Origin of Life

All cultures have developed stories to explain the origin of life. During medieval ages, for example, European scholars argued that small creatures such as insects, amphibians, and mice appeared by "spontaneous generation"—natural self-assembly of nonliving ingredients—in old clothes or piles of garbage. Italian physician Francesco Redi (1626?–1698) challenged this belief in 1668, when he showed that maggots come from eggs laid by flies, rather than forming spontaneously from the decaying matter in which they are found.

A series of experiments conducted in the 1860s by the French microbiologist Louis Pasteur (1822–1895) also helped to disprove the idea that life originated by spontaneous generation. Pasteur sterilized two containers, both of which contained a broth rich in nutrients. He exposed both containers to the air, but one had a trap in the form of a loop in a connecting tube, which prevented dust and other particles from reaching the broth. Bacteria and mold quickly grew in the open container and made its broth cloudy and rank, but the container with the trap remained sterile. Pasteur interpreted this experiment as indicating that microorganisms did not arise spontaneously in the open container, but were introduced by dust and other airborne contaminants.

Although Redi, Pasteur, and other scientists thoroughly disproved the theory of spontaneous generation as an explanation for the origin of present-day life on whatever scale, they raised a new question: If organisms can arise only from other organisms, how then did the first organism arise?

Charles Darwin (1809–1882), the famous English naturalist, suggested that life might have first occurred in "some warm little pond" rich in minerals and chemicals, and exposed to electricity and light. Darwin argued that once the first living beings appeared, all other creatures that have ever lived could have evolved from them. In other words, spontaneous generation did occur—but only a long time ago, when the first, minimally complex forms of life would have faced no competition from more-competent cells. Many of the laboratory experiments that would eventually be conducted to shed light on the origin of life have been variations on Darwin's "warm little pond." First, however, another influential suggestion regarding the origin of life was provided by Russian scientist Aleksandr Oparin (1894–1980) and English scientist J. B. S. Haldane (1892–1964). Oparin and Haldane suggested in the 1920s that the atmosphere of billions of years ago would have been very different from today's. The modern atmosphere is about 79% nitrogen (N₂) and 20.9% free oxygen (O₂), with only trace quantities of other gases. Because of the presence of oxygen, which combines readily with many other substances, such an atmosphere is termed oxidizing. Oparin noted that oxygen interferes with the formation of organic compounds necessary for life by combining with their hydrogen atoms and reasoned that the atmosphere present when life began must have been a reducing atmosphere, which contained little or no oxygen but had high concentrations of gases that can react to provide hydrogen atoms to synthesize the compounds needed to create life. Oparin and Haldane suggested that this primordial, reducing atmosphere consisted of hydrogen (H₂), ammonia (NH₃), methane (CH₄), and additional simple hydrocarbons (molecules consisting only of carbon and hydrogen atoms). Oxygen could not have been present in large quantities because it is chemically unstable, and is only maintained as a major ingredient of the atmosphere by the action of green plants and algae—that is, by life itself. Before life, the Earth's atmosphere could not have been strongly oxidizing.

According to this theory, energy for rearranging atoms and molecules into organic forms that promoted the genesis of life came from sunlight, lightning, or geothermal heat. This model of the early environment became especially popular among scientists after a U.S. graduate student of physics named Stanley Miller (1930–), then studying at the University of Chicago, designed an experiment to test it. In 1953 Miller filled a closed glass container with a mixture of the gases that Oparin and Haldane suggested were in the ancient atmosphere. In the bottom of the container was a reservoir of boiling water, and above it an apparatus that caused electrical sparks to pass through the gas mixture. After one week of reaction, Miller found that amino acids and other organic chemicals had formed from the gases and water. In the years since Miller reported his results, other researchers have performed more sophisticated "warm little pond" experiments, and have been to synthesize additional amino acids and even nucleic acids, the molecules that organize into RNA and DNA, which in turn encode the genetic information of organisms.

Subsequent research influenced by these experiments led many scientists to believe that the concentration of organic molecules in the primordial, nutrient-laden, warm "ponds" (which may have been tidal pools, puddles, shallow lakes, or deep-sea hot springs) increased progressively over time. Eventually more complex molecules formed, such as carbohydrates, lipids, proteins, and nucleic acids. The complexity gap between simple nucleic acids and self-replicating RNA or DNA is, however, large; therefore, some scientists have theorized that assembly of more complex compounds from simpler ones may have occurred on the surface of oily drops floating on the water surface, or on the surfaces of minerals—inanimate objects whose atomic structure might have provided a template for stringing together nucleic acids and giving them a place to "live" until free-floating cells protected by lipid membranes could evolve.

However, some scientists believe that the young Earth was too inhospitable a place for life to have developed on its surface at all; lacking O₂, the atmosphere would also have lacked its present-day stratospheric layer of ozone (O₃), which screens large quantities of harmful ultraviolet radiation from the surface. They believe that a more likely environment for abiogenesis (life from prelife) was in the vicinity of deep-sea vents, holes in the crust under the ocean from which hot, mineral-laden water flows.

Furthermore, many scientists today believe that the prelife atmosphere may not have been as strongly reducing as the one proposed by Oparin and Haldane and used in Miller's experiment. They assert that volcanoes added carbon monoxide (CO), carbon dioxide (CO₂), and nitrogen to the early atmosphere, which may even have contained traces of oxygen. Nevertheless, more recent experiments of the Miller type, run using a less reducing atmosphere, have also resulted in the synthesis of organic compounds. In fact, all 20 of the amino acids found in organisms have been created in the laboratory under experimental conditions designed to mimic what scientists believe the prelife Earth was like billions of years ago—whether using Miller's model or its less-reducing competitors.

But in the absence of life, how did these amino acids link together into more complex compounds? Living cellular chemistry links amino acids together using specific enzymes to form particular proteins. An amino acid is any compound which contains at least one amino group (NH₂) and one carboxyl group (-COOH). When amino acids are linked, a hydrogen molecule and a hydroxyl group (OH) are removed from each amino acids, which then link up into a protein chain, while the hydrogen and hydroxyl link up as a water molecule (H + OH = H₂O). Without enzymes, amino acids do not link up in this way—or, as a biochemist might describe it, polymerization does not proceed. How, then, could amino acids have joined to form proteins without the proteins termed enzymes to help them? One possibility is that amino acids may have joined together on hot sand, clay, or other minerals. Laboratory experiments have shown that amino acids and other organic building blocks of larger molecules, called polymers, will join together if dilute solutions of them are dripped onto warm sand, clay, or other minerals. The larger molecules formed in this way have been named proteinoids. It is easy to imagine some version of Darwin's "warm little pond"—a soup of spontaneously-formed amino acids—splashing onto hot volcanic rocks. Clay and iron pyrite have particularly favorable properties making them good "platforms" for the formation of larger molecules from smaller building blocks. One recently proposed theory of the origin of life suggests that tiny ( .01-mm diameter) hollows in iron sulfide minerals, such as are deposited in the vicinity of deep-sea hot springs, might have incubated the earliest life chemistry. Iron sulfide catalyzes the formation of organic molecules, and is used by some modern bacteria for this purpose. Sheltered in tiny iron-sulfide caverns, prebiotic chemistry might have developed at leisure, leaving this protected environment only after evolving a protective lipid membrane. This theory, however, like all theories of the origin of life, has its scientific opponents, and awaits the production of confirming or disconfirming laboratory evidence.

Proteinoids produced in laboratories can cluster together into droplets that separate, and that may protect their components from degrading influences of the surrounding environment. These droplets are like extremely simple cells, although they can not reproduce. Such droplets are called microspheres. When fats (i.e., lipids) are present, the microspheres that form are even more cell-like. If a mixture of linked amino acids called polypeptides, sugars called polysaccharides, and nucleic acids is shaken, droplets called coacervates will form. All of these kinds of droplets are called protobionts, and they may represent a stage in the genesis of cellular life.

The formation of amino acids and other organic compounds is presumed to have been a necessary step in the genesis of life; it is certain, at least, that somewhere along the line all life became dependent on DNA and RNA for reproduction. Scientists thus presume that the first self-replicating molecules were similar to the nucleic acids of modern organisms. (These early molecular systems need not have been as complex as the self-replicating systems that comprise modern cells. Researchers have recently shown, by deleting genes, that even the genetically simplest bacteria alive today can reproduce with much less than their full natural complement of DNA.) Once molecules that could self-replicate were formed, the process of evolution would account for the subsequent development of life. The particular molecules best adapted to the local environmental conditions would have duplicated themselves more efficiently than competing molecules. Eventually, primitive cells appeared; perhaps coacervates or other protobionts played a role at this stage in the genesis of life. Once cells became established, evolution by natural selection could have resulted in the development of all of the life-forms that have ever existed on Earth.