Two questions arise with aliens:
1) Why aren’t movie aliens very alien?
2) Do they exist?
Why aren’t they very alien?
Hollywood has difficulty with aliens. To keep audiences watching, and keep costs lower, aliens are usually either more or less human, or monstrous animals.
Given the course of life on Earth, most aliens should either be primitive life forms or fantastically advanced—and those met off their home planet should be in the latter category.
If intelligent life arises, should it resemble us? It is hard to argue that the resemblance would go beyond complex communication, a form of memory, and ability to manipulate the environment.
All the following are based on our brain and sensory structures, the result of accidents of evolution:
- emotion
- memory
- time perception
- sight, hearing, and the other senses
- verbal and visual communication
Consider communication. Intelligent aliens will need some means to exchange information, but it might be through modulation of magnetic fields rather than speaking Klingon.
Real aliens will be far stranger than Chinese or Yanomami: stranger even than dolphins.
Do they exist?
This more basic question is so far unanswerable. There is no clear evidence either way. There are equations that suggest that intelligent life should be common in the galaxy, but they are conjectural. Extrasolar planets are now being discovered regularly, but whether life arises readily is unknown. Given that our type of evolution is a byproduct of our biochemistry, it is also not clear that life arising elsewhere would evolve, or evolve at our pace.
If they do exist, we have to ask why they don’t seem to have visited. There are several possibilities:
- Interstellar travel is extremely difficult.
- They have visited, but not lately. If aliens have visited the Earth 10,000 times over its history, that averages out to one landing every 400,000 years. Only in the last few millennia would any human record of such a visit been left.
- They keep their visits secret, which is of course unarguable.