Abstract (from the original publication) :
It now seems unlikely that extraterrestrial living organisms could have reached the earth either as spores driven by the radiation pressure from another star or as living organisms imbedded in a meteorite. As an alternative to these nineteenth-century mechanisms, we have considered Directed Panspermia, the theory that organisms were deliberately transmitted to the earth by intelligent beings on another planet. We conclude that it is possible that life reached the earth in this way, but that the scientific evidence is inadequate at the present time to say anything about the probability. We draw attention to the kinds of evidence that might throw additional light on the topic.
- The authors, Francis H. C. Crick and Leslie E. Orgel, propose a theory on the origin of life on Earth, in which microorganisms were deliberately planted on the planet by extraterrestrial intelligent beings.
In the article, the authors explain that Darwin and a number of biologists had concluded that life appeared on Earth at a time when the conditions were more favourable to its appearance. Other scientists reached a different conclusion. Since life does not evolve, at the moment, from inanimate matter, then this may never have happened on Earth. Therefore, they argued that life must have come from another planet as an “infection” , a theory known as Panspermia. It was proposed that spores may have travelled from another solar system by radiation pressure  or through a meteorite. These theories were rejected since there is no radiation-resistant spore that would resist to such a long and challenging trip and it is unlikely that a massive object such as meteorite could escape from a Solar System.
Therefore, the authors propose the theory of Directed Panspermia, where an intelligent civilization from another planet would intentionally “infect” the Earth with microorganisms by means of a special long-range unmanned spaceship. The article, however, does not mention by what mechanisms the generation of complex organisms from the planted microorganisms would arise.
One should also note that Darwin’s book, “On the Origin of Species,” was published in 1859 and the structure of DNA was described by John D. Watson and Francis H. C. Crick (the latter being one of the authors of this article) in 1953.
We can conclude with a note from the authors: “It is difficult to avoid a personal prejudice, one way or the other, but such prejudices find no scientific support of any weight.”
-  A. I. OPARIN (1957). “The Origins of Life on Earth.” Academic Press, New York, New York (gives a general discussion on Panspermia)
 S. ARRHENIUS (1908). “Worlds in the Making.” Harper and Row, New York.
Read Also :
Summary of the of publication “Directed Panspermia”Originally issued by F. H. C. CRICK & L. E. ORGEL
(Received June 22, 1972; revised December 20, 1972)
Reprinted from Icarus, 19, 341-346 (1973)