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Mercury, March/April 2006 Table of Contents

by Bruce Dorminey

In an age of mail-order art based on one’s own DNA, it is easy to forget that for all the hubris that comes with sequencing the human genome, astrobiologists and geneticists alike are still cowed by some truly basic astrobiological questions. In what form did life first manifest itself? What came first—metabolism or genetics? And more fundamentally, if Earth saw life at least 3.5 billion years ago, when and where did the molecular precursors to DNA actually evolve?

Most of the astrobiological recipes for life have had a thorough airing in the popular science media—a Sun-like or cooler metal-rich star with an active planetary disk may enable the formation of terrestrial-mass planets from which the ingredients for life can possibly take root. From there, a mixture of planetary volcanism and a period of late heavy bombardment, hypothetically, would provide the energy and organics to actually reap micro-organisms.

But until his death a little over a decade ago, Francis Crick, the Nobel-prize-winning co-discoverer of DNA’s complete helical structure and genetic code, never seemed wholly convinced that the DNA macromolecule and its RNA counterpart were the product of altogether natural evolution. In fact, in a 1973 Icarus paper, he and colleague Leslie Orgel put forth the idea of "directed panspermia," in which natural panspermia is taken a step further. Crick echoed the hypothesis in his 1981 book, Life Itself. Typically, advocates of "directed panspermia" believe that life must be seeded from afar by some sort of extraterrestrial intelligence. Crick and Orgel simply concluded that it is "possible" that life reached Earth in this manner, but noted that the "scientific evidence was inadequate" to give any indication about directed panspermia’s probability.

This also begs the question—if our genome is the result of a form of extraterrestrial bioengineering, would they have also left any sort of intelligible bio-marker messages, as advocates of genomic SETI have postulated? And thirty years since, can we now safely conclude that our evolution has been completely natural?

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