Have you ever wondered why there were so many humanoid aliens in Star Trek? Well, according to Star Trek lore, the Preservers, an ancient and long dead super-race seeded the galaxy with humans, which eventually evolved into separate, but similar civilisations throughout the galaxy. While the Preservers were fictional, how true could the notion of other humanoid species populating the universe be?
I had read a long time ago that there could indeed be other humanoid races in our galaxy and that this is mostly due to the physics of our universe. While extraterrestrials can be based on other elements besides carbon like humans, interstellar space is itself seeded with organic hydrocarbons, which could literally form the basis of the backbones of many species on future worlds. Also, the stars themselves pump out many elements up to iron, with supernovas and other exotic stellar phenomena contributing the rest. So any alien species in its formative stage could receive the same basic elemental building blocks that we did and so start out on the same biological path.
Next is the Goldilocks’ hypothesis. So far, the only planet with life is ours and we have noticed that this is because of our position from the sun: not too close and not too far, but just right. Venus and Mars may have been more Earth-like in the past, but conditions proved that ultimately without any planetary checks and balances (e.g. cooling processes and stable atmosphere, respectively) the doomed worlds could not cope with their solar position. With over 200 extra-solar planets now discovered few of which are in the Goldilocks zone, astronomers are keen to seek that all-important small, stable rocky world. There have been some candidate solar systems with gaps in between the gas giants, but nothing has been observed so far. Hopefully this will change with the next generation of telescopes. Also, the sun would have to be an average star, yellow, maybe orange, and of a stable nature and temperature.
The size of the planet and any moons could also affect evolution. The size of humans has varied over the years, but averages are generally between 4-6 feet (1.2 to 1.8m) Gravity would limit the size of humans, so we would not have 12 foot giants roaming around. Maybe on Mars, man would grow taller and slimmer due to the lighter gravity and on a larger planet would be shorter and squatter, but could still be recognisably humanoid. Humans have an average life span of between 60-70 years and from this we could estimate generational information from the past and project guesstimates for the future, especially if new genetic technology is available to tweak our life spans even further. Our size, age, birth rate, death rate, resource use and other factors would determine our ability to survive and become a technologically advanced race and thereby able to explore other planets and stars. Also, our moon is large and in the perfect position to influence our planet through tides and its affects on human biology. It might also give us some protection from cosmic impacts. It certainly inspired human philosophy, religion and technology.
We humans are tetrapods, sharing a four-limbed existence with thousands of other creatures from land, sea and air. It seems to be the basic template from which more complex life evolved from. And since our world is the only example of this and since this evolutionary path has been copied for millions of years even after extinctions, it seems nature prefers this template as it is a successful formula for life. This includes multiple and independent evolution for eyes and other organs. Our sensory organs, appendages, and big brains are not the end-all of evolution, but another variation on a theme. Stalks, tentacles, tendrils, claws, wings, trunks, beaks, proboscises, fins, gills, flippers, etc, are evolutionary adaptations of tetrapods, but only humans have truly mastered technology over the millennia with our cleverly adapted limbs.
An extraterrestrial being, on another planet such as ours, around a stable star pumping out the same elements, even in an atmosphere different to our own could certainly end up looking humanoid. To build technology you would need to have appendages, whether hands (preferably with opposable digits) or tentacles to build and to manipulate things. You would need a big brain and senses to experience the world. Organs might not be the same, but to communicate you would need some kind of apparatus to do that, which could look similar to our eyes, mouth, nose and ears, whatever their size, location and effectiveness. Beings from ocean worlds may be amphibious in nature, but again, to build a lasting technical civilisation they would need to be beyond the form of frogs, toads and salamanders; likewise with mammals such as whales, dolphins, seals and walruses and any airborne species. A certain amount of dexterity would be needed to manipulate complex technology, so fingers, tentacles, or other fine-shaped appendages would be a necessity. This aspect of evolution could be borne out by theories that a certain dinosaur species, the Troodons, could have evolved into a dinosauroid, so two separate species could have shared one world, depending on evolutionary processes after the 65 million year crash. The odds for this would be astronomical. Could Earth really be this rich in life in a universe so barren?
There are alternatives to humanoids. Extraterrestrials evolving as telepaths or telekinetics able to communicate and move things with their minds, might not need a physical body such as ours. Also, there might be the possibility of energy beings, but they may have evolved from humanoids or could take the form of humanoids if needed. Aliens could be totally or partially machine in nature, maybe built by humanoids, so their form could be totally different depending on their function. Lastly, there could be shape-shifting aliens, so their form could vary, again according to function. These extraterrestrials may seem non-conformist, but could be the dominant form in the universe. Humanoids may be redundant, under-evolved or an evolutionary fluke.
So the physics of our universe could dictate the form of life available. In Star Trek, other humanoid races were usually highly distinguished through varying cranial ridges, stalks/antennae or pointy ears. Our real’ extraterrestrials will be different, but the general humanoid look, size, recognisable features and maybe an outstretched hand to shake would be welcomed.