Directed panspermia is the idea that life might have been intentionally spread throughout space and seeded on the surface of other worlds by a guiding intelligence. A detailed version of this hypothesis was put forward in 1973 by the molecular biologists Francis Crick (codiscoverer of the structure of DNA) and Leslie Orgel.1 The chances of microorganisms being passively transported from world to world across interstellar distances, they felt, were small. The probability of successful seeding would be greatly increased, they pointed out, if the fertilization were carried out deliberately by an existing technological civilization. Their argument depended first upon demonstrating that it was possible for an advanced extraterrestrial civilization to have developed in the Galaxy before life first appeared on Earth. This they were able to do (see extraterrestrial civilizations, ancient). As for the means of dispensation:
The spaceship would carry large samples of a number of microorganisms, each having different but simple nutritional requirements, for example, blue-green algae, which could grow on CO2 and water in "sunlight". A payload of 1,000 kg might be made up of 10 samples each containing 1016 microorganisms, or 100 samples of 1015 microorganisms.
Crick and Orgel further suggested that directed panspermia might help resolve one or two anomalies in the biochemistry of life-forms on Earth. One of these was the puzzling dependence of biological systems on molybdenum. Many enzymes, for example, require this metal to act as a cofactor. Such a situation would be easier to understand if molybdenum were relatively abundant on Earth (see elements, terrestrial abundance). However, its abundance is only 0.02% compared with 0.2% and 3.16%, respectively, for the metals chromium and nickel, which are chemically similar to molybdenum. Crick and Orgel commented:
If it could be shown that the elements represented in terrestrial living organisms correlate with those abundant in some types of star-molybdenum stars, for example-we might look more sympathetically on "infective" theories.
A second example they give concerns the genetic code:
Several orthodox explanations of the universality of the code can be suggested, but none is generally accepted to be completely convincing. It is a little surprising that organisms with somewhat different codes do not coexist. The universality of the code follows naturally from an "infective" theory of the origin of life. Life on Earth would represent a clone derived from a single set of organisms.
There might be a variety of reasons why an advanced civilization would wish to intentionally initiate life elsewhere: as an experiment in astrobiology using an entire world as a laboratory; to prepare a planet for subsequent colonization (see terraforming); or, to disseminate the genetic material of the donor world to ensure its survival in the event a global catastrophe (see extraterrestrial civilizations, hazards to).
1. Crick, F. H. C., and Orgel, L. E. "Directed Panspermia," Icarus, 19, 341 (1973).