themselves could be conscious. Many of these ideas are being increasingly discussed by scientists7. Lovelock imagined that Earth, as a complex planet, is itself conscious and alive, with the whole atmosphere forming its biochemistry (his idea was named The Gaia Hypothesis)8. The point is that we have no grounds on which to assume that life-as-we-know-it is the only form of life.
The human imagination is limited. Science fiction writers have presented nearly all of the above forms of life at some time. John Wyndham writes of living planets, early Star Trek episodes contained gaseous and electronic life in various forms, and multiple films contain psychic life of emanating from unknown mediums interacting with Humanity in obscure, and very unlikely, psychic ways.
We must face the truth that as life as we know it is derived from the cold, unthinking laws of chemistry, which are themselves bound to pure physics, it is possible that quite exotic life forms could emerge through similar processes, but using a completely different medium than organic chemistry.
Alien life could be so different it is unrecognizable. We might be as invisible to it as it is to us. For example our bodies are colonies, containing millions of bacteria performing useful jobs for us. Our bodies teem with life. You can't get closer to a different species than bodily symbiosis. But most of our history, we had no idea that that was the case. We couldn't perceive life even when it was literally under our own noses. Likewise exotic forms of life might perceive us Humans, yet we might not be perceiving them in return. Or we might one day discover alien life and find we simply do not know how to communicate, or how to make them notice us at all.
“The Gaia Hypothesis is the idea that the Earth's biosphere itself is a complex, conscious, self-regulating living being10. It was named by James Lovelock in the 1980s. Lovelock, born 1919, was a British scientist who specialized in atmospheric chemistry and an environmental theorist. The concept of "emergent properties" and our Human (all-too-biological) biases may mean that we are poorly equipped to understand, or perceive, life-forms such as Gaia11. After all, if all the paint daubs of a painting were sentient, they would never be able to perceive themselves as part of a greater living picture. Lovelock writes that the way many parts of the Earth's ecosystem are self-regulating and balanced is so intriguing that it seemed to behave in the same way as a single organism12.”
Observant readers will realize that Gaia isn't actually alien life at all. It is native. However the Gaia Hypothesis is certainly a thought-provoking starting point in considering what life out there in the Universe could be like.
Due to the radiation given off by our local Sun we find that the chemical chlorophyll is most efficient at absorbing energy from it. Chlorophyll absorbs all visible light except green, which it reflects. If a sun gives off different levels of radiation, chlorophyll could be useless. The visible spectrum that we are accustomed to is a result of the frequencies of light reflected from chlorophyll and other plant skins. In order to see our food, photoreceptors (eyes!) on Earth evolved to be able to view a particular range of frequencies of light - the 'visible spectrum'. Another important factor is the way the light from the Sun is refracted through our atmosphere. The frequencies of light that we evolved to perceive therefore result from a mix of factors, from the makeup of the atmosphere to pure chance.
With a star with even slightly different radiation emissions to our sun the local life could use a visible spectrum completely different to ours. It would mean that we could not view their "colors" as they do, and they would see us completely differently. The range of colours we can see with our eyes is a very tiny part of the whole electromagnetic spectrum. We might find all the colours we use to communicate are invisible to them without scientific equipment, and vica-versa. They may not see any of our color coding, it would appear different perhaps just as noise, depending on the material's other emissions. In communications we would have to avoid using any specific range of colors under the assumption that they can see them. A piece of metal with a story printed on it could appear to them as a plain piece of metal because by eyesight they cannot detect the same range of electromagnetic outputs as we.
Alien life forms might use a different frequencies of the electromagnetic as their principal visible spectrum. But they might not use light at all. Even on Earth, many species are blind to light. Aliens could use very different methods of communication to those that we're accustomed to. Birds navigate using magnetic fields, bats use sonar, some fish use electromagnetic fields to communicate and fight with each other. We have discovered some of these methods of communication only by chance. Contrast our vocals with those of whales and dolphins. Due to the different quality of the medium they live in, species will develop locally effective ways of communication.
We could pass life right by. What we view as noise, or what we can't detect at all, could be rich communication. Think of the hormone- and pheromone- rich world in which some animals live... and all of it is completely undetectable to us. How much more undetectable and unobvious could alien communication be on a different planet? It could be unimaginably obscure!
"Homocentricity or Anthropocentrism: Why Do Religions Think Humanity Is Central to God and Creation?" by Vexen Crabtree (2003)"From the Big Bang Theory to Multiverses: How and Why Does the Universe Exist?" by Vexen Crabtree (2014)
“Mainstream theologians and intellectuals within the Abrahamic suite of religion have not given much time to discussions of the implications of alien life. However some have, and Erman McMullin warns his fellows that "religion which is unable to find a place for extraterrestrial persons in its view of the relations between God and the universe could find it increasingly difficult to command man's assent in times to come"13.
Some saviour religions actually cater for aliens. The Urantia movement holds that God has 700 000 sons incarnated on various worlds in the universe; Jesus is, of course, one of those divine sons14. The early scientist and astronomer Huygens argued that "the planets must be inhabited because otherwise God had made worlds for nothing"15. The religions that seem best placed to function in a Universe with more than species of intelligent, sentient beings are those that hold to universalist ideals; that is - the creator of the universe saves all living beings, and everyone goes to heaven. Other positions - that God punishes members of one species for doing 'evil' things, while punishing members of a different species for doing completely different things according to local circumstances, does not make sense if God embodies any set of absolute morality. In other words - deist, pantheist, universalist and non-religious forms of deism appear to be able to explain aliens much better than Abrahamic religions (Judaism, Christianity and Islam).”