Directed panspermia refers to a theory of exogenesis that involves the sentient, purposeful seeding of life throughout the universe. Because it requires a guiding intelligent hand, it differs from standard theory of panspermia in which life might be accidentally transferred between bodies in the solar system (for example between Mars and Earth) or between stars in the interstellar medium.
Given that the Milky Way galaxy is some 13 billion years old, and that our solar system is less than 5 billion years old, it is possible that an intelligent civilization evolved elsewhere in the galaxy before life emerged on Earth. This window provides more than enough time for an intelligent race to seed the galaxy, even using relatively slow-moving vessels.
Some of the earliest proponents of the directed panspermia hypothesis were chemist Leslie Orgel and biologist Francis Crick, who in a 1973 paper proposed that an advanced ET civilisation could have seeded life on Earth. The authors write:
[T]here is adequate time for technological society to have evolved twice in succession. The places in the galaxy where life could start, if seeded, are probably very numerous. We can foresee that we ourselves will be able to construct rockets with sufficient range, delivery ability, and surviving payload if micro-organisms are used. Thus the idea of Directed Panspermia cannot at the moment be rejected by any simple argument. It is radically different from the idea that life started here ab inito without infection from elsewhere. We have thus two sharply different theories of the origin of life on Earth. Can we choose between them?
There is, in other words, no straightforward way to falsify the proposition that life on Earth originated from elsewhere. The authors propose a number of means that we may test the hypothesis, for example by attempting to match the elements needed for life to operate on Earth with elements that may or may not be plentiful on this planet. Life as we know it is reliant on some trace elements such as Molybdenum, found rarely on this planet, suggesting that it could have originated from a star system where that element is more plentiful.
The authors acknowledge that interstellar seeding of life is technically achievable. It is possible that humanity has already unwittingly engaged in a primitive form of directed panspermia. Probes sent to the Moon , Venus and Mars could have carried stowaway bacteria on board that may have survived – however briefly – on the surface of these planetary bodies. NASA Researchers claim that a strain of Streptococcus mitis bacteria returned to Earth on the Surveyor 3 spacecraft having survived a round trip to the moon. The inhospitable surface environment of these destinations makes it unlikely that life could multiply and spread across the surface, but humanity will have to take additional care to sterilize probes on future missions to places like Europa or Enceladus that could potentially become contaminated with Earthly bacteria.
Problems with the theory
One problem is that we don’t yet understand the mechanism by which life emerges from inanimate matter (abiogenesis). The theory of directed panspermia simply defers the problem of abiogenesis to some precursor civilisation. In other words, even if aliens seeded the Earth with life, who or what made them?
There are also a number of practical concerns. Building a space probe capable of maintaining the viability of genetic material over immense distances of time and space is much easier than maintaining a life support for living crew, but still challenging. A probe containing biological spores could travel more slowly, but it would need to be able to sustain the integrity of the ship and its frozen cargo over long periods of time – longer than any earth-bound human structure has endured, perhaps hundreds of thousands of years. Carl Sagan and other researchers place an upper limit on the survivability of genetic material in space at ~ 1 million years. Once a probe reached its destination, it would need to retain the capability to ‘wake up’, direct itself at the target planet, revive and release its biological cargo. All of this might be within the technological capabilities of a highly advanced race, but a program of directed panspermia remains beyond human capabilities for the near future.
Finally, any theory of alien intervention on Earth must contend with Fermi’s Paradox. If precursor civilizations exist in our galaxy, why have we not seen evidence of them? A civilization powerful enough to seed the galaxy would surely be capable of interstellar travel and perhaps stellar engineering, the results of which we might detect. The theory of directed panspermia rests upon the existence of extraterrestrial life, and we cannot confirm the latter.
Why would a civilization engage in directed panspermia at the galactic scale? Perhaps all species are driven by a biological imperative to spread their genotype beyond their home planet, thus ensuring its longevity. Perhaps an alien world in catastrophic danger initiated a program to seed life beyond their doomed planet, believing that they alone bore responsibility for preserving life in the universe. The mind can wander endlessly over the possibilities.
However, to accept the directed panspermia hypothesis is to concede that we owe our existence to some other, higher intelligence. While this precursor civilization might not be ‘God’ in the metaphysical sense, they would nonetheless be our creators. What purpose might they have for humanity, or for life on this planet? What would be the nature of our relationship if we ever made contact with them? Owing to the enormous time scales involved in human evolution, any precursor civilization would already be at least 3.5 billion years ahead of us. Their motives might be inscrutable and their existence might be undetectable to our primitive senses.