Origin of Life | Research & Encyclopedia Articles

Origin of Life

No one knows exactly how life originated on Earth. The planet is approximately 4.6 billion years old. Fossils of microorganisms similar to blue-green bacteria are present in rocks three billion years old. This suggests that life evolved within the first billion years after the Earth was formed, at a time when the planet's environments were very different from those of today. Experiments performed over the past 50 years suggest that many important ingredients of organisms, including amino acids and nucleic acids, may have formed under pre-life conditions existing on Earth; their presence must have facilitated the actual genesis of the first organisms.

Background of the origin of life

All organisms are made of chemicals rich in the same kinds of organic, or carbon-containing compounds. Moreover, the same 20 amino acids combine to make up the enormous diversity of proteins occurring in living things. In addition, all organisms have their genetic blueprint encoded in nucleic acids, either DNA or RNA. These nucleic acids are encoded with the information needed to synthesize specific proteins from amino acids. One class of proteins, known as enzymes, acts to regulate the activity of nucleic acids and other biochemical functions essential to life. Enzymes do this by greatly increasing the speed of specific chemical reactions (that is, they are catalysts). In addition to acting as enzymes, proteins provide structure for cells and assist in obtaining nutrients, functions which cells require for survival. These two types of molecules, nucleic acids and proteins, are so essential to life that many scientists assume that they, or closely related compounds, were present in the first life forms.

Theories of the origin of life

All cultures have developed stories to explain the origin of life. During Medieval times, for example, southern Europeans believed that small creatures such as insects, amphibians, and mice appeared by "spontaneous generation" in old clothes or piles of garbage. The Italian physician Francesco Redi challenged this belief in 1668, when he showed that maggots come from eggs laid by flies, and not spontaneously from the decaying matter in which they are found.

An series of experiments conducted in the 1860s by the French microbiologist Louis Pasteur 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 to indicate that the microorganisms did not arise spontaneously in the open container, but were introduced by dust and other contaminants that could not reach the container with the trap.

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

Charles Darwin, 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 believed that once the first living beings appeared, all other creatures that have ever lived could have evolved from them. Many of the laboratory experiments that have been conducted to shed light on the origin of life have been variations of the "warm little pond" that Darwin mused about.

Another early, influential explanation of the origin of life was provided by the Soviet scientist Aleksandr Oparin, and the British scientist J.B.S. Haldane. Oparin and Haldane suggested in the 1920s that the atmosphere of billions of years ago was very different from that which exists today. The modern atmosphere contains about 79% nitrogen and 20.9% oxygen, and only trace quantities of other gases. Because of the oxygen gas, which is a relatively reactive compound, this is an oxidizing atmosphere. Oparin noted that oxygen interferes with the formation of organic compounds necessary for life, by oxidizing their hydrogen atoms. Oparin therefore reasoned that the atmosphere present when life began was a reducing atmosphere, which contained little or no oxygen, but had high concentrations of gases that can react to provide hydrogen atoms to synthesize compounds needed to create life. Oparin and Haldane suggested that this primordial, chemically reducing atmosphere consisted of hydrogen, ammonia, methane, and additional simple hydrocarbons (molecules consisting only of carbon and hydrogen atoms).

According to their theory, energy for rearranging atoms and molecules into organic forms that promoted the genesis of life is thought to have came from sunlight, lightning, and geothermal heat. This model of the environment of genesis became popular among scientists after a graduate student named Stanley Miller, 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, pre-life atmosphere. In the bottom of the container there was a reservoir of water, and above it an apparatus caused electrical arcs to crackle. After one week of reaction, Miller found that dilute concentrations of amino acids and other organic chemicals had formed from the contained 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 the building blocks of nucleic acids, the molecules that organize into RNA and DNA, which 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, or shallow lakes) increased progressively over time. Eventually, more complex molecules formed, such as carbohydrates, lipids, proteins, and nucleic acids.Energy driving these reactions was probably supplied by ultraviolet radiation or electricity. The assembly of more complex compounds from simpler ones may have occurred on the surface of oily drops floating on the water surface, or on mineral surfaces.

Some scientists believe that the young Earth was too inhospitable a place for life to have developed on its surface. They believe that a more likely environment for genesis was the vicinity of deep-sea vents, or holes in the crust under the ocean from which hot, mineral-laden water flows.

Many scientists today believe that the pre-life atmosphere may not have been as strongly reducing as the one proposed by Oparin and Haldane and used in Miller's experiment. They believe that volcanoes added carbon monoxide, carbon dioxide, 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 pre-life Earth was like billions of years ago.

Cellular metabolism links amino acids together using specific enzymes to form particular proteins. When this happens, a hydrogen molecule and a hydroxyl group (OH) are removed from the amino acids, which then link up into a protein chain, while the hydrogen and hydroxyl link up as a water molecule. Without enzymes, amino acids do not link up in this way, or as a biochemist might describe it, the polymerization reaction does not proceed. If this is true, then how, before life began and cells existed, could amino acids join to form proteins? One possibility is that amino acids may have joined together on hot sand, clay, or even rock. 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 rock. The larger molecules formed in this way have been named proteinoids. It is easy to imagine Darwin's "warm little pond," complete with 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.

Proteinoids can cluster together into droplets that separate, and that may protect their components from degrading influences of the surrounding environment. In this way, the droplets are like extremely simple cells, although they can not reproduce. Such droplets are called microspheres. When fats (also known as 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 prebiotic droplets are called protobionts, and they may represent a stage in the genesis of life.

The formation of amino acids and other organic compounds is presumed to have been a necessary step in the genesis of life. However, another step in the process must have happened soon after: self-replicating molecules would have to be capable of forming if life was to exist. Scientists presume that the first self-replicating molecules were similar to the nucleic acids of organisms.

Once molecules that could self-replicate were formed, the process of evolution would account for the subsequent development of life. As such, the particular molecules best adapted to the local environmental conditions would have duplicated themselves more efficiently and more often than competing molecules. Eventually, primitive cells appeared, and 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.

Most living cells today store genetic information in DNA. The information of DNA is transferred to RNA by a process known as transcription, and the RNA then forms proteins, including enzymes, by translating the information dictated by the DNA. The enzymes facilitate the biochemical cellular functions necessary to maintain life and reproduce it. Many scientists believe it is unlikely that all of the components of this complex sequence of events, DNA to RNA to protein, evolved at the same time. Some scientists propose that, in fact, RNA appeared before DNA. This view has been strengthened by the discovery that some forms of RNA, called ribozymes, can act like non-protein enzymes to catalyze biological reactions. These scientists suggest that RNA was capable of ordering the sequence of amino acids, forming proteins, and replicating itself in a type of "RNA world," in which RNA was more important than DNA.

Scientists who favor the hypothesis of an "RNA world" suggest that RNA might have been able to self-replicate even before DNA and protein enzymes had evolved. As such, single-stranded RNA might have been able to assume a shape that allowed it to line up amino acids in specific sequences to create specific protein molecules. RNA molecules capable of causing amino acids to link up to form a protein could have had an advantage in replication and survival, compared with other RNA molecules. At that point, molecular evolution and natural selection could have taken over in furthering the development of life. The RNA that produced useful protein enzymes, for example, would have survived better than those that did not.

Critics of these ideas say that the evidence for self-replicating RNA is weak. Instead, they suggest that other organic molecules, rather than nucleic acids, were the first self-replicating chemicals capable of storing genetic information. According to this idea, these simple hereditary systems were later replaced by nucleic acids during the course of evolution.

Radio astronomers have found that organic molecules, which might have played an important role in the formation of life, are present in dust clouds in outer space. Organic molecules are also known to be present in meteors that have fallen to Earth's surface. These observations provide further evidence that chemicals important for the genesis of life may have been present on the pre-life Earth. The presence of complex organic compounds outside of our solar system suggests that the formation of compounds important for life is more likely than once thought.

The presence of organic compounds in outer space also suggests to a few scientists that life may not have actually originated on Earth. Instead, they suggest that genesis may have occurred somewhere in outer space, and organisms later arrived on Earth. Most researchers discount this hypothesis, because they feel that ionizing radiation and the great extremes of temperature in space would have killed any organisms before they could have reached the Earth. The suggestion of an extraterrestrial origin of life also suffers from the drawback that it merely shifts the compelling questions about genesis from Earth to another place in the universe.

A theory known as panspermia suggests that organic precursors to life arrived to Earth with meteors. Once here, the organic compounds arranged themselves into molecules that eventually led to the development of life. This theory simplifies the problem of explaining the origin of life, by suggesting that the formation of simple organic compounds did not have to take place on Earth.

Nevertheless, the genesis of organisms from simple organic compounds, however diverse and abundant they may have been, is not yet satisfactorily explained by this or any other theory of the origin of life.

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