The Humanoid Trope

Aliens have typically been conceived as consistent with the humanoid trope: bilateral symmetry, erect posture, bipedal locomotion, two grasping forelimbs, and a head with a large brain case. Other humanoid features may optionally be added: opposable thumbs, two eyes situated for binocular vision, a singular mouth with a hinged jaw, a pair of nostrils between the eyes and the mouth, mammalian skin, and a homologous musculoskeletal structure complete with pectoral, abdominal, brachial, and femoral muscle groups. The question remains as to the merit and viability of the humanoid trope, as well as whether we are justified in anticipating this morphology from civilized aliens. Note that we are now principally concerned with alien species which may become technological, as they are the creatures we are most likely to encounter at our present state of development; we can expect alien life in general to be significantly more diverse than the few species which develop technology.

Bilateral Symmetry

Bilateral symmetry is the most likely to emerge in complex organisms. The primary cause is metabolic expense—there is no plausible reason that an organism would support four, six, eight, or more eyes, nostrils, ears, mouths, and limbs, as is required of radial and spherical symmetry, when only one or two may be sufficient for its needs. This caveat is important—there may be exceptions where radial symmetry and the plurality of organs it supports is adaptive, probably in hostile environments. As a rule, however, the evolutionary filter for bilateral symmetry applies particularly to larger organisms, whose mass compromises the efficacy and efficiency of numerous limbs, and more expensive forms of life, such as endotherms.

Furthermore, the physical laws of this universe typically requires only two of each major sensory organ to augment their functionality. Two eyes are sufficient to perceive depth and position in three-dimensional environments. Two nostrils are sufficient to detect scent molecules which may rapidly decay or slowly propagate. Two ear canals are sufficient to triangulate the origin of a particular sound. Two antennae are sufficient to detect and triangulate vibrations in the earth. There are a few exceptions to this rule: electroreception requires clusters of sensory organs, magnetoreception is thought to function in tandem with a protein known as cryptochrome, and non-neurological senses as with plants may be distributed throughout the fundamental structure of the organism. Aliens making sole or extensive use of these senses may therefore be liberated from the expectation of bilateral symmetry.

Erect Bipedalism

Erect posture and bipedal locomotion are not necessarily related. On earth, the progenitor of most extant vertebrates was quadrupedal; its descendants have therefore had to operate with this inherited musculoskeletal structure. Some aquatic species have fused the hind legs into a tail and developed fins from the forelimbs. Some avian species have formed wings from the forelimbs. Some mammalian species, notably primates, have formed grasping arms from the forelimbs. The mathematics of their morphology have not changed as a result of this radial evolution; the form and function of the appendages has simply changed. Thus, when hominids developed grasping arms, they only had two remaining limbs to use for locomotion—bipedal locomotion by definition.

Yet erect posture is not a direct consequence of this limb orientation. Theropods such as the tyrannosaur also developed bipedal locomotion, but due to their large balancing tails they were able to retain prone, or horizontal, posture.

The tyrannosaur’s heavy tail would have compensated for its unusual posture

The development of erect posture was principally concerned with the utility of the grasping arms—walking upright enabled hominids to free their hands for manipulation and invention. It is precisely this point which informs our expectation of erect bipedalism in technological aliens—humans are the only known technological species (although tool use in chimps and octopi has been noted), and we are also the only species which consistently exhibits erect bipedalism.

This will not necessarily be the case for aliens with a different ancestral history. If an ancient hexapod underwent radial evolution to produce most extant species within the alien’s clade, we would similarly expect the alien to have six legs. They may therefore remain hexapodal, become quadrupedal with the development of two grasping arms, or become bipedal with four grasping arms. They will not necessarily evolve erect posture with any of these means of locomotion—the hexapod and quadruped will require at least partially prone posture, and the biped may remain prone if a posterior appendage such as a tail balances the weight of the anterior body. If their musculoskeletal system allows for adequate flexibility and balance, and if their grasping arms have sufficient reach, prone posture is no obstacle to technological development.  

However, the metabolic cost of numerous limbs remains a strict evolutionary filter for such organisms. Unless there is a demonstrable environmental need for a structure to exist, it will not predominate, or else it will become vestigial and disappear when it is no longer useful. Evolution does not often favor complexity—its prime objective seems to be the simplification and streamlining of extant life. Thus we would expect the simpler and less expensive bipedal and quadrupedal stature to predominate. In this sense at least, we may have good reason to believe that most technological aliens will have similar musculoskeletal structure to the human organism. Nevertheless, we cannot rule out the existence of bipeds which shatter the posture assumption or the existence of quadrupeds and hexapods which contradict the locomotive assumption.

Grasping Arms

Arms are not integral to the evolution of a technological species. Dexterous appendages, however, are essential. This makes the arm a logical and effective answer to the technological problem—the large muscles of the shoulder and upper arm allow broad articulation whereas the small muscles of the hand and its digits enable delicate movements and adjustments. This duality allows the arm to address a variety of demands, from the application of force to controlled positionality to fine manipulation. This level of coordination is not seen in any other grasping appendages on earth. It is by no means the only appendage capable of this coordination, however.

The availability of grasping limbs distinct from arms, such as tentacles, further complicates the supposed normativity of the humanoid trope. Tentacles may only be a viable means of locomotion in aquatic environments, but in terrestrial and aquatic environments alike they may be coordinated sufficiently well to exhibit the fine motor control requisite for technological development. Further, tentacles are sufficiently simple that their metabolic cost may be lesser than complex arms and digits, facilitating their emergence in bilateral, radial, and spherical organisms. They may also emerge in numbers greater than classical arms at comparable metabolic cost, enabling a wider range of gross and fine motor control.

If tentacles develop on the ventral side of the organism, analogous to forelimbs, we may expect either erect or prone posture. The typical length of tentacles would ensure effectiveness in either orientation. If, alternatively, tentacles develop on the dorsal side of the organism, similar to the dorsal fin in fish, we may only expect prone posture. Erect posture with this orientation would place the grasping limbs behind the organism, ineffective for reasons we will examine in the following section. Accordingly, tentacles may support technological development in the prone hexapodal aliens discussed previously. We must understand, however, that tentacles are unlikely to be incorporated into the skeletal structure of the organism, else they would suffer obstruction of their essential flexibility. Therefore they are most likely to occur in aliens which do not have an endoskeleton, traditional or otherwise.


The central nervous system is, in most vertebrates and some invertebrates, the coordinative center of metabolic processes and higher order faculties. The impetus for cranial development, therefore, is the protection and facilitation of these functions. Endoskeletal organisms house the brain in a thick skull; invertebrates, alternatively, shield it behind a hard shell. Sensory organs cluster around the brain to ensure rapid and efficient sensory input and response, forming the morphology of the face.

In most organisms, there is still another impetus for the development of a head: differentiation into anterior and posterior bodies. This is characteristic of creatures with bilateral symmetry; such differentiation is categorically impossible in radially and spherically symmetric creatures.

Bilateral symmetry enables the differentiation of the creature along several planes

The anterior body is the portion of the organism which encounters the environment first, suggesting consistent unidirectional movement. Because the anterior body is involved in navigation, it requires specialized organs to inform this process; hence, eyes, nostrils, and ear canals form on the anterior body. And because the anterior body is the first to encounter environmental space, the mouth is located here to facilitate efficient feeding. These influences together form the morphology of the face, even in organisms which lack a central nervous system positioned in the anterior body. Hence symmetry is equally as important as neurology in the development of the head.

From these joint influences we may make a few tenable assumptions: the central nervous system entails the development of a head in bilaterally and radially symmetrical organisms, except in the case that the central nervous system is concealed in the body of the organism; cranial development is especially likely in bilaterally symmetric creatures with an anterior body; and an anterior head may develop even if it does not contain the central nervous system.

Thus we may imagine two hypothetical morphologies which support a head: the first is classically earthlike, with bilateral symmetry and an anterior head arranged with eyes, nostrils, a mouth, and ear canals; the second is deviant, with radial symmetry and an upper head arranged with electroreceptors and cryptochrome. We may also imagine a deviant morphology which does not require a head: radial symmetry with a central nervous system enclosed in the main body of the organism. Having identified the significance of bilateral symmetry to metabolic efficiency, we may reasonably expect most technological aliens to have an anterior head, consistent with the humanoid trope.

Evolutionary Value of the Humanoid Trope

In summation, we are justified in anticipating the humanoid trope to remain consistent among technological aliens. Such a generalization, however, fails to capture the full spectrum of evolutionary possibilities. There are likely to be a significant minority of civilizations whose morphologies betray the humanoid trope in biologically and socially important ways. We must be prepared to recognize that deviant morphology in extraterrestrial species does not prohibit intelligence comparable to or greater than human intelligence.

View the full text here: An Analysis of Alien Prospects.pdf

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