The use of aliens in science fiction is very common. Aliens may be hostile such as the Martians in War of the Worlds by H.G.Wells, aliens may also be altruistic such as the Hain in Ursula Le Guin's novels about the Ecumen and sometimes the altruistic intentions of aliens backfire as in Frederick Pohl's novel Gateway. The aliens may even be humans who have grown up on a different planet such as in Stranger in a Strange a Land by Robert Heinlein. However, the one thing all aliens have is that they provide the storyteller with an fresh perspective on humans. Colin Wilson terms such persons and alien characters outsiders in his 1956 book called simply The Outsider. Since antiquity people have looked into the night sky and wondered what sort of people lived on the planets and on the Moon. Ben Johnson even featured lunar creatures called the Volatees his Masque News from the New World Discovered in the Moon. The astronomer Johannes Kepler speculated in 1609 that the Moon was inhabited by large serpentine worms that could travel back and forth to Earth and even abduct humans! Kepler's speculations were not published until 1634 in his book Sumnium. William Shakespeare does not use space aliens in any of his works but he does create a very strange creature in his play The Tempest called Caliban. I saw a performance of the Tempest when I was about 12 or 13 and I was terrified by Caliban at the time and thought that Prospero was a kind old man who was trying to educate Caliban. I saw the same play nearly forty years later at the RSC with Antony Sher as Prospero and John Kani as Caliban and left the theatre with a different point of view. Caliban was now an uneducated boy who followed his instincts and desires with no regard for the consequences and Prospero was a scheming manipulative character (the name Prospero is an anagram of oppresor). Caliban is described as being half fish and also a mooncalf, or Cavaluna. In the Tudor period is was thought that the Moon could exert a detrimental effect on cow embryos whilst they were developing in the womb and cause them to be born deformed. Caliban's mother was the witch Sycorax who was exiled from Algiers and his father is never completely defined. It could have been Setebos but Shakespeare does not make this clear. In my 1989 edition of Shakespeare's plays (edited H. Staunton publisher Routledge) Caliban is depicted as a wild savage clothed in animal skins and with pointed teeth.
Caliban does try to rape Miranda and he does not even think that this is wrong. Miranda is still a virgin at the end of the play (Act IV, Scene 1) so Caliban did not manage carry out his intentions. He also hatches a ludicrous plot to kill Prospero which will never succeed and he although many commentators describe Caliban as a wild man he lacks the cunning that many wild animals possess. He is an uncivilised man who does not know the difference between right and wrong. However, he can appreciate the beauty and wonder of the uncharted island he lives on (Act III, Scene 2) "Be not afraid the isle is full of noises..." also Caliban always speaks in blank verse. So let us now look at the other more "civilised" characters in the play who do know the difference between right and wrong, yet still behave appallingly.
Antonio and Sebastian try to kill Gonzalo and Alonso.
Stephano is persuaded by Caliban to try to kill Prospero.
Antonio and Alonso set Prospero and Miranda adrift on a raft to die. Luckily for them Gonzalo leaves them supplies and Prospero's books on the raft.
Prospero uses his skills in the dark arts to enslave Ariel and Caliban and to get them to do his bidding. Although he sets Ariel free at the end of the play
Antonio wants to take Caliban back to Italy to exhibit him in circus for money.
Shakespeare has a fascination in how society and nature interact. He may have heard stories about strange creatures and indigenous peoples from sailors who had travelled to Africa and North America and used them as a basis to create Caliban. To my mind The Tempest is one of Shakespeare's finest plays and it has undergone a new lease of life in modern post colonial times. At the end of the play Prospero throws away his magician's tools and charms and I have always thought this was Shakespeare himself announcing to the audience that he too was ending his career as a playwright (Let your indulgence set me free - Act V, Scene 1). Although Shakespeare did go on to write Two Noble Kinsmen, Cardenio and Henry VIII most modern scholars think that he collaborated on these plays with other authors. Shakespeare's Caliban is unique creation, he is a true outsider and quite possibly the inspiration for another strange creature, Gollum in Tolkein's magnum opus Lord of the Rings.