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SOURCE: Journal of the British Interplanetary Society
Vol 32, pp.99-102,1978
TITLE: THE PHYSICAL APPEARANCE OF INTELLIGENT ALIENS
AUTHOR: N.J. SPALL
Bearsted, Nr. Maidstone, Kent, England
There can be little doubt that one of the most important factors that will
determine the manner in which our society reacts should contact ever be
established with intelligent extraterrestrial (ET) life forms will be the
physical appearance, or morphology, of the alien. All the prejudices, the
fears, the mistrust and the bigotry that exists amongst the races that make up
mankind will be focussed into this reaction. Thus, speculating on the
morphology of an intelligent alien is important for the future of space
exploration. Serious efforts are now being made around the world in the field
known as Search for Extraterrestrial Intelligence (SETI) and the manner in
which our society reacts to contact will depend to a great extent on the
appearance of the alien. Anticipation of the possibilities now may reveal
whether a shock for the world is likely. It is also useful to consider alien
morphology in terms of gaugin how likely the chances of intelligent aliens
evolving really are.
1. AN APPROACH TO THE SUBJECT
The problem of trying to anticipate the physical appearance of the ET is at
first sight ludicrously impossible. To start with, we don't even know if
intelligent ETs exist, let alone what their planet of origin is like or what
their morphology may be.
Our task is therefore limited to using what knowledge we have of the evolution
of intelligent life on Earth, considering possible extraterrestrial planetary
environments and making a series of reasonable assumptions. A combination of
biology, zoology, and anthropology is required as well as the newer science of
exobiology. Most important, the overriding thought when considering the
subject should be "how would this imagined alien become intelligent?"
2. THE TWO VIEWS
Conveniently, disagreement over the likely appearance of intelligent ETs
divides itself into two opposing camps. On one side are those who take a
rather anthropormorphic view of the ET and believe that it would basically be
humanoid in shape with two arms, two legs, a head at the top of the body and
the main sense organs located on the head. Opposing this view are those
exobiologists who believe that the intelligent ET is bound to appear
exotic because the creature would inevitably have taken a totally
different evolutionary path from man and would have arisen in a very
un-earthlike planetary environment.
This article will show, however, that the case put forward by the non-humanoid
ET protagonists will not stand up to the example of the evolution of
intelligent life on Earth, nor the necessities of morphology that a creature
requires to become intelligent. It is therefore suggested here that any
intelligent life across the galaxy will have evolved into a basically humanoid
3. EXOTIC BIOLOGY
A possibility often suggested by more radical exobiologists is that
extraterrestrial life might depend on a chemistry that does not require the
carbon atom. Bracewell  has proposed that life could make use of the
chemistry of the silicon atom rather than the carbon atom. Silicon based
organisms would, for example, breathe out silicon dioxide (sand) instead of
carbon dioxide. The rock eating creature has often been suggested as a product
of this biological system. [An example of this can be seen in the STAR TREK
episode about the Horta. AB]
The problem is that silicon polymers of the protein type are unlikely to form
the compounds essential for chemical evolution. Bieri  points out that
the energy requirements for duplicating a living system are fulfilled only by
carbon and the hight energy phosphate bond.
It is very difficult to envisage any life other than that based on the carbon
compounds forming in water. Unfortunately this limits the planetary
considerations necessary for the evolution of larger sized organisms somewhat
severely -- in fact it restricts planets that may have intelligent to those
with broadly Earth-like surface temperatures and pressures. (It also restricts
the type of star that may shine on life producing planets -- the DNA molecule
is sensitive to high levels of radiation, particularly the ultraviolet.
What of possible creatures that could get by without requiring the
availability of an Earth-like oxygen rich atmosphere? The conjectured
'balloon' creatures floating in the gas belts of Jupiter and using, instead of
oxygen, a metabolism of hydrogen -- could they ever become intelligent ETs?
And what is wrong with with Fred Hoyle's "Black Cloud," an intelligent gas
cloud thousands of kilometres across? The answer lies in our prime question,
"how could this creature become intelliegent?" Intelligence, it is argued
later, will probably only arise from astimulating predatory existence in a
harsh but survivable physical environment.
Conceding defeat to the necessity for life to be based on carbon in a water
medium, the exotic morphology ET supporters suggest that there are enormous
variations open to chance evolution even under Earth-like conditions. Slight
differences in surface pressure, temperature, gravity or solar radiation, they
argue, will produce widely divergent evolutionary trends . Steen 
suggests that intelligent ETs might be insect like, bird like, fish like or
even plant like. They may be spherical in shape, glutinous jelly-like
creatures, such as "Quatermass" might meet, or possibly even a planet sized
oceanic intelligence such as that in Stanislaw Lem's novel "Solaris."
For less bizarre (but still very exotic) alien creatures proposed for
extraterrestrial life bearing planets, the exhibits on display at the National
Air and Space Museum's "Life in the Universe" section in Washington, DC
provides some good examples of exotic aliens . Biologist Bonnie Dalzell
has designed for a dry Earth-like world the "hexalope," a six legged antelope.
For a high gravity planet, we are presented with the "bandersnatch," a
monstrous herbivore with eight legs, a large mouth in its chest, two eyes
on stalks and ears along the side of its body -- the creature weighs 30,000
lbs. on its 3-G world! The intelligent ET that Dalzell presents us with is
a six legged toad like creature.
Life on Earth shows us just how strange creatures can become in the chain of
evolution. The giraffe is a good example of this. But it is highly unlikely
that these creatures could ever become intelligent.
4. THE ANTHROPORMORPHIC VIEW
The problem ignored by exotic ET protagonists is that speculation on the
morphology of the ET must take account of the lessons taught us by
evolutionary development on Earth.
(The argument for humanoid ETs given here is based on the works of Robert
Bieri , N.J. Berrill  and Robert Puccetti )
In the early period of the development of life on Earth, organic matter based
on carbon compounds began in a water medium before the invasion of the land.
The early sea bound creatures developed a critical characteristic that would
decide the future form of land dwellers -- that of bilateral symmetry in the
shape of the body. This shape reduced water resistance and turbulence to a
minimum and became the characteristic of all the higher creatures of the sea.
It can be seen that adoption of a predatory way of marine life has
developed bilaterally symmetrical creatures as diverse as the
squid, the penguin, the seal, the otter and the large fish. Radially symmetric
ocean dwelling creatures all adopt a relatively stationary way of life, jelly
fish, sea anenomae etc., having a loss of sensitivity and degeneration of the
nervous system when compared to the more active predators.
Bieri points out that predatory animals with complex nervous systems and
bilateral symmetry possess the largest and most important sensing and grasping
organs close to the mouth. Also, digestion and excretion is most convenient
with an anterior mouth and posterior anus for an active hunting animal. In
order to reduce time for nerve impulses to travel from the sensing organs,
the brain is at the head.
5. CONCEPTUALISM AND INTELLIGENCE
Conceptualisation, it would seem, can arise only in a land animal. Birds
cannot possess brains large enough for this due to the fact that they must be
light in weight and have hollow bones to fly. A large intelligent brain
requires a considerable amount of blood and therefore a heavy cardiovascular
system -- both these factors would lead to an impossible power to weight ratio
for an intelligent airborne creature. It is also difficult to imagine an
intelligent ET evolving from gliding winged creatures such as the flying
squirrel (which glides from trees with the use of membranes under its front
legs) -- it is too small to evolve intelligence. It is doubtful that even a
gliding creature as large as the extinct Pterodactyl could ever develop a
large enough brain.
The question of intelligence arising in sea animals is somewhat more complex
due to the fact that the whale family happens to possess large brain capacity,
a very advanced system of communication and displays remarkable feats of
intelligence. However, conceptualisation, as Puccetti attempts to define it,
seems to arise in conjunction with a social existence, speech and the use of
tools. The development of tool usage undersea is extremely difficult due to
the density and viscosity of water. Predatory sea animals rely on their
natural hunting equipment -- teeth, streamlining, speed, etc. -- rather than
weapons and tools. Only semi-land creatures, such as beavers and otters (both
mammals) possess any sort of manipulating appending and these they use on the
How the whale family came to develop such a large cerebral capacity tends to
cast some doubt on the whole question of conceptualisation development. Here
it is assumed that whales are clever, but do not conceptualise on their
An encounter, therefore, with a race of intelligent aliens who are either
aquatic, reptilian or are creatures capable of flight and who developed
conceptualisation characteristics with a high level of technology, seems
highly unlikely. Our intelligent ETs would have to be land dwellers.
6. THE PREDATORY SUPREMACY
It should be emphasized that it seems most likely that all intelligent
conceptualising creatures in the galaxy will have their own origins in
predatory animals. Man's origins appear to stem from herbivore apes that,
faced with climatic and vegetation changes, left the trees, became omniverous
and adapted to running on the savannah, hunting other animals in groups and
using their ability to grasp and manipulate to develop weapons, tools and
eventually a basic technology. It is difficult to imagine an animal bothering
to use weapons and tools, firstly if it was a fully adapted herbivore and
secondly if it was already a competent predator, such as the lion or tiger.
Arthur C. Clarke describes this critical path that the early hunting apes had
to take extremely well in 2001: A SPACE ODYSSEY (although of course he did not
let his apes develop their technology purely on their own initiative.)
Man has remained the only creature with a technology on this planet because of
his predatory hunting nature, despite the basic ingenuity of creatures such as
the ant with its ingenoius city like hills, chimpanzees which can fish out
termites with sticks, birds that can break shells with heavy stones, and
the sea otter that can break open shells by floating on its back and beating
them against stones on its chest. These creatures have stretched their
manipulative abilities to the limits. [??]
7. MOVING AROUND
The development of legs, arms and grasping appendages is critical to our
conceptual ET's road to intelligence. A primitive technology will require the
ability to hold and manipulate, with some degree of sensitivity, basic tools
and weapons such as clubs, spears, knives and twine. The intelligent ET must
have this manipulative capability combined with speed of movement, otherwise
it will remain in its comfortable environment (as did the dolphin) and we
would certainly never meet it stepping out of a star ship.
As a method of movement, sliding, wriggling and rolling are all much too slow
for the land predator. As Puccetti points out, walking is the only viable
means of moving at high speeds and for long distances. The wheel was never
used as a means of locomotion by nature except in some tiny bacteria. Although
the reciprocating knee joint in the human leg can put up with large shock
loads and the shoulder and hip joints can rotate through a considerable arc,
it is difficult to imagine an organic bearing that could rotate through 360
Insect like appendages are unlikely. Insects possess legs that are basically
hollow cylinders with muscles and tendons inside the skeletal tube. The
problem with this arrangement is that if the creature grows in size the tube
will constrain the inner muscle size -- hence the Tarantula being the largest
land insect left since prehistoric times. Hard levers and struts surrounded by
muscles and tendons, as in land walking vertebrates, is a much more likely
arrangement in the predator land dwelling alien.
The question of the number of legs is one of the most contentious when
discussed by those speculating on the morphology of the intelligent ET. The
four legs that we have are the product of genetic inheritance from our
earliest mammal ancestors; but this inheritance allowed us great speed of
movement and thus played a major factor in the development of intelligence. One
leg is out of the question -- the creature could never get up if it ever fell
over. Odd numbers are unlikely because of balance problems. More than four
can only be found in insects. Galloping after prey with six legs is too
complex for land predators (and herbivores, as we have established, are
unlikely to become intelligent). Each leg has to swing through a wide arc for
speed and with more than four this becomes very difficult.
Monkeys and apes can use their two legs for manipulation but have to run on
both arms and legs together. Indeed the ape cannot use weapons to hunt whilst
running on all fours. It is difficult to imagine the development of an
intelligent hunting animal such as man running on two sensitive
grasping appendages. Thus we have the evolutionary step of the conversion of
one pair of legs to manipulating, pushing and pulling devices and the other
pair to movement. In this way the creature optimises between high speed
movement and delicate manipulation.
8. SENSORY ORGANS
So far we have formulated the picture of an intelligent ET with a body much
like our own. Its sensory organs, however, show characteristics that are
somewhat different, though not greatly.
Sense organs would largely depend on the characteristics of the aliens
planetary environment and the illumination provided by the local sun.
More than two eyes is rare in land creatures -- the spider possesses multiple
eyes, but they are of doubtful sensitivity, and would confuse a large hunting
creature. Stereoscopic vision near to the brain and high on the body is the
most suitable. Binaural hearing would seem the most logical. This is required
for location bearing -- and thus the ET requires just two ears. Again these
would be on the head. Only one mouth is needed with the smell sensor close to
it and taste sensors inside it. The smell sensor can be used for breathing,
whilst the mouth is occupied with eating and drinking.
Additional sensory devices such as bat like acoustic ranging systems or infra
red sensors similar to those possessed by the rattlesnake, are possible. But
as Bieri points out, they imply a corresponding reduction of vision in the
normal sun illuminated spectrum. As we have established above that carbon life
probably only develops on planets with suns much like our own we can assume
that the visual spectrum would be similar to that on Earth for the alien ET.
Although, therefore, the sensors of the ET are similar to our own, the
placement on the head and their form might be quite different. Odd shaped
heads are likely, different ear shapes and sizes most probable and eye size
and colour would be different.
9. THE LIKELY ET APPEARANCE
The argument presented above gives backing to the anthropormorphic view of the
intelligent ET -- that is that the creature would be basically humanoid. But
this only a starting point. What would the intelligent ET look like in detail?
This question is, of course, even more difficult to contemplate than
speculating on the ET's likely basic form. However, here are a number of
possible variables to consider:
1. SIZE AND BUILD -- The height and build of the alien has often been
suggested as being related to the gravity on the creature's planet of origin.
A planet slightly larger than Earth, with a subsequently higher gravity would
result in the alien being squatter, with heavy bones and a powerful physique
-- in other words, something like a gorilla. On the other hand, a lower
gravity planet would result in taller, more spindly aliens. This argument is a
little simplistic in its conclusion and does not explain the wide range in
the sizes of Earth creatures -- for example, why is there such a large
variation in the size and build of the apes, all of which are fairly clever
It seems probable that one can draw parameters about the ET's size, the likely
range being between the smallest of the human races (the pygmy) at about 4
1/2 feet tall and the upper limit being around 7 1/2 feet tall. If the alien
is very much heavier than man, he would have problems with running for long
distances in pursuit of prey in his early development as a land predator and
would require a very large supply of readily available food to maintain
One interesting point about man is that we appear to be getting taller due to
our evolution, our bodies are losing their broader muscles and our heads
changing shape. It is more than likely that the humanoid intelligent alien
also experiences this form of slow morphological evolution due to changes in
dietary nutrition and life style. There is, of course, no guarantee that the
alien will meet man as we appear now. An intelligent alien basing his
conception of what man looks like from previously discovered spacecraft
message devices (such as those carried by Pioneers 10 and 11), or picked up TV
images in, say, 50,000 years time, may be in for a surprise when he meets a
hairless, chinless, towering egghead from Earth!
Equally interesting is the question of the differences between the male and
the female of the intelligent aliens' species. Would the two be quite
different morphologically as in the case of homo sapiens, or would the two be
virtually indistinguishable as with some creatures on Earth?
2. SKIN COLOUR -- The wide variation in skin colour and tone with creatures on
the Earth is enough to indicate the extreme range that could occur with the
intelligent ET. Indeed, why would the ET have a smooth skin? It is possible
that fur may cover the alien having been left behind after an evolution
stemming from a bear like creature, for example. (Indeed, it is interesting to
wonder whether whiskers, or some sort of delicate sensory feelers may remain
with an intelligent creature after it has begun to rely on its hands).
3. FACIAL ARRANGEMENT -- This, as already stated, is mainly constrained by the
smell and taste sensors being close to the mouth and by the need for stereo
vision and binaural hearing. Beyond this the facial arrangement possibilities
would be reasonably wide.
4. NUMBER OF FINGERS/TOES -- Again, variations could be wide although beyond
ten fingers or toes on each hand or leg would seem excessive and difficult for
the brain to coordinate. Less than four fingers on the hand would make basic
technology difficult to manipulate.
5. INTERNAL CHARACTERISTICS -- The internal digestive, cardiovascular and
pulmonary systems inside the intelligent ET would most likely be quite
different and it is not possible to list all the variations within the
confines of this article.
10. THE LIKELY REACTION
Our immediate impressions of the intelligent ET will be critical to how
society later reacts to the contact. The theme of this article is that,
because of the evolutionary demands to become intelligent and the probable
similarity between Earth and the alien planet, the intelligent ET will be
basically humanoid in form. Therefore, our reaction will most likely not be
Various questions, however, remain. For example, how far will the ET have
evolved beyond the humanoid morphology?
It is unlikely that prosthetics will change the basic form of the ET. In
general, artificial limbs (and bionics) are intended to resemble those
currently possessed. The aliens' view of good looks will be determined by the
most perfect and healthy of its species. Consequently any artificial aids will
be designed to blend with the pure form of the alien -- contact lenses
replacing glasses is a good example of this.
It is difficult to imagine the advanced alien ever giving up its basic body
appearance. Some writers have suggested that semi-immortality might be
achieved by removing the brain from the failing body and installing it in a
machine, thus creating the cyborg. If this is ever done it is likely that man
would want the new machine body to resemble the original organic body shape. An
even more radical idea is that once the alien has developed very high levels
of knowledge and consciousness, the mind may even be liberated from the body.
If this occurred we might never discover its original appearance.
A final question is to what degree will alien clothing and cosmetics mask the
basic morphology? Fashions can enhance and emphasise body shapes in certain
cases with our own current civilization -- possibly the same will occur in the
intelligent ET's society. Hair styling, however, is an example of how
sometimes fashion can seriously alter the shape of the body. Also, any
spacesuit or breathing apparatus might appear unusual.
Unfortunately, only through the discovery of artifacts or through contact
itself will we ever learn what the actual morphology of the alien may be.
Indeed, the chances are that the first close encounter with an alien
civilization will be via the radio telescope. Video pictures will in this
situation have to suffice for many years in the place of face to face contact.
It is the conclusion of this paper that these images of the intelligent ET
will not shock us; they may surprise and intrigue us, but it is unlikely that
mankind will find the alien fearful in physical appearance.
Hopefully, the ET will feel the same way about us.
1. R.N. Bracewell, "Life in the galaxy," reprinted in INTERSTELLAR
COMMUNICATION, ed. A. Cameron (Benjamin, NY 1963).
2. Robert Bieri, "Humanoids on other planets?" AMERICAN SCIENTIST, LII
3. P.M. Molton, "Is anyone out there?" SPACEFLIGHT, 15,p.250, July, 1973
4. S.W.P. Steen in the review of Freudenthals "Lincos" language, BRITISH
JOURNAL OF THE PHILOSOPHY OF SCIENCE, 336, (1962)
5. D. Dooling, "Speculating on man's neighbours," SPACEFLIGHT, 17, p232,
6. N.J. Berrill, "Worlds without end," Chapters 9 and 10
7. Roland Puccetti, "Persons: a study of possible moral agents in the
universe," Macmillan, 1968
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