Theories of Origin of Life on Earth include the following:
Creationism: life on earth was designed and CREATED in all its splendor and diversity by an divine intelligent being, which, since then has been regulating its fate.
intelligent alien design: life on earth was designed by non-divine intelligent aliens. After this creation-act,life on the planet was left to evolve and to its fate…! Some theosophists believe that this Alien is equivalent to GOD and the “prophets” of our times like Jesus, Mohamed or Krishna were either one of those aliens or their messengers..!
Life is Cosmopolitan: Supporters of this hypothesis suggest that life originated in some distant planets and was carried along with asteroids and meteor/cosmic dust to Earth. The search for life on nearby planets that resemble earth (at least theoretically) is a way to test the credibility of this hypothesis.
Abiogenesis or spontaneous origin of life: This is the current neo Darwinian view held by science as most credible of all.
In the phenomenal Sci-Fi CONTACT, Carl Sagan tells about a fantastic contact made by a human representative and an extra-brilliant alien civilization. This ancient civilization is imagined as to have survived several catastrophes and as successful in harnessing the power from the Stars to the fullest. This novel tells us about the protagonist and her accomplice an international space crew) shaking hands with their long lost friends and relatives..! Sagan being a strong supporter of Neo-Darwinism says nothing about life on earth being created by those aliens. Yet, the novel partly favored the concept of Intelligent Design.
But such theories, are rooted in human mind’s wishful thinking. And theories of UFOs and crop circles stay on far more shakier grounds than Theory of Evolution and Spontaneous genesis of Life.
The complexity of the designs of organisms is what has bewildered scientists all these centuries. It has spawned a bunch of ideas ranging from Pure Creationism to Pan Genesis and Alien-Designed life forms as mentioned above. All this arise because we under estimate the power of Natural Selection (NS). We also often forget that the chief mechanism of nature’s filtration is differential extinction of bad and incompatible designs. Its just like admiring an artist for his best works when all his bad works have been destroyed. Similarly, we look at the best and most complex designs of nature with awe while all the poorer designs have been cleansed by extinction.
It has to be remembered that more than four-fifths of all the species that ever lived on this planet are extinct due to incompatibility or poor design, which is a very bad record for even an average architect, not to mention a super-intelligent Alien (or GOD) creator.
More over, a theory like “Intelligent Design” can make little testable predictions. That’s why we accept Abiogenesis as the best theory around.