Dan Haycock

Eris Collective

Alien Life: Does Extraterrestrial Intelligence Imply a Humanoid Body Plan?

Some of the content of this article is taken from my book,

Humanoid aliens are common in science fiction, but what are the odds that intelligent life – if it were to evolve on other planets – would resemble human beings? I suggest that “intelligence” implies a human psychology, which in turn implies a humanoid body plan.

Intelligent vs. Conscious vs. Sentient

It is necessary to first draw a distinction between intelligence, consciousness and sentience. For the purposes of this article, “sentience” refers to the information processing capacity of a system. The psychologist and physicist Robert Freitas developed the concept of the sentience quotient (SQ - analogous to IQ), based on the relationship between the size of each processing unit, processing speed per unit and the overall number of units.

Humans and other animals, computers and even plants and fungi can all be assigned a sentience quotient, since all process input and generate output (see also my article, ).

The word "consciousness" has several different, related meanings. It is used to refer to sensory awareness, self awareness or the ability to reflect on things, a state of alertness (contrasted with unconsciousness) and a level of the mind (contrasted with the subconscious).

For the purposes of this article, I will use the term “consciousness” to refer to sensory awareness – the ability to decode qualia from sensory input, and thus construct phenomenological representations (see also my article, ).

In contrast, “intelligence” refers to an aptitude for a particular set of cognitive abilities, including reasoning, planning, understanding, communicating and problem solving. In humans, intelligence is associated with verbal reasoning, spatial awareness, mathematical ability, etc.

Alien Intelligences

It seems plausible that highly sentient organisms might evolve on other planets, but those organisms may not be conscious. Sessile, plant-like organisms, for example, may be capable of generating output to vast amounts of input (e.g., adapting growth patterns to changing levels of nutrients), without having any sensory awareness whatsoever.

Equally, organisms may evolve with a vast capacity for sensory awareness, but without possessing any aptitude for cognitive tasks like reasoning, planning, problem solving, etc. Such beings would be highly conscious, and yet we would not consider them intelligent.

I hold that alien life forms with a high degree of sentience and/or consciousness could evolve with such radically different morphologies to human beings that we may not even be sure whether they should be classified as organic or inorganic.

However, in order to possess the cognitive abilities that constitute intelligence, an organism would need to possess certain prior characteristics (e.g., it would have to use tools). The evolution of these characteristics are, in turn, dependent on a certain form of body plan.

Evolution of Intelligence

The size to which a single celled organism can grow is limited by the fact that it must absorb nutrients through its body surface. Since surface area does not scale proportionally with volume (e.g., increasing the volume of a cube by 8 times only yields a surface area 4 times greater), large single celled organisms would be unable to adequately nourish themselves. Any large, complex life forms must therefore be multicellular.

In turn, the need for each of an organism’s cells to respire, metabolise and discharge waste products creates a selection pressure for simple multicellular organisms to develop body plans which allow each cell to be in close contact with the surrounding environment. Simple multicellular organisms thus display an abundance of porous, hollow, branching and tubular forms, ensuring the cells in the middle of the body do not die.

Alien life forms would be constrained by this same selection pressure, and so the simplest aliens would most likely be tubular (worm-like), branching (plant-like), porous (sponge-like) or hollow (anemone-like).

Mobility, Predation and Cephalisation

Intelligence is more strongly selected for in organisms which are mobile rather than sessile, and also in organisms which are predatory, since both mobility and predation involve moving through different environments and anticipating events. Predation also involves outsmarting prey, providing a selection pressure for problem solving and future planning.

Of the simple multicellular body plans, tubular forms – with a mouth at one end and an anus at the other – most lend themselves to predation and mobility. All bilaterians (the group which includes all insects, molluscs and vertebrates, including humans) have body plans which are essentially innovations on this tubular design. Intelligent alien life would most likely be both predatory and mobile, and would thus also have a body plan loosely based on this simple tube structure.

Another trait that intelligent alien life forms would most likely share with humans is cephalisation: the clustering together of sensory organs and the bulk of the nervous system (i.e., a brain) to form a head. This minimises transmission time between input and output, and allows different senses to be easily cross referenced. For predators, it also makes sense for cephalisation to occur at the feeding end of the body.

In molluscs, this processing centre is built around a ring of nerves encircling the gut, while in insects and vertebrates it is focused at one end of the organism. Intelligent aliens would thus probably also have heads and faces – either on the end of a neck, like us, or centrally, like an octopus.

Cognitive Abilities

Beyond this, many of the human cognitive abilities which we associate with intelligence are dependent on three things: tool use, social interaction and language. Without these characteristics, it is unlikely that we would have developed much of a capacity for abstract or associative thought, spatial manipulation, reasoning, problem solving, etc. It is these three abilities that mark the human race out when compared to the rest of the animal kingdom.

Kathleen Gibson, an expert on the evolution of the brain, and Tim Ingold, a social anthropologist, have argued (for example) that the increased human information processing capacity evolved in response to increasing tool use, language capabilities and social behaviour, all of which are mutually interdependent.

All of these behaviours depend on an ability to break down concepts, actions and perceptions into subunits which can then be hierarchically recombined to form larger mental structures. This ability underlies the cognitive capabilities associated with human intelligence.

Tool use is only possible if there is some kind of appendage for manipulating objects: e.g., arms and hands, tentacles, a trunk, etc. Any intelligent alien life form would thus need to possess these kinds of appendages.

Although it is possible to communicate with gestures, using sounds to communicate provides a stronger survival advantage than using gestures, since spoken language can be utilised over long distant, in the dark, or without interrupting motor tasks.

Likewise, although some organisms communicate through odour, human-like manipulation of semantic signalling relies on discrete, sequential units of communication. Some kind of sound based language thus seems necessary for the development of intelligence.


These lines of reasoning could be criticised on the basis that intelligent, terrestrial organisms exist which do not fit this body plan. Dolphins, elephants, cephalopods, etc.

However, as intelligent as these animals are, it is unlikely that life forms with similar body plans could advance to the degree that humans have: dominating entire planets and building whole civilisations. For example, much human technological advancement was dependent on the manipulation of fire – something which would be impossible for an aquatic species. It thus seems unlikely that dolphin-like or cephalopodic life forms could ever undergo the selection pressures provided by extensive tool use – selection pressures which are necessary to develop human-level intelligence.

In summary, for intelligent alien life forms to evolve – rather than simply sentient or conscious alien life – it seems likely that those life forms would develop an essentially humanoid body plan, consisting of an extended body with a head and face, appendages for tool manipulation and the capacity for a vocal language.

This article contains content taken from my book, , available now on Amazon.

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   Title: Being and Perceiving
   Publisher: Manupod Press
   Language: English
   ISBN-10: 0956962106
   ISBN-13: 978-0956962102