According to the astronomer and American astrophysicist, Frank Drake, we are not alone in the space: there are 10.000 civilizations in our galaxy, existing 100.000 million of galaxies. Drake, father of the project Seti (search of signals of other worlds), says that it is difficult to detect signals of other civilizations, because we receive their radio waves every 200 years and because their inhabitants would not see with good eyes that we waste our energy, while they preserve it conscientiously. For the same reason, primitive civilizations as ours would be easy to detect. In 1974 Drake, used the radiotelescope of Arecibo/Puerto Rico, to send messages to a star located to 25,000 year-light of our solar system. The Message of Arecibo, explained in numbers the molecule of DNA, the characters of the human genetic code, a summary of our solar system, the location of the Earth and an explanation of our metric system. He also sent messages to aliens in the probes "Pioneer" (1972) and "Voyager" 1977 (greetings in 59 modern languages and an hour and a half of good human music).
According to Drake, it would be a bad news that advanced civilizations than ours professed religious cults. The only suspicion of having captured an extraterrestrial signal happened in 1970, when Jerry R Ehman, captured a brief signal, of intensity 25 during 16 seconds. It will be difficult that aliens attack us because for them it would have a high cost. It is very probable that they are constituted of carbon, although their designs were different. For our part we are well designed: we are biped, we have the head on the shoulders, the eyes near the brain, hands to capture preys. It is probable that to 100 meters the alien seem human, but closely not. More operative maybe to have 4 arms and a distance but near between the mouth and the stomach.