I should have known what fruit would spring from such a seed.
Lord Byron, Childe Harold
In the previous section we considered the idea that ETCs might have encoded messages in the DNA of terrestrial organisms. Although this is a remote possibility, a broader version of the idea is, paradoxically, more plausible. With each breakthrough in the study of genetics it becomes increasingly apparent that all life on this planet is deeply related. Perhaps individual species are not alien, but we cannot discount the possibility that every species came from the same extraterrestrial source. Perhaps we are all aliens.
The idea that life originated elsewhere and was somehow transported to Earth is an old one. The notion of panspermia — literally "seeds everywhere" — probably dates back to Anaxagoras. Some of the best scientists of the 19th century discussed various forms of panspermia, but it was a book by Arrhenius in 1908 that popularized the notion. Arrhenius supposed that the Universe is full of living spores that are driven through space by the pressure of starlight. Such spores fell on the early Earth, flourished, and evolved into the life we see today.55
As we shall discuss in more detail later (see page 189), one of the deep mysteries of the origin of life is the indecent haste with which it arose on Earth. There scarcely seems enough time for random physical and chemical processes to generate life from lumps of inanimate matter. The panspermia idea is attractive, since it removes the problem of timescales: life dropped "ready-made" onto Earth. Nevertheless, the Arrhenius hypothesis quickly fell from favor. One reason why the idea was shelved was the difficulty of imagining spores hardy enough to withstand the rigors of an aeons-long journey through space; in particular, radiation would surely prove deadly to spores. Another reason was that it merely removed the problem of the ultimate origin of life from Earth to somewhere in space (although of course it would be nice to know where life originated, if only to settle a fact of history).
The idea that there may be microbial life out in space did not entirely disappear. For example, Hoyle and Wickramasinghe championed the idea that microbes travel to Earth on comets, causing occasional mass outbreaks of disease.56 The claim was lent some credence by the discovery that bacteria traveled to the Moon on unmanned lunar landers, and were still alive when brought back to Earth by Apollo astronauts. Clearly some microorganisms can survive the harsh environment of space, if only for a few years.
Furthermore, the much-hyped announcement in 1996 that the Martian meteorite ALH84001 might contain bacterial microfossils led some scientists to suggest that life began on Mars. Microbes subsequently traveled to Earth inside meteorites, which would protect them from the harsh environment of space. It is an attractive suggestion: conditions on early Mars may well have been more conducive for the emergence of life than those on early Earth. However, skepticism is in order. Recent work suggests that the so-called microfossils may be artifacts of the procedures used to view the rock at extreme magnification. The ALH84001 affair is perhaps yet another example where Mars has led scientists astray, causing those who are working at the very limits of observation to see patterns that are not there.
Although panspermia is not in the mainstream of biological thought, it is a possibility that has certainly not been ruled out. If it turns out to be true, then the chances of life being a frequent occurrence in the Universe are greatly increased (though it does not necessarily say anything about the existence or otherwise of intelligent life and ETCs). In 1973, however, Crick and Orgel published the idea of directed panspermia:57 panspermia plus intelligence, as Dyson put it. Crick and Orgel felt that the chance of viable microorganisms landing on Earth after an interstellar journey measured in light years was small. But deliberate seeding is different. Directed panspermia is the suggestion that an ancient ETC may have deliberately aimed spores toward planets with conditions favorable to the survival of life. Maybe primitive life did not arrive here haphazardly inside a mete orite; maybe it was sent here via a probe. (Why would an ETC seed planets in this way? Perhaps they were preparing planets for subsequent colonization, but somehow failed to get round to colonizing Earth. Perhaps they were performing grand astrobiological experiments. Perhaps they faced a global catastrophe, and wanted to ensure the survival of their genetic material. Who can tell?)
It is difficult to know how to test the hypothesis of directed panspermia. Billions of years after the event, how can we distinguish between primitive life emerging from the primordial ooze, primitive life arriving inside a meteorite, or primitive life arriving by space probe? (In their paper, Crick and Orgel argued that directed panspermia could resolve certain puzzles. For example, why is there only one genetic code on Earth? A universal code follows naturally if all life on Earth represents a clone derived from a single set of microorganisms. Another example relates to the dependence of many enzymes on molybdenum. This metal is rather uncommon — it ranks 56th in order of abundance of the elements in the crust of the Earth — and yet it plays an important biochemical role. This slightly odd state of affairs would be less surprising if life on Earth derived from a system in which molybdenum was much more abundant. Of course biochemists have more orthodox answers to these puzzles, and so the evidence in favor of directed panspermia is weak.) If biologists develop a convincing theory of how life originated naturally from the materials available on the primordial Earth, then panspermia — directed or otherwise — would be unnecessary. Or Crick and Orgel might some day be proved true: we may meet the ETC that seeded our part of the Galaxy with spores. Until it is shown to be true or false, the hypothesis of directed panspermia remains on the table as a possible resolution of the Fermi paradox: ETCs exist because we sprang from their seeds. Where are they? They are here, because we are aliens.
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