Panspermia is the general hypothesis that microorganisms, spores, or bacteria attached to tiny particles of matter have diffused through space, eventually encountering a suitable planet and initiating the rise of life there. The word itself means "all-seeding."

In the 19th century, the Scottish scientist Lord Kelvin (Baron William Thomson [1824-1907]) suggested that life could have arrived here on Earth from outer space, perhaps carried inside meteorites. Then, in 1908, with the publication of his book Worlds in the Making, the Swedish chemist and the Nobel laureate Svante August Arrhenius (1859-1927) put forward the idea that is now generally regarded as the panspermia hypothesis. Arrhenius said that life really did not start here on Earth but rather was "seeded" by means of extraterrestrial spores (seedlike germs), bacteria, or microorganisms. According to his hypothesis, these microorganisms, spores, or bacteria originated elsewhere in the Milky Way galaxy (possibly on a planet in another star system where conditions were more favorable for the chemical evolution of life) and then wandered through space attached to tiny bits of cosmic matter rather than moved under the influence of stellar radiation pressure.

The greatest difficulty most scientists have today with Arrhenius's original panspermia concept is simply the question of how these "life-seeds" can wander through interstellar space for up to several billion years, receive extremely severe radiation doses from cosmic rays, and still be "vital" when they eventually encounter a solar system that contains suitable planets. Even on a solar-system scale, the survival of such microorganisms, spores, or bacteria would be difficult. For example, life seeds wandering from the vicinity of Earth to Mars would be exposed to both ultraviolet radiation from the Sun and ionizing radiation in the form of solar-flare particles and cosmic rays. The interplanetary migration of spores might take several hundred thousand years in the airless, hostile environmental conditions of outer space.

Nobel laureate Francis Crick (1916-2004) and Leslie Orgel (1927- ) attempted to resolve this difficulty by proposing the directed-panspermia hypothesis. Feeling that the overall concept of panspermia was too interesting to abandon entirely, in the early 1970s, they suggested that an ancient, intelligent alien race could have constructed suitable interstellar robot spacecraft; loaded these vehicles with an appropriate cargo of microorganisms, spores, or bacteria; and then proceeded to "seed the galaxy" with life, or at least the precursors of life. This life-seed cargo would have been protected during the long interstellar journey and then released into suitable planetary atmospheres or oceans when favorable planets were encountered by the robot starships.

Why would an extraterrestrial civilization undertake this type of project? It might first have tried to communicate with other races across the interstellar void. Then, when this failed, the alien civilization could have convinced itself that it was alone. At this point in its civilization, driven by some form of "missionary zeal" to "green" (or perhaps "blue") the Milky Way galaxy with life as that intelligent alien species knew it, alien scientists might have initiated a sophisticated directed-panspermia program. Smart robot spacecraft containing well-protected spores, microorganisms, or bacteria were launched into the interstellar void to seek new "life sites" in neighboring star systems. This effort might have been part of an advanced-technology demonstration program—a form of planetary engineering on an interstellar scale. These life-seeding robot spacecraft may also have been the precursors of an ambitious colonization wave that never came—or perhaps is just now getting under way.

In their directed-panspermia discussions, Crick and Orgel identified what they called the theorem of detailed cosmic reversibility. This theorem suggests that if humans can now contaminate other worlds in our solar system with microorganisms that are hitchhiking on terrestrial spacecraft, then it is also reasonable to assume that an advanced, intelligent extraterrestrial civilization could have used its robot spacecraft to contaminate or seed other worlds (including Earth) with spores, microorganisms, or bacteria sometime in the very distant past.

Other scientists have suggested that life on Earth might have evolved as a result of microorganisms that were left here inadvertently by ancient astronauts themselves. It is most amusing to speculate that humans may be here today because ancient space travelers were "litterbugs," scattering their garbage on a then-lifeless planet. This line of speculation is sometimes called the extraterrestrial garbage theory of the origin of terrestrial life.

Sir Fred Hoyle (1915-2001) and Nalin Chandra (N. C.) Wickramasinghe (1939- ) have also explored the issue of directed panspermia and the origin of life on Earth. In several publications, they argued convincingly that the biological composition of living things on Earth has been and will continue to be influenced radically by the arrival of "pristine genes" from space. They further suggested that the arrival of these cosmic microorganisms, and the resultant complexity of terrestrial life, is not a random process but is one carried out under the influence of a greater cosmic intelligence.

This brings up another interesting point. As scientists and engineers here on Earth develop the technology necessary to send smart machines and human explorers to other worlds in this solar system (and eventually beyond to other star systems), should future generations of humans initiate a program of directed panspermia? If our descendents became convinced that human beings might really be alone in the galaxy, then strong intellectual and biological imperatives might urge them to start the process of "greening the galaxy" or to seed life where there is now none. (See chapter 10.)

Perhaps late in this century, robot interstellar explorers will be sent from humans' solar system, not only to search for extraterrestrial life but also to plant life on potentially suitable extrasolar planets when no life is found. This may be one of humans' higher cosmic callings—to be the first intelligent species to develop to a level of technology that permits the expansion of life itself within the galaxy. Of course, the human race's directed-panspermia effort might only be the next link in a cosmic chain of events that was started eons ago by a long-since extinct alien civilization. Millions of years from now, on an Earth-like planet around a distant sunlike star, other intelligent beings will start wondering whether life on their world started spontaneously or was seeded there by an ancient civilization (in this case, terrestrial) that has long since disappeared from view in the galaxy. While the panspermia or directed-panspermia hypotheses do not address how life originally started somewhere in the galaxy, they certainly provide some intriguing concepts regarding how, once started, life might "get around."

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