How Long Did It Take

The fossil record tells us that abundant organisms capable of responding to light and able to build mounds existed on Earth 3.5 billion years ago, as evidenced in rocks from the Warrawoona region of Australia. Yet we know that only 300 million years before that—somewhere around 3.8 billion years ago—Earth was still being bombarded by giant asteroids and comets during the "heavy bombardment" phase. This seems like an awfully short period of time for the first life to evolve. Stanley Miller (the chemist who, with Harold Urey, showed in the 1950s that amino acids could be made in a test tube) has in the 1990s derived an estimate on how long it took to go from inorganic chemicals to life. Miller thinks the transition from "prebiotic soup" to cyanobacteria (the microbes we find today sliming swamps and ponds) may have taken as little as 10 million years.

Miller based his conclusion on three lines of evidence: the rate of plausible chemical reactions leading to the formation of the building blocks of life,- the relative stability of these building blocks, once made (the number of years they remain intact before decomposition),- and the rates of new gene formation through "amplification" in modern bacteria.

The first of these, the rate of amino acid synthesis, is very fast—from minutes to tens of years at the most. Once formed, most organic compounds (such as sugars, fatty acids, peptides, and even RNA and DNA) can last from tens of years to thousands of years. Thus these are not rate-limiting steps, putting the pieces together was what took time. Miller sees three bottlenecks: (1) the origin of replicating systems—essentially the formation of RNA and then that of DNA, which could duplicate itself,- (2) the emergence of protein biosynthesis, or the ability of the RNA molecule to begin synthesizing proteins, the actual material of cells,- and (3) the evolutionary development of the various, essential cell operations, such as DNA replication, production of ATP (the energy source within cells), and other basic metabolic pathways. In a 1996 article written with Antonio Lazcano, Miller argues that the time necessary to go from soup to bugs may have been far less than 10 million years. Making life may be a rapid operation—a key observation supporting our contention that life may be very common in the Universe.

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