Arrival of the Fittest

Wagner

Life's early appearance on earth demands a theory of its chemical origin. Among the earliest ones is the "primordial soup" theory, usually credited to Alexander Oparin and J. B. S. Haldane [..] who wrote about it in the 1920s [..] The primordial soup remained speculation for decades, until 1952, when it received a huge boost from Stanley Miller, a graduate student in the laboratory of Nobel laureate Harold Urey at the University of Chicago. Based on an informed guess at the composition of the gases that were present in the early atmosphere, Miller sealed these gases in a container, showered them with electric sparks to simulate primordial lightning, and washed the mixture in a rainfall of condensing water. After mere days, many organic molecules—those normally created by living organisms—had appeared in Miller’s miniature world. This was a monumental discovery, for it showed how organic molecules could have emerged from inorganic matter during the turbulent youth of our planet. And Miller’s primordial ocean produced not just any organic molecules. It created amino acids such as glycine and alanine, basic building blocks of modern proteins. Later experiments produced many other of life’s construction materials, including sugars and parts of DNA. But even more important was that Miller’s experiments moved life’s origin from philosophical speculation to the realm of hard, experimental science.

In September 1969, the world learned something that Miller had not known in 1952: Life’s molecules can emerge in environments even more hostile than that of the early earth. That September an exploding fireball briefly created a second sun in the sky over Murchison, an Australian town of a few hundred souls some one hundred miles north of Melbourne. After fracturing, the meteorite left a trail of smoke and smaller fragments, the largest of which fell harmlessly into a barn. This cosmic accident happened two months after man had first walked on the moon, at a time when scientists were itching to study extraterrestrial rocks.

As they scratched this itch, they found that the Murchison meteorite ferried a most unusual cargo. As old as the earth itself, it had wandered through outer space for eons, yet it contained several of the amino acid building blocks of proteins, as well as purines and pyrimidines, which are important DNA building blocks. Later work used twenty-first-century spectroscopy to show that it harbored more than ten thousand different kinds of organic molecules, although many of them in exceedingly small quantities.

The Murchison meteorite, it is important to know, iş not a freak of nature. Similar meteorites have landed on earth, and countless other rocks ferry organic cargoes through the heavens. [..] Incidentally, the single most abundant three-atom molecule in interstellar clouds is water, another blow to the notion that we and our planet are oh so very special.

Life’s simpler building blocks are so prevalent in the universe that molecules from space may have seeded life on earth itself. Meteorites and comets, especially those that bombarded the early earth, discharged ten times more water than currently fills all of the earth’s oceans, and a thousand times more gases than its present-day atmosphere.

What is more, they also delivered the rich buffet of organic molecules we find in interstellar space, and in a staggering number of servings. At least ten trillion tons of organic carbon, and perhaps a hundred times as much, have entered our atmosphere from outer space. That is at least ten times more than all the carbon that circulates in living cells today [..]