Imagine landing on a strange planet. Your landing site is one of the many volcanic islands dotting an ocean covering the whole of the planet. The light in the sky is dim, with 20 percent less starshine than you are used to, and there is a single, very large moon in the sky.
The day—the cycle of light and dark—is short, less than 15 hours. In the dark, you can see comets in the sky, along with numerous meteorites streaking toward the planet. In the light, you step onto the barren landscape. You are sealed in a space suit, because the dense outside atmosphere, composed mostly of nitrogen (N2) and carbon dioxide (CO2), but almost devoid of breathable oxygen (O2), would suffocate you quickly. As you step out, you notice that the day is very hot, more than 49° C (120° F).
Welcome to the earth of four billion years ago. This was a time when continents were just forming, the sun was dimmer than today, the moon was closer, and the earth was spinning faster. On the early earth, there was almost no free oxygen in the atmosphere, and a possibly immense volume of the greenhouse gas CO2 kept the planet much warmer than it is today. This is the environment in which life originated on our planet.
This knowledge about conditions on the early earth derives from what scientists have learned in the fields of physics, astronomy, biology, chemistry, and geology. For example, astrophysicists have learned that small stars, like our sun, become brighter over billions of years. Therefore, four billion years ago the sun must have been dimmer.
The laws of physics dictate that tidal forces, produced largely by the moon, should slowly reduce the speed at which the earth spins. As the spin of the earth slowed, the day became longer. But four billion years ago, it took only 15 hours for the earth to revolve once on its axis. Paleontologists have confirmed this gradual slowing by measuring daily growth rings in the shells of marine animals that lived millions of years ago. In addition, the physical law of conservation of momentum states that the moon must move away from the earth as the earth's rate of revolution slows, so four billion years ago the moon must have been much closer.
Geologists have found evidence in ancient rocks indicating that there were only traces of free oxygen in the atmosphere. Scientists also believe that there was abundant volcanic activity because radioactive material in the early earth would have given off heat that would have produced volcanoes.
Under these conditions, how could life have evolved on such an inhospitable planet? Scientists have debated this question in earnest since the early 1920s when a Russian chemist and an English geneticist independently proposed that the basic building blocks of life could have formed from simple molecules in the earth's early atmosphere as a result of being energized by lightning. Since then other scientists have suggested that the basic building blocks of life could have been transported to earth onboard comets or meteorites. And more recently, some scientists have considered the possibility that life may have originated elsewhere—on another planet or celestial object—and arrived on earth after a cataclysmic collision sent a fragmentary meteorite from that planet or object hurtling to earth. Still another idea postulates that life may have developed around hot water vents on the ocean floor.
These and other theories have occupied various groups of scientists for many years. And in the course of their investigations, they have also come to find that they do not always agree on what life is.
The Earliest Evidence for Life on Earth
The earth itself and the rest of the solar system are about 4.6 billion years old. The earliest time that life forms could have emerged on earth appears to have been four billion years ago. This is because asteroids and comets heavily bombarded the earth from the time it formed until four billion years ago. Scientists know about this early bombardment because they can date impact sites on the moon that have not eroded away, like those on earth have. Judging from the size of these lunar craters, some of the asteroids and comets that collided with the early earth were huge. They were so large that the energy released as they crashed onto the earth's surface would have heated any early ocean to such a high temperature that the water would have boiled and vaporized away. This would have made the planet uninhabitable because water is essential for life.
Just recently, a group of scientists announced that they had found indirect evidence that life was present on earth as early as 3.8 billion years ago, not long after the end of the great meteorite bombardment. This suggests that life emerged rapidly once conditions that would support life were in place.
Scientists from the Scripps Institution of Oceanography in San Diego, California, reported in a November 1996 issue of the journal Nature that the composition of carbon in ancient rocks from an island off Greenland indicated that life processes were occurring 3.8 billion years ago. This ancient rock near Greenland is believed to be just slightly older than the Greenland rock formation known as the Isua Group, which contains the oldest known rocks composed of sediments deposited by water. Radiometric dating indicates that the Isua Group is 3.8 billion years old. (Radiometric dating uses the rate of decay of radioactive elements within a sample to estimate the age of the sample.)
The indirect evidence of life in this ancient rock consisted of a distinctive ratio of lighter to heavier carbon isotopes. Isotopes are sets of atoms that have the same number of protons but different numbers of neutrons, giving each isotope a different atomic weight (the sum of the protons and neutrons).
The rock studied by the Scripps team contained much more carbon-12 than would be expected from nonbiological processes. Living organisms tend to favor lighter isotopes of an element just slightly over the heavier ones. Thus, for example, organisms use carbon-12 slightly more than carbon-13 (which contains one additional neutron), and oxygen-16 slightly more than oxygen-18 (which contains two additional neutrons).
The first direct evidence for life on earth consists of the oldest known fossils, which date from about 3.55 billion years ago. These fossils consist of microscopic, degraded strings of cylindrical or elliptical cell walls. The outlines of these cell walls were preserved in rock in the form of films rich in carbon.
The fossilized cell walls in the rock, known as chert, looked quite similar to younger fossils of bacteria dating from 2.5 billion to 0.55 billion years ago. In addition, the fossils in the chert were indistinguishable from fossils of cyanobacteria (sometimes called “blue-green algae”) that are less than 2.5 billion years old. This suggests that the earliest known fossils were from fairly highly derived organisms—that is, many other life forms originated before these organisms.
These earliest fossils were discovered during the 1980s in independent investigations, first by biogeologist Stanley Awramik of the University of California, Santa Barbara (UCSB), and then by paleobiologist William Schopf of the University of California at Los Angeles (UCLA). They collected samples of chert from a rock formation in Western Australia known as the Warrawona Group. Radiometric dating of volcanic rocks associated with these cherts provided the age estimate of about 3.55 billion years.
So the fossil evidence indicates that life began at least 3.8 billion years to 3.55 billion years ago. The geological evidence establishes that the earliest forms of life must have first developed in the absence of molecular oxygen. There were at most only minute traces of free oxygen in the atmosphere about four billion years ago. The evidence for this lack of oxygen lies in the sedimentary rocks, such as sands, clays, and cherts deposited by ocean waters on the early continents, and preserved in present-day Australia, South Africa, northern Canada, and Greenland.
Many of these ancient sedimentary rocks are far more abundant in iron than are modern marine sediments. The waters that deposited modern sediments are rich in dissolved oxygen, and iron in the presence of oxygen quickly turns to rust, in a process called oxidation. Rust does not dissolve in water. In contrast, unoxidized iron does dissolve and can move into the oceans in waters flowing down rivers. Very minute traces of oxygen could cause this iron to precipitate out of the water and fall to the ocean bottom, without turning it to rust. The abundant iron in very ancient sedimentary rocks therefore suggests that there was very little free oxygen on the early earth, either in the atmosphere or dissolved in the oceans.
The scarcity of free oxygen on the early earth may actually have been a boon to early life forms: Strange as it may seem, oxygen is a potent poison to living organisms. Oxygen rapidly breaks down the complex molecules in cells. Human cells, and the cells of all animals, plants, and fungi, have complex chemical machinery that prevents oxygen—most of the time—from harming molecules within the cell.
Many kinds of primitive bacteria that survive today do not have this protective machinery and die quickly when exposed to oxygen in the air. Such primitive bacteria are the holdovers from the ancient times of earth's history, and they give scientists clues to the processes that produced life.
Primitive Bacteria: “Living Fossils?”
The most primitive organisms in existence today include bacteria that live in terrestrial hot springs and in deep-ocean hot water vents created by volcanic activity. It was only relatively recently that scientists traced the evolutionary significance of these organisms.
The discovery of the primitive nature of these organisms was made possible by the development over the past several decades of new biotechnology techniques that allow scientists to determine the sequence of pieces within the genetic code of an organism. Scientists may then estimate the number of changes in this code that separate two organisms. Based on the assumption that organisms sharing a more recent ancestor will have more of this code in common, it is possible to put together a map showing evolutionary connections. Some very sophisticated computer programs aid in the construction of this “tree of life.”
The results of these genetic comparisons indicate that living organisms fall into three fundamental groups, known as domains. The domain of creatures known as eukaryotes (organisms whose cells have a defined nucleus containing genetic material) includes animals, plants, fungi, and the algae and protozoans. Another domain includes all true bacteria (single-celled organisms that lack internal cell membranes). The third domain comprises the small, strange group of single-celled, microscopic organisms known as the archaea (formerly known as archaebacteria). Archaea and true bacteria differ in their biochemistry. The archaea include some of the earth's most primitive and oldest creatures. There appear to be no organisms surviving from the time before the three domains diverged.
The most primitive living bacteria and archaea share two very interesting characteristics. They are autotrophic, which means they make their own food, synthesizing complex molecules using chemical energy that they capture from the outside environment. The primitive bacteria and archaea also live in hot environments that would scald to death most other organisms. The autotrophy of these primitive organisms was a great surprise. Theories about the origin of life proposed in the early 20th century had suggested that the earliest living organisms would be heterotrophs (organisms that absorb complex molecules as food rather than synthesizing them). Animals are heterotrophs, while green plants and cyanobacteria, like the primitive bacteria and archaea, are autotrophs.
The primitive autotrophs are quite different from green plants and cyanobacteria in the manner in which they synthesize complex molecules, however. Green plants and cyanobacteria capture the energy of light from the sun and use this energy, CO2, water (H2O), and oxygen (O2) to synthesize complex molecules. In contrast, such organisms as archaea that live near hot water vents capture chemical energy from substances such as hydrogen sulfide (H2S) emanating from the vents in order to create complex organic (carbon-containing) molecules.
The fact that the most primitive organisms existing today live in very hot environments suggests that life, or at least cellular life, may have originated in such environments.
Four billion years ago, there must have been many more hot vents on the ocean floor than exist today. Scientists believe that there was an abundance of radioactive atoms in the early earth. Many of these early radioactive atoms quickly broke down into nonradioactive substances in the earth, producing heat as they decayed. The heat produced would have been transferred to the earth's surface in a myriad of volcanoes. Those volcanoes that were located underwater would have produced numerous hot vent environments.
Definition and Properties of Life
To explore further how early life might have first come into existence requires a definition of “life.” Most scientists agree that a living thing must have two qualities: metabolism and reproduction. First, a living organism has metabolism—that is, it captures energy from the outside environment, stores that energy in the form of complex organic molecules such as carbohydrates (for example, starches, sugars), and breaks down these molecules in order to release energy for use by the organism. The outside energy may be in the form of light or chemical energy, as in the respective cases of green plants and archaea, or, in the case of heterotrophs such as humans and other animals, in the form of food energy gained by eating other organisms.
Vast numbers of different chemicals are created and altered in the metabolism of a living organism. The life processes of all organisms on earth are based on chemical reactions involving carbon-based molecules, known as organic chemical reactions. The chemical bonds of carbon to oxygen, hydrogen, nitrogen, and other elements in organic molecules are weaker than most other chemical bonds. These weak chemical bonds mean that an organism can easily rearrange the structure of complex carbon-based compounds, building them up or breaking them down as needed. Heat, light photons, and radioactive energy can also easily alter the structure of carbon-based compounds.
The second function that defines “life” is that it reproduces itself. All life on earth uses two molecules—deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)—as blueprints for reproduction. DNA and RNA are the largest and most complicated molecules in living organisms. Without these molecules, an organism could not make a copy of itself. Both DNA and RNA molecules are shaped like extremely long, twisted ladders and have six basic components that are repeated throughout the molecule: a sugar group, a phosphate group, and four different nucleotide bases. These nucleotide bases form pairs on the “ladder rungs” of the DNA. The sequence of these nucleotide bases makes up an organism's genetic code.
When a cell reproduces, its DNA makes a second copy of itself within a cell, and causes the cell to divide, with each of the two new cells receiving one set of DNA. The sequence of nucleotide bases in DNA governs what proteins are made in the cell and in what order they are made. RNA, a molecule similar to DNA, translates the molecular instructions encoded in DNA and relays the instructions to the proper sites within a cell for the construction of proteins.
Proteins—organic molecules that can be very long and complex—are one of the chief components of living organisms. Like DNA and RNA, proteins consist of several basic components repeated in sequence in a long chain. The basic components are known as amino acids. There are 20 specific amino acids that form the building blocks of all proteins in living organisms on earth. Some proteins are categorized as enzymes—molecules that speed specific chemical reactions within a living organism.
It is not always simple to define what is “alive.” For example, some scientists include viruses among life forms, and other scientists do not. A virus, which consists of DNA or RNA molecules coated by molecules of protein, can reproduce itself. A virus does not capture energy from its environment, however. A virus can only reproduce by entering the cell of a living organism and instructing that cell to make copies of the virus.
When DNA and RNA make copies of themselves, the copy is not always perfect. Sometimes there are mutations—changes in the genetic code. These mutations are important in the evolution of life because sometimes these changes lead to traits that favor the survival of an organism or allow more prolific reproduction. In this process, which is called natural selection, organisms that possess these favorable traits proliferate and pass on these qualities to their offspring.
The fundamental unit of life is the cell. All known living organisms are composed of cells, except for viruses (assuming that viruses are considered “alive”). Cells, the organic “factories” that construct more complex molecules from simpler ones, are enclosed in membranes made of walls of organic molecules. The cell membrane permits the selective transport of molecules into and out of the cell.
Theories about the Origin of Life
In even the most simple single-celled organism, very complicated molecules are taking part in the numerous intricate chemical processes that constitute life. The development of even the most primitive form of life, therefore, required many steps. First, the organic chemical “building blocks” of life, such as the amino acids, must have been available from some source. Second, these building blocks somehow assembled into longer, more complex organic molecules, and at some point DNA must have come into existence. These complex organic molecules somehow became contained within cells. In the last several decades, scientists have learned a great deal about these steps and have developed a number of new theories about where, how, and in what form early life might have developed.
The first theoretical basis for modern research into these questions was developed by the great Russian chemist, A. I. Oparin, and the great English geneticist, J. B. S. Haldane, in the 1920s. Independently they argued that the basic organic building blocks of life could have formed from simpler molecules in the primitive atmosphere of the earth—an atmosphere without molecular oxygen—when energized by lightning or similar kinds of energy.
According to Oparin's and Haldane's hypotheses, the complex organic molecules created in the atmosphere would then rain into the primitive ocean, forming an “organic soup.” The two scientists reasoned that as molecules came together in this soup, natural selection would favor any group of molecules that could devour other molecules for chemical energy, growth, and reproduction. Thus, Oparin and Haldane suggested that life began as simple heterotrophs—cells that ate outside organic compounds. Only when the organic soup was depleted of complex organic molecules would some primitive life forms stumble by mutation upon autotrophy and begin using captured physical energy, instead of chemical energy, to create organic molecules.
The first part of Oparin's and Haldane's ideas was tested experimentally in the early 1950s by chemist Stanley Miller, then a graduate student under the direction of the Nobel Prize-winning physicist Harold Urey at the University of Chicago in Illinois. Using the atmosphere of Jupiter as then understood as a model of a primitive planetary atmosphere, Miller placed a mixture of the gases methane (CH4), ammonia (NH3), water vapor (H2O), and hydrogen (H2) into a pair of closed flasks connected by glass tubes. He rigged this experimental setup so that he could send electrical sparks through one of the flasks, simulating lightning in a primitive atmosphere.
Miller's results were amazing: The electrical discharges caused the gases to react to form fairly complex organic molecules that began to rain out of the artificial atmosphere. Among the molecules were amino acids, the building blocks of proteins and thus of all life. The initial parts of Oparin's and Haldane's hypotheses—that the building blocks of life were created in the early atmosphere in the presence of lightning or some other energy source—seemed to be vindicated.
Miller and many other scientists have continued to perform experiments to determine where and how the building blocks of life may have formed. These scientists have shown that organic compounds could be produced by many kinds of energy: ultraviolet light (as from the sun), heat (as from volcanoes), and even shock (as from impacting meteorites). Their experiments have produced all of the 20 amino acids synthesized by life today as well as various sugars and phosphates, including the ones that form the backbones of DNA and RNA.
Yet, some portions of Oparin's and Haldane's hypotheses and Miller's experiments have come into question in recent years. For example, the most primitive known organisms that live today have turned out to be autotrophs, not heterotrophs—that is, they do not devour other molecules for food energy, as Oparin and Haldane predicted.
In addition, scientists now believe that the composition of earth's early atmosphere was quite different from the mixture of gases used by Miller in his experiments. The mixture of gases that Miller and others pumped into their flasks was highly reduced (a chemical state that is the opposite of oxidized). Geological and theoretical studies in recent decades have come to the conclusion that the earth quickly lost any primitive, highly reduced atmosphere.
By the end of the great meteorite bombardment about four billion years ago, the earth's atmosphere probably contained mostly N2, CO2, water vapor (H2O), and perhaps some carbon monoxide (CO). The Miller-Urey experiments with this mixture of gases did not yield abundant quantities of the building blocks of life.
Organic Compounds from Space
In recent years, most scientists have reached a consensus that the building blocks of life came from outer space. Space probes and rendezvous missions with Halley's Comet discovered abundant organic compounds in space. In fact, the body of Halley's Comet (as opposed to its glowing tail and corona) proved to be the darkest object ever observed in the solar system: It is covered by organic gunk. So are some of the moons of Jupiter and Saturn. Therefore, it seems that even if the synthesis of organic molecules that was observed in the Miller-Urey experiments could not have occurred on the early earth, there were many other places in the solar system where it could have happened, including comets.
Recent experiments and calculations, including some carried out in astronomer Carl Sagan's lab at Cornell University in Ithaca, New York, have demonstrated that fairly complex organic molecules, including amino acids, could survive as a comet crashed into the earth. Thus, meteorites that fell to earth during the waning stages of the great bombardment may have imported the building blocks of life.
The Creation of Information-Carrying Molecules
These considerations still do not solve the question of how DNA and RNA arose. How can such complex “informational” molecules come into existence from nonliving processes? Nor do these considerations address the ultimate question about the origin of life: how DNA, or its precursor molecules, came to control and produce the little organic factories called cells.
In recent years, scientists have focused on hypotheses that assume that RNA came into existence prior to DNA. DNA does not do much except store molecular information. In the cells of living organisms, other molecules such as enzymes make DNA reveal its information. The information is then translated into RNA molecules that carry the instructions encoded in DNA elsewhere in the cell.
RNA is capable not only of passing on molecular information but also of stimulating, or “catalyzing,” some of the chemical reactions that produce proteins. In all life today, RNA cannot stimulate chemical reactions by itself; it can do so only with the help of other enzymes, known as coenzymes. Some of these helper enzymes are passed on to each cell during reproduction in the seamless continuity of life through time.
One scenario that has emerged in recent years—based on the idea of a primitive, “RNA world,”—is that life began as “selfish” molecules that existed only to reproduce themselves. The first selfish molecule would have been a simple RNA with a very short sequence of nucleotide bases. Unlike modern-day RNA, these hypothesized early RNA molecules could catalyze chemical reactions, snatching chemical building blocks out of surrounding water, and using the building blocks to make copies of themselves. If random changes—mutations—in the molecular structure of a single, primitive RNA allowed it to produce an attached enzymatic protein that happened to speed the RNA's reproduction, the pair of molecules would proliferate at the expense of all others.
This scenario is not unreasonable. Present-day viruses and bacteria frequently mutate and develop into more virulent or hardy strains, which can quickly multiply. The average bacterium can reproduce itself every 20 minutes in a favorable environment, and the envisioned short RNA is many millions of times simpler than even the simplest of living bacteria. In the right environment, reproduction time for the primitive molecule might have been a matter of seconds.
This concept of a selfish molecule that accidentally adds to itself information that helps it reproduce more efficiently is very different from Oparin, Miller, and Urey's ideas of globules of molecules slowly advancing toward a living state. The “selfish” RNA theory emphasizes reproduction and evolution. The ideas of Oparin, Miller, and Urey emphasize metabolism—growing a cooperative group of molecules that can gain energy and other molecules.
How the “selfish” RNA theory gets to metabolism—gathering energy from the outside environment—is simple in concept. If any other enzymatic molecule that a selfish RNA produces by random chance further speeds reproduction, that mutated RNA will proliferate at the expense of others. If new helper enzymes can capture some kind of energy from the environment that aids in reproduction, the governing RNA will multiply further.
And finally, if an RNA and its helper enzymes begin to synthesize molecules that form a sealing membrane—a cell wall—that protects them from the variability of the outside environment, the RNA might be able to proliferate yet further. And then something that could be called life would exist.
One emerging idea about how primitive life or its precursor molecules may have gained the initial energy to stimulate chemical reactions—and thus the beginnings of a metabolism—suggests that life may have originated as a surface film of organic molecules. Carbon-based molecules readily form such films. This line of investigation, which is being led by chemist Günter Wachtershäuser in Munich, Germany, is based on the observation that very primitive existing life forms are autotrophic and are associated with sulfide minerals.
Electric properties of the surface of certain sulfide minerals, such as common pyrite (“fools gold,” FeS2), which is common in marine environments, can both attract complex organic molecules and possibly keep electron energy flowing through them so that the organic molecules could react and become more complicated. This could take place in an environment scarce in molecular oxygen, as the early earth was.
Life originating near underwater hot vents would make a certain amount of sense from an evolutionary perspective. Organic chemical reactions move faster in hotter environments, and errors in reproducing genetic material would have been more common. Thus, in a given interval of time, more mutations would have been produced in primitive aggregates of complex molecules that resided near hot vent environments than would such aggregations existing in the cooler waters of the surrounding ocean. The mutations would have sped the process of evolution.
Where does DNA come into all this? DNA is a stronger molecule than is RNA. DNA is less likely to be broken down by heat, ultraviolet light, or unhealthy chemical reactions. Thus, it is envisioned that as the “RNA world” became more complex, at some stage DNA would have become useful in preserving important molecular information, by slowing the rate of mutation. DNA may have been the chance molecular invention that allowed functional information to be better archived.
Life from Outer Space?
Another emerging idea is that life did not originate on earth but rather emerged elsewhere in the solar system and was then transported to our planet. This is not entirely absurd; the most primitive bacteria and archaea have extreme environmental tolerances and some possibly could survive travel through space.
In 1996 a team of National Aeronautics and Space Administration (NASA) scientists announced tentative evidence of primitive life forms preserved in a 4.5-billion-year-old meteorite from Mars.
If the evidence holds up, scientists will have to seriously entertain the possibility that life on earth may have arisen first in Martian environments. Mars has much weaker gravity than the earth, making it easier for rock to be blasted into space off Mars than off the earth. Furthermore, because the earth has stronger gravity, it is more likely for the earth to capture a meteor than is Mars. We may all be the descendants of Martians.
Of course, it is entirely possible, and perhaps probable, that life may have arisen independently on the earth and Mars, especially if organic-rich comets were crashing into both planets about four billion years ago. Resolution of this question will come from: (1) continued chemical experiments on the molecular precursors of life; (2) planned exploration for Martian fossils at the beginning of the next century; and (3) continued searching for a source of regular, complicated radio signals from outer space. Discovery of an intelligent signal would confirm that there is cognizant life elsewhere and that there are many planets on which life originated independently.
About the author: J. John Sepkoski, Jr., was a professor of paleontology at the University of Chicago. His research focused on large-scale evolutionary biodiversity and extinctions among life forms.