than those of related substances. Cooling shrinks and heating expands most materials, which is why bridges and buildings have expansion gaps. Water is anomalous in contracting until just above the freezing point where expansion occurs. The result is that ice floats on the surface of water, a property with dramatic consequences. If ice were heavier and denser than liquid water, as most solid phases are, then the ice would collect in the deepest recesses until cooling eventually turned all the lakes and the entire ocean into a solid mass. Life would be extremely difficult in a massive ice-block. Instead ice floats and, unlike water, is a poor heat conductor. Ice creates an insulating barrier between cold air above and the water below which therefore remains liquid.
Water can absorb more heat than almost any organic compound. Heat from the sun is absorbed by the oceans and lakes, providing a vast heat reservoir which moderates changes in temperature. Much energy is required to vaporize water, which makes water an excellent coolant by evaporation. Land animals make extensive use of this for cooling by sweating. An average person running for an hour would experience a fatal temperature increase of about 10 °C if they couldn’t sweat. The body’s five quarts of blood, largely water with a high heat absorption, counters the temperature increase and effectively cools the body through perspiration, allowing a modest overall rise in body temperature, but without frying the brain. Water has a host of unique properties: specific heat capacity, surface tension, and thermal conductivity properties, all of which conspire to make water a prerequisite for life.
Prebiotic Evolution on an Early Earth
Sometime close to 3.5 billion years ago, the earth’s surface cooled to less than 100 oC allowing water to condense into vast oceans. The oceans provided a haven for simple organic molecules that would have degraded at higher temperatures. Various forms of energy bathed the primitive earth—lightning, geothermal heat, atmospheric shock waves generated by meteoric impact, ultraviolet light from the sun, and others—driving reactions in the atmosphere and ocean to form a wide variety of simple organic molecules. Among the energy options, thunderstorms are proposed as a particularly important energy source for prebiotic chemical evolution because of the efficiency of the resulting shock waves in chemical synthesis. Shock waves surpass ultraviolet light by more than a million fold in efficiently producing amino acids, leading to the conclusion that shock waves may very well have been the principal energy source for prebiotic synthesis on the early earth.
In the upper zones of this primitive atmosphere there was no ozone layer to filter living things from lethal doses of ultraviolet light. Instead, ultraviolet light irradiated the gaseous atmosphere and formed simple organic molecules; formaldehyde, hydrogen cyanide, and ammonia among others. Conversion of these simple and sometimes toxic precursors into amino acids, the building blocks of life, seems remarkably unlikely and yet is supported by some equally remarkable experiments. The classic apparatus in the famous Miller-Urey experiment consisted of a small boiling flask containing water, a spark discharge chamber with tungsten electrodes, a condenser, and a water trap to collect the products and two or more of the following gases: methane, ethane, ammonia, nitrogen, water vapor, hydrogen, carbon monoxide, carbon dioxide, and hydrogen sulfide. 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
