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The Beginnings of Life on Earth

Christian de Duve

Building Blocks

The early chemists invented the term "organic" chemistry to designate the part of chemistry that deals with compounds made by living organisms. The synthesis of urea by Friedrich Wöhler in 1828 is usually hailed as the first proof that a special "vital force" is not needed for organic syntheses. Lingering traces of a vitalistic mystique nevertheless long remained associated with organic chemistry, seen as a special kind of life-dependent chemistry that only human ingenuity could equate. The final demystification of organic chemistry has been achieved by the exploration of outer space.

Spectroscopic analysis of incoming radiation has revealed that the cosmic spaces are permeated by an extremely tenuous cloud of microscopic particles, called interstellar dust, containing a variety of combinations of carbon, hydrogen, oxygen, nitrogen and, sometimes, sulfur or silicon. These are mostly highly reactive free radicals and small molecules that would hardly remain intact under conditions on earth, but would interact to form more stable, typical organic compounds, many of them similar to substances found in living organisms. That such processes indeed take place is demonstrated by the presence of amino acids and other biologically significant compounds on celestial bodies—for example, the meteorite that fell in 1969 in Murchison, Australia, Comet Halley (which could be analyzed during its recent passage by means of instruments carried on a spacecraft), and Saturn's satellite Titan, the seas of which are believed to be made of hydrocarbons.

It is widely agreed that these compounds are not products of life, but form spontaneously by banal chemical reactions. Organic chemistry is nothing but carbon chemistry. It just happens to be enormously richer than the chemistry of other elements—and thus able to support life—because of the unique associative properties of the carbon atom. In all likelihood the first building blocks of life arose as do all natural chemical compounds--spontaneously, according to the rules of thermodynamics.

The first hints that this might be so came from the laboratory, before evidence for it was found in space, through the historic experiments of Stanley Miller, now recalled in science textbooks. In the early 1950s, Miller was a graduate student in the University of Chicago laboratory of Harold Urey, the discoverer of heavy hydrogen and an authority on planet formation. He undertook experiments designed to find out how lightning—reproduced by repeated electric discharges—might have affected the primitive earth atmosphere, which Urey believed to be a mixture of hydrogen, methane, ammonia and water vapor. The result exceeded Miller's wildest hopes and propelled him instantly into the firmament of celebrities. In just a few days, more than 15 percent of the methane carbon subjected to electrical discharges in the laboratory had been converted to a variety of amino acids, the building blocks of proteins, and other potential biological constituents. Although the primitive atmosphere is no longer believed to be as rich in hydrogen as once thought by Urey, the discovery that the Murchison meteorite contains the same amino acids obtained by Miller, and even in the same relative proportions, suggests strongly that his results are relevant.

Miller's discovery has sparked the birth of a new chemical discipline, abiotic chemistry, which aims to reproduce in the laboratory the chemical events that initiated the emergence of life on earth some four billion years ago. Besides amino acids and other organic acids, experiments in abiotic chemistry have yielded sugars, as well as purine and pyrimidine bases, some of which are components of the nucleic acids DNA and RNA, and other biologically significant substances, although often under more contrived conditions and in lower yields than one would expect for a prebiotic process. How far in the direction of biochemical complexity the rough processes studied by abiotic chemistry may lead is not yet clear. But it seems very likely that the first building blocks of nascent life were provided by amino acids and other small organic molecules such as are known to form readily in the laboratory and on celestial bodies. To what extent these substances arose on earth or were brought in by the falling comets and asteroids that contributed to the final accretion of our planet is still being debated.





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