ETI refers to Extra Terrestrial Intelligence. For our purposes, we can consider "Life" to include any organisms brought forth via evolution by natural selection. More broadly, life may include creatures that do not evolve, as a creature that maintains metabolism and homeostasis and so on might very well be considered alive. It may also include non-natural, artificial, selection - artificial life. And it may include stranger forms of evolution, as for example a process by which a certain organism is always improved and changed rather than breeding and competing with its descendants. However, let us limit ourselves to an understanding of life as the arisal of evolutionary processes against the background of a lifeless natural universe.

Even this limited vision leaves a lot of room. There are many, many options and possible scenarios for such life, with greater or lesser plausibility. This page will be concerned with exploring the ramifications of a single development - the arisal of an intelligent, technological civilization. As it is unlikely that we'll be ever capable of discovering more that fragmentary and partial information about any non-intelligent life form, and even less likely that we'll be able to learn from it, it is this scenario that is of greatest interest. It is generally accepted that an intelligent technological civilization should be rare in the galaxy, even rarer than extraterrestrial life which may itself be quite rare. It may be that no such civilization other than our own's exists. Nevertheless, we will take it as a given.

In this page, we will argue for a highly 'limited' view of exobiology, under which the constraints posed by physics, the nature of the evolutionary process, and the requirement of ending up with a technological civilization severely limit the variability of life. We will present arguments to show that many aspects of biochemistry, anatomy, psychology, and social structure can be derived from these general principles with a high degree of assurance. It is necessary to make clear, however, that even a very likely element need not be actually correct in practice. Since there are a myriad of aspects, each one independent of the other, we would not expect every single likely feature to be exhibited. Rather, we would expect most likely features to be exhibited, with every specific intelligent species missing some of the likely features at random.

As a final note on our methodology, these are based on our current understanding of physics and the laws of nature in general. There is no point in positing hitherto unknown principles. Furthermore, we will adopt the view that convergent evolution serves as a powerful argument for the universality of a given biological feature. We will also make the rather vain assumption that if something developed in a certain way in our world, it is at least a priori likely that it is a fairly regular way for life to evolve. Arguments to the contrary will need to show why earthbound conditions will not be typical for extreterrestrial life, not vice versa.

Life Will Be Chemical

There are four forces in nature - the gravitational force, the electromagnetic force, the weak force, and the strong force. Our life is founded on the electromagnetic force, as it is exemplified in the forces between atoms (which are ubiquitous in nature). In other words, our life is based on chemistry. Is it likely that other life will be based on any of the other forces?

Gravity is an unlikely choice, as it is only attractive[1]. This prevents the construction of elaborate structures and dynamics that are vital to life. There also doesn't seem to be any scenario under which the natural evolution of the universe, itself driven by gravity, will result in the evolution under natural selection of gravitationally-bound structures. We can therefore safely reject gravitational life.

We can also reject life based on the strong and weak forces. These forces typically operate only at very small distances, of the scale of the atom's nucleus. In order for them to have any affect on structures larger and more complex than atoms, then, we need to pack up the stuff of the nuclei together tightly. This is accomplished in rare astrophysical locales, such as white dwarfs and, to a greater extent, neutron stars. Even if life does develop in these exotic locales, it doesn't seem possible for such 'matter' to exist beyond these environments of extreme gravity. We can therefore simply ignore this possibility without further analysis - if such life exists, it will be localized and with current scientific knowledge we cannot detect it.

This leaves us only with chemical life. Atoms and molecules are ubiquitous and display complicated interactions so that different atoms or molecules will be attracted a distance towards each other, yet be repelled if they come too close - this is the essence an atomic or molecular bond, keeping two atoms (in the simplest case) at some particular distance from each other. Chemical bonds are also very dynamic, allowing a rich plethora of processes and changes in state and structure. There is just no competition - only the chemical interactions offer the rich, diverse building blocks from which the complex structures required for evolution can be built.

Note that this does not mean that the other forces will be irrelevant. It is entirely possible that the weak or strong forces will be utilized as sources of energy, for example - indeed, they are, in terrestrial life. But life will be built from chemicals and have an essentially chemical metabolism.

Life Will Be Organic

Of all the atoms, carbon is ideal for constructing complex structures. Over a wide range of temperatures, carbon compounds are stable and its double and triple bonds are powerful and robust. With four valent electrons, a carbon atom can be used to form exquisitely complex structures. It is also fairly common in the universe, especially but not only on planets. It therefore makes sense that is will form the key building block of life - life will be 'organic' (which means 'based on carbon compounds').

There important caveats here. First, other materials can also be used, with Silicon being a relatively close competitor. However, none of these other materials can come close to Carbon's advantages. So it is possible that Silicon will be used instead, but less likely, and even less likely that less-ideal materials would be used. The use of Carbon is not necessary, however; it is merely more likely. See Alternative biochemistry.

Furthermore, in order to be used Carbon has to be freely available for recombination, which in the early stages of life would literally mean it has to be floating around - floating around in some solvent, or perhaps in air. The range of conditions under which this is possible is unclear. It is certainly possible on earth-like planets, and perhaps on considerably colder planets with different (organic!) solvents. It is possible that in extremely cold environments, which are ubiquitous in space, Carbon will freeze solid and will just not be usable. Perhaps in such environments other elements, like silicon, are more likely. We will proceed however on the assumption that this is not generally the case, so that where Carbon isn't available other useful elements are likewise not available.

Life Will Be Aqueous

As already mentioned, in order for early life to develop a solvent is required. Only a solvent provides the mobility of materials and the stable chemical background against which metabolism and homeostasis can first develop. Cellular membranes and protective sheeting can take life to drier locales one it has developed them later, but initially a solvent environment is required.

The ideal solvent is water. In addition to its various chemical properties that make it the 'wonder' material, water is fairly common in our and presumably other solar systems. It stands to reason that in those environments where water is liquid, then, it will serve as a liquid. In most environments in the solar system, however, water is frozen solid (if it is present at all), and in others it will boil away. What about those environments where water isn't present? Would they be devoid of life?

I don't know enough about chemistry to say whether other solvents would make sense under these conditions. Methane seas seem to be fairly common, and there are places with sulfuric acid and many places with various gaseous environments. So it isn't clear. It may be that life will not use water as its solvent.

Whichever solvent life does use, however, it will use it consistently. Just like our bodies are still mostly water, so too will other life carry its solvent with it even if it leaves its primordial oceans. That this solvent will be liquid water seems like a good bet, but this is immaterial in the greater picture, as we shall see below.

Life Will Use Molten Rock

Life requires an influx of energy. Being chemical, early life requires chemical energy. Mechanisms to exploit more esoteric types of energy, such as sunlight, require sophisticated structures that can only arise once life has established a foothold. Early life requires a more basic source of energy, chemicals rich in energy that are regularly replenished in a chemical environment that doesn't decompose them efficiently. There must be a simple chemical pathway that will offer a lower-energy end product, so that life may serve as a catalyst to enable it and harvest the released energy to do the work required to maintain and propagate itself.

The only source of such chemical energy that we are familiar with is molten rock. Whether due to fission-powered tectonic activity (as in hydrothermal sea vents) or due to tidal forces (producing very similar tectonic activity), rock can be melted and later frozen in highly-energetic compounds. It is therefore most likely that early life will draw their energy from chemical compounds in molten rock (hydrogen sulfide, ferrous iron, and so on).

This has ramifications for the energy economy within life.

Life Will Use Phosphorus. Or Arsenic.

Life Will Use Amino Acids

Perhaps of different chirality, though.

Life Will Be Cellular

Life can never escape its ancestral environment. To go elsewhere, it must find a way to take that environment with it - and hence the importance of cells. Cells also allow a measure of isolation and protection, and membranes enable hitherto untenable chemical complexity and staged processes. The development of a cell memberane is hence to be expected.

Later cells may develop further protection to shield them from even harsher environments, like the cell wall of plant-life. Further inner membranes may also develop within the cell.

Life Will Be Genetic

It goes almost without saying, but life will carry with it a genetic code. There doesn't seem to be any real reason why DNA will be needed to carry that code; it can be carried by anything, some organic molecule. It will certainly not have the same genetic code as we do, even if for some reason it does use DNA.

Life Will Develop Autotrophic Capabilities

Early life will make use of available organic materials, hence probably amino acids. But these will run out, especially as life moves to new environments. Hence, life will find means to create the neceassary amino acids (and other structures) from more basic raw materials.

Life Will Develop Photosynthesis (or chemosynthesis)

Chemical energy sources are limited in their extent, while light flows over the entire planet. If there is even the possibility for it, sooner or later photosynthesis will be developed, creatures that draw their energy from the sun. Of course, this won't develop outside the surface of a planet.

Ecology Will Develop

From very early long, entire ecologies will develop. At the bottom will be autotrophs gaining energy from chemical reactions and, perhaps, the sun. Parasites will pray on them, giving them disease, and there would be predators. Other organisms will develop that will live in symbiosis with others. The full plethora of ecological relationships will develop.

Interlude: Taking Stock

What we have at this stage are autotrophs using chemical and, perhaps, solar energy to contruct organic compounds and thereby multiply and sustain themselves. A larger ecological food pyramid sits on top of them. All of this is done in an aquous environment, which probably means water at around our own's living temperatures. This is all done in water oceans, perhaps underwater near tectonic cracks with perhaps an offshoot at the water's surface, under the glare of the sun. Numerous other chemical elements are surely involved in all of this, to allow rich metabolism, and these are involved in cycles (such as the 'nitrogen cycle', 'sulfur cycle') and so on of metabolization and replendishing.

We have "slime world".

Cellular Respiration?

Biosynthesis on earth involves taking the Carbon out of carbon dioxide, hence creating oxygen (O2) that can, in turn, be used to burn organic materials. The latter, aerobic, processes are very effective. Will a similar process occur on other planets?

Multicellularity Will Develop

Multicellularity developed independently several times on earth.

Eukaryote Cells Will Appear

Eventually, symbiont uni-cellular organisms will merge into an "eukaryotic" cell, with different roles taken up by the different organisms, now turned into 'organelles'. This degree of specialization within cells seems critical to the development of complex multicellular organisms, I'm not sure why.

Gut and Anus

Intelligence requires size. As large multicellular organisms will develop, perhaps the first development will be of a digestive track. It is most effective to have a small indentation to concentrate digestive enzymes. This develops in time into a specialized digestive track, with an anus. Hence, life will have a single mouth and a single anus, connected with a specialized digestive track. This pattern has developed mulltiple times independently on earth.

The only other pattern that seems worthwhile is multiple mouths, used to cover especially large prey. But this appears less efficient, especially for an intelligent species which is expected to have manipulators.

Circulatory and Respiratory Systems

As the creature grows larger, it becomes necessary to develop respiratory and circulatory systems to move nutrients and other metabolic necessities to and from all its cells. Of course, the circulatory system will be used to ferry everything: the nutrients from the digestive system, the replendished air/water from the environment (or relevant molecules of it, such as oxygen and carbon dioxide), waste, and so on.


The development of such systems on large organisms is impossible without muscles. It takes pumps, constriction, and so on to move food, blood, and air (or water) around. Hence, the only way truly large organisms can develop is if they develop muscles.

This conclusion seems to be denied by plants. Certainly, some trees are huge. But plants are hardly specialized. Since they lack muscles, they lack capable circulatory systems as well which greatly prevents the development of the complexity required for intelligent life. Almost all cells in a plant are part of a very few tissues, and rates of metabolism are low as they are based on chemical diffusion. Creatures can grow to be large without muscles, but only in a 'simple' way. High-metabolism, active creatures are unlikely under such a scheme; and our intelligence requires such high rates of metabolism and activity, as slower ones will die in the "active creature" niche.


Coordination of muscles requires nerves. Only this will allow the complex movements required to prey effectively, especially once senses develop.

The Six Senses

Creatures will develop several senses.

The first sense will probably be of local, cellular, variables such as ambient heat, chemical concentration, and so on, creating local, single-cell level, responses. Hence, there would be glands and tissues responding, not more. But ultimately they will get connected up with a network of nerves for coordinated response.

They will develop hearing, as sound waves travel well in whatever medium (air or water, or even earth) they're in. They will develop sight, if life indeed reached the surface or if life is for some reason luminiscent. They will develop smell and taste. They will develop echolocation. They will develop electrosense, the sense of the electric and/magnetic field.


Coordinated movement is done best with bone, or something like it. Animals will develop a skeleton, exoskeleton, or at least cariligous ligaments and anchor points.

Water based life may manage without bone as octopuses of Earth manage. On Earth soft bodied land animals are not larger than a slug. Some tropical slugs can be quite big. It looks unlikely that a soft bodied land animal could be larger than a rat. If a soft bodied land animal were to manage without a skeleton it would need a planet with either low gravity or a dense atmosphere. A low gravity planet like Mars cannot hold onto a dense atmosphere. Therefore intelligent land dwelling aliens probably have some type of rigid suport. Jointed tentacles as flexible as the vertibrate spine are possible.

Exception, octopuses can squeeze through small cracks and openings that an animal with hard body parts cannot enter, in an environment like a Payrus swamp where there are many small gaps a soft bodied animal would be able to get to resources that hard bodied animals cannot reach and escape predators through small gaps that hard bodied animals cannot use. Therefore soft bodied animals just might be able to compete with hard bodied animals.


But will it be eusocial?


Development is based on altering cellular patterns, and hence symmetry is key. The different body plans that will develop will show symmetries, much like the various terrestrial body plans.


Once a creature's nervous system coordinating his senses and hormonal and muscle responses gets sophisticated enough, central processing becomes vital. The creature will develop a central nervous system, a brain.

Basic Emotions

Hunger, Lust, Curiosity, and so on.

Learning Capacity

Flexibility of the nervous system allows for flexibility of thought and learning, and is vital to minimize genetic information as there is too much to select on.


Some means of moving.


The heavy metabolic requirements of intelligence means that it has to be a predator eating large pray, hence jaws are a must. Eating mats of plankton is also a possibility, however.


hands or possibly tentacles are useful for toolmaking.

Weak Physique

Due to fire, tools, and so on.


Necessary for communication which is necessary to build on past achievements.

Social Emotions

Love, Hatred, Pride, Courage....


Perhaps the hardest issue to wrap our mind around is the nature of our own rationality. It is evident from animal studies that certain basic mental faculties associated with general pattern recognition and information processing arose independently several times in living taxa, and probably in extinct ones as well. These include physical intuitions such as the concept of objects that persist even when you're not looking, and are different from each other; a sense for numbers, capable of comprehending and directly perceiving small integers; a sense for size, capable of comparing sizes and amounts in "bigger than" relations; and so on.

What isn't clear is whether there is any further mental faculty that can be identified with rationality itself. While research is rudimentary, it seems that such a faculty does not exist. Instead, our Reason is composed of a myriad of separate "logical" intuitions like those mentioned above. What creates the illusion of a single, unified Reason is the reflective nature of our thought and especially the recursive nature of language, which allows us to contemplate our own basic intuitions and weave them into a single consistent framework.

Based on this understanding of reason, it is eminently likely that other sapient creatures will develop similar base mental faculties and will, because of the very consistency of the world, develop the very same reasoning rules and modes of thought that we have. While their specific mental faculties and algorithms will vary, the nature of our Reason is dictated most of all by the requirement of weaving different intuitions together in a consistent manner. Hence, the consistency of the world is at the root of our rationality, and we should expect sentient beings to arrive at reasoning algorithms that are consistent with and compatible with our own. This is only possible if they are isomorphic to our own; if they encode the same basic logic, even if in a different axiomatic order.

Where alien Reason might differ is in foibles and biases on the one hand, and in the actual algorithms used on the other. No human thinks things through intuitively through by using logic; instead, we impose logical thought onto our intrinsically associative thinking. Only the basest logic is preserved and abstracted by our evolutionary intuitions. An alien mind, however, might be endowed with inherently different modes of intuitive thought. Like us, it will restrict and structure them based on the same logic, for the reasons mentioned above. An intriguing possibility, however, is that it will also incorporate complex logical mental algorithms into its mental faculties. Associative reasoning is most easy to develop by evolution, as it is naturally incremental in its dividends and applicable to a wide assortment of environments and changing mental calculation levels. In contrast, complex logical deductions have little value in plausible environments and hence are not under evolutionary pressure to develop. On the other hand, they are increasingly useful under artificial conditions, especially in conditions of competing artificial intelligence. Hence, it is very plausible that such faculties will only be developed within Artificial Evolution.

Spatial Intuitions and Folk Physics

Intentionality and Folk Psychology


We have argued that an extraterrestrial intelligent technological species will be in many ways, perhaps disappointingly, like us. It will live within an ecology much like ours, with plants, herbivores, carnivores (or their equivalent) and so on. It will be a lagre, multicellular creature that is probably a predator and at the least a herbivore. It will live in large socieities that use language and writing to build up on the past and gradually create science, knowledge, and technology. It will be driven by our drives and share much of our psychology, including trivial things such as pride and honor and chastity, and will have criminals, liars, thieves, murderers, and so on.

In many ways they would be unlike us. Perhaps they will have four arms, an exoskeleton, and bug eyes. Perhaps they will live in a methane environment. Perhaps they will be completely herbivores. Perhaps they will not share our sense of humour.

See also

  1. Over large distances and in certain exotic circumstances gravity may actually be repulsive. We shall ignore such exotic physics in our discussion.
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