By far the most widely accepted explanation for the numerous reports of encounters with nonhuman entities is that they are aliens, who have arrived on Earth from some other planet in the Universe. This view is most strongly adhered to in the United States, and is known as the extraterrestrial hypothesis. Over the fifty or so years since the ‘modern era’ of UFOlogy began, an astonishing variety of aliens has been encountered. Contrary to popular assumptions, the Greys, while they have come to e regarded as the quintessential alien being, are merely one species out of many that have been seen, met and communicated with by puzzled or terrified humans.

The sheer variety of alien physical types is enormous, so much so that it has been cited in opposition to the extraterrestrial hypothesis for nonhuman encounters. Even the Greys themselves can be split into several different subgroups, based on height, skin colour and texture, the number of fingers on their hands and their eyes. Other aliens are described as being indistinguishable from humans, and yet some of these seem to require breathing apparatus in our atmosphere, while others do not. There are yet more entities that bear absolutely no similarity to the humanoid form whatsoever.

This diversity of form has led many researchers to speculate that Earth is the object of exploration of numerous space faring races from diverse planets, and this point of view has resulted in several attempts to classify aliens into groups. The most comprehensive of these classification systems was formulated in the late 1960s by the Brazilian researcher Jader U. Pereira, who chose as his principal characteristic the ability, or otherwise, to breathe in our atmosphere. As the American researcher Patrick Huyghe states, this is logical enough, since entities able to breathe oxygen would be profoundly different, biologically, from those who could not. However, this criterion results in a large number (the majority, in fact) of physically diverse entities who are able to breathe our atmosphere being placed in the same group, and it is unsatisfactory for this reason.

Pereira himself accepted the fault in his system, and so added other criteria, such as the presence and length of hair, types of clothing worn (if any), language and the use of tools. Once again, however, Patrick Huyghe points out the inadequacy of this system, since these characteristics are ‘cultural rather than somatic’.

The American folklorist Thomas Bullard of the University of Indiana has also attempted to formulate an alien classification system, based on abduction reports. His results reveal three basic types of entity: the entirely human, the humanoid (similar to humans, but with noticeable differences) and the nonhumanoid. Here, however, we encounter the same problem that beset Pereira’s system: each group of entities (especially the nonhumanoid) contains numerous subgroups which are not adequately addressed, and which may constitute entirely different species. Indeed, as Huyghe notes, most researchers have come to the conclusion that the formation of any completely comprehensive alien classification system is all but impossible; such is the astonishing diversity of aliens encountered. Huyghe himself has made a brave attempt at classification, diving reported aliens into Classes, Types and Variants. The Humanoid Class contains five Types: Humans, Short Greys, Short Non-Greys, Giants and Nonclassics; the Animalian Class contains another five Types: Hairy Mammalian, Reptilian, Amphibian, Insectoid and Avian; the Robotic Class contains two Types: Metallic and Fleshy; and the Exotic Class also contains two Types: Physical and Apparitional. Each of the Types is further divided in Variants. The result is perhaps the most comprehensive and inclusive system to date.

This amazing variety of alien forms is perhaps no more than we should expect: given the size of our own Galaxy (not to mention that of the whole Universe) and the likelihood that there are many millions of inhabited planets out there, it seems logical enough that the denizens of those worlds should differ widely in their physical form. However, we are left with a serious problem with regard to the majority of reported aliens: their humanoid appearance. Many evolutionary biologist maintain that humanoid life on other planets is incredible unlikely. The reason for this is to be found in the physical attributes of our own world, which has had a direct bearing on the form into which human beings have evolved over the past three billion years or so. We are the result of a great many factors, including the period of Earth’s revolution around the Sun, it means distance from its parent star, its axial rotation period, its average radius and its mass. Then there is the composition of the atmosphere and the resulting amount of solar radiation (the engine of evolution) that is allowed through to the surface. There is also the presence of a large Moon to consider, without the steadying influence of which, the Earth would have oscillated wildly in its axial rotation, and the seasons would have been so diverse as to make life itself impossible. When these and many other factors are taken into account, it does indeed seem extremely unlikely that a being from a world differing even slightly form our own would look anything like us- the more so when we consider the many thousands of random genetic mutations that have given rise to the human form, mutation that could not be repeated identically on another world, no matter how similar to Earth it is.

And yet, we are nevertheless face with many different alien races which, while sharing the basic humanoid form (two arms, two legs, torso, head, etc.), display sufficient difference to imply origins on many different planets. However, other evolutionary biologists suggest that ‘parallel evolution’ may occur throughout the Universe, with life forms arising as a result of essentially the same chemical and genetic processes. According to Huyghe, ‘The chemistry of the compounds making up the genetic code seems to demand that all life be similar’. Thus a successful oceanic predator would look pretty much like a shark, no matter which planet it happened to inhabit; and a successful intelligence being on an Earth-like planet would possess two legs for rapid locomotion (any more would use up valuable processing capacity in the brain. And would have been selected out through evolution), a head placed at the highest point on the body in order to see as much of the environment as possible, and so on. It therefore seems at least feasible that some alien life-forms visiting Earth would possess basically humanoid forms, while still retaining variation on that genetic theme.