Although the early earth is not thought to have had a boiling ocean, the boiling action of Miller’s apparatus provided a convenient means of circulating gases past the spark discharge. Perhaps even more important is the trap, which provides an efficient method for removing the mixture of products. About 2 percent of the resulting mass was in the form of amino acids. In the history of simulating prebiotic events, electrical discharge experiments have been repeated many times and consistently found to yield amino acids, the simplest building blocks required for protein synthesis.
On primordial earth, the diverse mixture of simple compounds formed in the atmosphere may have been washed down by rain into the oceans. Here life’s basic units may have accumulated along with the products of ocean reactions. Further reactions inevitably took place in this reservoir, and eventually the precursor chemicals reached the consistency of a “hot dilute soup.” Innumerable smaller bodies of water provided a mechanism for thickening the soup. None other than Charles Darwin first suggested a “shallow sun-warmed pond” as a place in which concentration occurred.22 Equally likely are lakes and shoreline lagoons, with alternate flooding and evaporation to provide a constant source of chemical ingredients and concentration to allow the molecules to come together and form larger biomolecules.
The hypothetical concentration is easily envisaged in small pools, perhaps screened from ultraviolet light by overhanging rock and situated in a warm environment as occurs naturally in countries with geothermal activity. This environment is commonly encountered in pools around Rotorua, New Zealand, and in Yellowstone National Park, although these places are inadequate for concentrating volatile substances such as aldehydes and HCN. Further concentration could occur by the accretion of organic compounds on sinking clay particles in shallow water basins. The surface of these clays can catalyze a variety of chemical reactions and could potentially condense these precursors into ever-larger molecules such as proteins and DNA.
Prebiotic evolution is not without problems. For example, carbon makes up almost 20 percent of the body’s mass and yet comprises only 0.03 percent of the earth’s crust. Similarly, DNA requires phosphorous in the form of phosphate, but this is one of the rarest light elements with a concentration in the earth’s crust of around 1000 ppm and about 1.5 ppb in the earth’s surface water. Phosphates are key constituents of not only nucleic acids but of many cell-signaling molecules. They also act as the storehouses for cells’ metabolic energy. However, phosphate readily forms insoluble complexes with several metal ions, particularly calcium, thought to be present in the early earth’s oceans. Access to soluble phosphates in the primitive ocean is problematic because of the prevalence of calcium and magnesium ions that readily form insoluble phosphate salts. How did such a relatively inaccessible essential element become incorporated into DNA?
The phosphorous problem and the success of the spark-discharge experiments encapsulate a fundamental principle in origin of life experiments. There is currently no direct demonstration by which simple organic molecules form selectively and then assemble into vast biopolymers having the functions found in living systems. Remarkable experiments demonstrate the viability of generating simple organic molecules, such as the amino acids from spark discharge experiments, and are suggestive of life-conferring processes. Many molecules found in living organisms are delicate, high energy species that are created by complex molecular machines, usually enzymes, that are without parallel. How these molecules formed in the absence of cellular machinery is one of the most puzzling questions for pre-biotic evolution.
Life’s Building Blocks
Ingenious experiments suggest mechanisms by which simple molecules coalesce into biomolecules. Scientists might not have created life, but the synthesis of life’s precursors has been clearly demonstrated in the lab. A corresponding condensation of life’s building blocks from an oceanic soup would be expected to be evident from rich seams of amino acids and DNA precursors—purines and pyrimidines—all over the earth in deep sediments of great age. No confirmation of an oceanic broth has been found.
Equally important to discovering how key building blocks formed is their rate of degradation. During the Hadean era, the energy required to form prebiotic molecules would also facilitate their degradation unless some sorting mechanism were available. Several atmospheric gases are polymerized or degraded under the conditions of early earth while others would have been quickly and irreversibly converted to organic salts in the alkaline ocean. Amino acids generated at high altitudes are estimated to require roughly three years to reach the ocean, during which they are degraded by UV radiation. In one estimate no more than 3 percent are expected to survive the passage to the ocean.
Most prebiotic molecules have a limited lifetime. One recent estimate for the four core monomeric building blocks of DNA suggests lifetimes ranging from nineteen days to twelve years. At temperatures near zero Celsius, the lifetime is extended to 17,000 years. Complex molecules tend to be fragile, leading some experts to speculate that life must have formed relatively quickly after earth cooled sufficiently. Distinguished origin-of-life researcher Leslie Orgel was overheard saying, “It would be a miracle if a strand of RNA ever appeared on the primitive earth.”23
Organic compounds degrade ever more quickly as the structures become more complex. In essence, macromolecular DNA has a much shorter “sell by” date than the small constituent nucleotide precursors. Although temperatures near freezing would give a better chance for the accumulation of the sufficient concentrations of organic compounds in the ocean, -21oC would be ideal for chemical evolution. If the early earth were some 20oC cooler than today because of less sunlight, there would be far fewer thunderstorms on the earth because thunderstorms are generated by warm, moist air coming into contact with cold, dry air. But thunderstorms are proposed as the most efficient energy source for generating prebiotic molecules. Origin of life research is plagued by this type of quandary; thunderstorms provide the right type of energy for condensing the basic building blocks of life, but the ideal conditions for preventing the degradation of the biopolymers, DNA, and amino acids, occurs at low temperatures where thunderstorms are extremely unlikely.
The difficulty in identifying an efficient mechanism for assembling prebiotic molecules has led some people to suggest that these molecules came from outer space. Meteorites, such as the Murchison meteorite in Australia, have been found to contain amino acids. The most predominant was the simplest amino acid glycine, comprising about 40 percent of the total amino acids in the case of the Murchison meteorite. The exact amount of amino acids arriving from meteorites is under dispute but has been estimated at 0.5 g each year during the time when life first appeared. Further complicating these estimates is the contamination of meteorites with amino acids already present in earth’s environment, particularly bacterially derived amino acids present in groundwater.
Life on earth is based on proteins and DNA. Forming these polymeric units involves assembling numerous precursors in a specific sequence in order to create functional biomolecules. Currently no broadly agreed sorting mechanism exists by which the correct sequence might be achieved. A similar puzzle exists at the atomic level where the formation of amino acids and nucleotides requires new molecules to form from atoms of low prevalence in the earth’s crust. Evidence from spark discharge experiments provides a tantalizing mechanism to explain the formation of amino acids and at the other end of the biological spectrum, all of life rests on proteins and DNA or RNA. The transition between these points remains a great mystery.
Divinely Guided Evolution?
Most scientists regard faith as something relegated to religion and are surprised to learn that science rests on several assumptions that amount to articles of faith. Belief forms the basis of scientific advances because, in proposing any hypothesis, scientists are effectively stating a belief about the world’s structure. The belief may be true or contain truth, and the refining nature of the scientific method leads to an understanding based on evidence that may be far from the original belief. Scientists operate on several axioms taken on faith:
1. Nature is Orderly. Nature has an underlying order shown in patterns and regularities that can be discovered. The orderly structure of nature is often thought to be self-evident; yet awareness of that order is relatively recent. Kepler (1571–1630)