March/April 2006 Table of Contents
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?
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.
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.
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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