Every once in a while, I would read an article that claimed that life originated somewhere else and then came to earth on an asteroid. This claim puzzled me, because it sounded like an eternal regress. If life began on some other planet and then came to earth, how did it originally develop on that other planet? It turns out that I misunderstood the claim, and I have been set straight by a recent article called "Cosmic Blueprint of Life," by Andrew Grant, published in the November 2010 edition of Discover Magazine (this particular article is not yet available on the Internet). The claim is not that life developed on some other planet and then eventually came to earth on asteroid. Rather, the claim is that many of the basic chemicals necessary for life were manufactured in space, and then showered upon earth (and presumably other planets where--presumably--life exists). In this article, Grant writes that:

[The notion that the] underlying chemistry of life could have begun in the far reaches of space, long before our planet even existed, used to be controversial, even comical. No longer. Recent observations show that nebulas throughout our galaxy are bursting with prebiotic molecules. Laboratory simulations demonstrate how intricate molecular reactions can occur efficiently even under exceedingly cold, dry, near vacuum conditions. Most persuasively, we know for sure that organic chemicals from space could have landed on Earth in the past--because they are doing so right now. Detailed analysis of a meteorite that landed in Australia reveals that it is chock-full of prebiotic molecules. Similar meteorites and comets would have blanketed earth with organic chemicals from the time it was born about 4.5 billion years ago until the era when life appeared, a few hundred million years later. Maybe this is how Earth became a living world.

According to Grant, there's two ways to look at the famous 1953 experiment by Stanley Miller and Harold Urey. They prepared a closed environment with the gases they assumed constituted the early Earth atmosphere (methane, ammonia, hydrogen and water). They then simulated lightning strikes through the use of electric sparks. Within a week, the process had produced a variety of prebiotic compounds. As Grant points out, however, the experiment did not show that "all the building blocks of life could have emerged on Earth from non-biological reactions."

Even the simplest lifeforms incorporate two amazingly complex types of organic molecules: proteins and nucleic acids. Proteins perform the basic task of metabolism. Nucleic acids (specifically RNA and DNA) encode genetic information and pass it along from one generation to the next. Although the Miller-Urey experiment produce amino acids, the fundamental units of proteins, it never came close to manufacturing nuclear bases, the molecular building blocks of DNA and RNA.

Grant points out that space was long considered to cold and too low-density to form molecules, but this has now been disproved. Scientists have now found ammonia molecules near the center of the Milky Way using a radiotelescope. They have also found formaldehyde, formic acid and methanol. Laboratory simulations of the environment of outer space had produced "dozens of prebiotic molecules, among them the same amino acids that Miller and Urey found." Further, these experiments have produced "intricate molecular rings containing carbon, nitrogen and hydrogen: fatty acid like molecules that look and behave like the membranes protecting living cells; and nucleic acids or nucleotides, the primary components of RNA and DNA. [More . . . ]