Introduction to The Early Earth
Descriptions such as alien, hostile, and violent only hint at the conditions you would encounter if you could travel back in time 4.6 billion years to the newly formed Earth. At first, you would see a waterless surface pockmarked with craters and littered with volcanic rocks. Over the next billion years, you would watch as rocky debris left over from the formation of the solar system—some of it the size of a small planet—pummeled Earth. You would see volcanoes spewing clouds of noxious gases and monstrous rivers of lava. But sooner than you might imagine in such an unstable world, you would notice a thick atmosphere enveloping the planet and water pooling in low places. Then, about 3.5 billion years ago, you would discover that you were no longer the only life form on Earth.
Until the early 1970's, information about Early Earth was so sparse that geologists did not even have a name for the planet's first billion or so years. Many geologists now refer to this period as the Hadean Eon—after Hades, the ancient Greek underworld.
By 2006, scientists had developed a better understanding of the forces that had transformed Earth from an arid wasteland to a world whose oxygen-rich atmosphere nourishes an astonishing variety of life. They had been able to create a rough time line for the major events of the Hadean Eon, though many of the details remained unclear. They had discovered just how severe, violent, and long-lasting the bombardment period was. And they had found evidence that Earth's life forms and water had appeared much earlier than previously believed.
Most important, they had radically changed their theories about the origin of life. In the mid-1900's, scientists commonly assumed that life appeared in the warm, shallow seas of a primitive Earth whose temperatures were similar to those of today. By 2006, however, they had learned that some organisms, known as extremophiles, can thrive under conditions that would instantly kill human beings and most other life forms. At one time, scientists assumed that all life depended on the sun for energy, but they now know that some extremophiles get their energy from chemical reactions in hot springs bubbling up from the ocean floor. Other extremophiles live in oxygen-free environments deep underground. Under which of these—or other still unknown conditions—did the first life forms appear? Finally, scientists no longer wonder how Earth came to be so suitable for life. They know that from its first appearance, life has profoundly shaped the atmosphere, oceans, and other physical conditions on Earth.
The World In A Grain of Zircon
Trying to reconstruct Hadean-Eon conditions is challenging because our planet's geologic history is a story of unceasing change. Rock is continually destroyed by weathering, erosion, and other surface processes and recycled into new forms by heat, pressure, and the movement of the tectonic plates that make up Earth's outer shell. All this geologic activity has given Earth beautiful landscapes and a great variety of habitats for living things. However, this activity has also destroyed—and continues to destroy—the evidence of Earth's geologic past. Just as with human history, the further back in time scientists go, the more incomplete this record becomes. In fact, by 2006, geologists had only the tiniest bit of physical evidence from the early Hadean. The oldest known material of any kind on Earth consists of microscopic grains of the mineral zircon found in Australia; they date from about 4.4 billion years ago. The oldest known Earth rock is an outcrop (section of exposed bedrock) called the Acasta Gneiss in Canada's Northwest Territories. This coarse-grained rock formed about 3.9 billion years ago deep inside Earth from even older rock that was altered by heat and pressure. The discovery of the zircon has given geologists hope that they may find even older rocks. Rocks are far more complex than minerals and convey much more detail about the conditions under which they formed.
Scientists also look to outer space for clues about conditions on Early Earth. Certain types of meteorites, called chondrites, are the remains of rocks that formed early in the history of the solar system. Chondrites, which remained unchanged until they fell to Earth, are a valuable source of information about the geologic and chemical conditions on Early Earth.
In 1969, United States astronauts on the Apollo space missions began bringing back rock samples from the moon. Since then, scientists have discovered that some meteorites actually originated on the moon or even on Mars. They were blasted into space by giant meteor impacts, eventually landing on Earth. All these samples of extraterrestrial terrain were unaffected by weathering and other processes that alter rocks over time on Earth. As a result, they preserve an early record of conditions on the moon or Mars. Scientists presume the same sorts of events occurred on these celestial bodies as well as on Earth during the Hadean Eon.
Earth and the Moon Are Formed
The formation of Earth probably began with the violent death of a star called a supernova. During this gigantic explosion, most of the dying star's matter was blasted into space. This expansion compressed molecules of gas and microscopic grains of dust nearby, forming a giant, rotating cloud called the solar nebula. The gravitational attraction between the molecules and grains caused the cloud to start contracting. As the nebula contracted, it spun faster and flattened into a disk. Most of the material in the solar nebula collapsed toward the center. Eventually, this central mass grew dense and hot enough for hydrogen atoms to begin fusing to helium atoms—the basis of nuclear fusion—and the central mass became a star, the sun.
The remaining material swirled around the sun at a faster and faster rate. As the dust grains collided, they stuck together to build larger objects—a process called accretion—and their gravity grew strong enough to sweep up more and more of the dust and gas molecules. Collisions between larger objects led to the formation of chunky rocks called planetesimals. Some planetesimals combined to form the planets, including Earth. Other planetesimals formed moons, asteroids, and comets.
Scientists believe that shortly after Earth formed, it collided with another planet-sized object in an event known as the Giant Impact. The impact shot a cloud of vaporized rock off Earth's surface and into orbit around Earth. Over time, the cloud cooled and condensed into a ring of small, solid bodies, which then stuck together, forming the moon. The Giant Impact would have melted most of or even all of Earth and the moon. According to some estimates, Earth would have been as hot as the sun for about 10,000 years afterward.
By about 4.5 billion years ago, Earth and the moon had cooled enough for light silicate rocks to rise to Earth's surface, forming the earliest crust. But that crust did not last long. In the interior, the decay (breakdown) of radioactive elements and increasing pressure generated great quantities of heat that melted the interior and rose to the surface. At the same time, frequent volcanic eruptions may have covered much of the planet's surface in red-hot flows of lava. Finally, asteroids, comets, and other chunks of space debris left over from the formation of the solar system continually bombarded the young planet. Each time an object hit the planet, the impact created a crater, melted some of the crust, and spread a blanket of debris around the crater. Scientists believe that the impacts occurred so frequently that debris from one impact buried the rocks heated by previous impacts. Erosion and the movement of the tectonic plates that make up Earth's outer shell have destroyed all evidence of these impacts. The heat from the interior and the bombardments became so intense that the crust melted, forming a magma ocean. Scientists have found evidence of a magma ocean in rocks brought back from the moon. They assume that the same process occurred on Earth, though no traces of a terrestrial magma ocean have been found.
A Series of Atmospheres
Our oxygen-rich atmosphere, so essential to life on Earth, is the latest in a series of atmospheres that have surrounded our planet since its formation 4.6 billion years ago. The newly formed Earth probably did not have a true atmosphere. Instead, gases escaping from the developing Earth may have formed a thin haze around the planet. But if this atmosphere existed, it was almost certainly stripped away by the impact that created the moon. Indeed, many of the larger impacts that occurred during the accretion of Earth would have effectively blasted any accumulating gas into space. As a result, Earth probably did not have a significant atmosphere until the end of the heavy bombardment period.
The development of Earth's magnetic field was another factor that helped in the creation of a dense atmosphere. After the sun ignited, eruptions began occurring on its surface, sending a continuous flow of electrically charged particles streaming into space. This solar wind swept away any haze that may have formed from gases escaping from Earth's interior. As iron and other heavy materials sank to Earth's center and began to melt, electric currents in the newly formed core generated magnetic fields, and a region of strong magnetic forces called the magnetosphere surrounded Earth. This magnetic field shielded the planet from the solar wind, allowing the formation of an atmosphere.
In 2006, scientists held two main theories explaining the formation of the first atmosphere. One theory involves a process called volcanic outgassing. According to this theory, as the interior of Earth became hotter, the heat caused certain elements to rise to the surface in volcanic rock. Some of these chemicals—including ammonia, carbon dioxide, carbon monoxide, hydrogen, methane, nitrogen, sulfur dioxide, and water vapor—became the gases of the atmosphere. Additional gases, such as argon, were added by the decay of radioactive elements within Earth.
The presence of argon 40 in the atmosphere offers one of the strongest arguments for the outgassing theory. About 1 percent of the atmosphere consists of this isotope (form) of argon, a gas that does not readily combine with other elements. Argon 40 is created by the decay of a radioactive isotope of potassium, a relatively abundant element that makes up nearly 2.5 percent of Earth's crust. The abundance of argon 40 in the atmosphere implies that a large amount of this gas escaped from Earth's interior.
A second theory suggests that as comets slammed into Earth during the heavy bombardment phase, they released their cargo of frozen gases. These gases then formed Earth's first true atmosphere. Some scientists believe the zircon grains discovered in Australia provide some support for this theory. These grains show signs of having been weathered out of their original rock, transported by water, and then redeposited in newly formed sedimentary rock (rock composed of layers). In other words, the grains indicate that liquid water has existed on Earth for at least 4.4 billion years.
The presence of liquid water on Earth so long ago—perhaps only 150 million years after the planet's birth—raises difficult questions. According to most theories on how stars form, our infant sun 4.4 billion years ago was only about 70 percent as bright as it is today. As a result, the sun would have provided only about 70 percent of the heat that it does today. Accordingly, Earth would have been too cold for liquid water, which would have frozen.
Liquid water could have survived, however, if a dense atmosphere had surrounded Earth by that time. This atmosphere would have trapped enough heat to keep Earth's temperature above water's freezing point. Some scientists argue that volcanic outgassing could not have supplied enough gas to create such a dense atmosphere so fast. They suggest that the atmospheric gases, particularly water vapor, must have come from comets. Comets, which pummeled Earth in large numbers for millions of years, consist mainly of frozen gases.
Scientists were still exploring both the outgassing and comet theories in 2006. Both may well have played a role in the creation of the atmosphere. However it was created, Earth's atmosphere 4.4 billion years ago was much thicker than it is now. Most scientists agree that it contained little oxygen. Some scientists believe Earth's early atmosphere contained ammonia, helium, hydrogen, and methane, much like the present atmosphere of Jupiter. Others believe it may have contained a large amount of carbon dioxide, as does the atmosphere of Venus. If Early Earth resembled Venus, speculates geochemist Stephen Mojzsis of the University of Colorado at Boulder, the sky would have been reddish because of scattered sunlight, and any oceans would have been olive-green because of metals dissolved in them.
The Oceans Appear
For many years, some scientists believed that comets were also the sole source of the water that formed the oceans. By 2006, however, this theory had been largely discredited. One study that cast doubt on the comet theory involved the use of radio telescopes to analyze the chemical makeup of three comets as they came close to Earth in the 1980's and 1990's. These studies revealed that comets contain from 2 to 20 times as much heavy water as ocean water does. (Like normal water, heavy water contains one oxygen atom and two hydrogen atoms; however, it also contains at least one atom of deuterium, an isotope of hydrogen with twice the mass of ordinary hydrogen.) The higher proportion of deuterium in comet ice suggests that comets could not have been a major source of seawater. If comets had supplied water to the Earth's oceans, the oceans would be much richer in heavy water than they are.
Volcanic outgassing almost certainly created the oceans, most scientists believe. According to this theory, Earth's water originated from water molecules in the cloud of gas and dust that gave rise to the solar system. The water molecules were added to the accreting Earth with all the other materials that make up the planet. Later, the water vapor boiled out as steam. For hundreds of millions of years, volcanoes ejected the water vapor along with chlorine and other hot gases into the Earth's atmosphere.
Rain began to fall once the bombardment stopped and Earth cooled to a temperature below the boiling point of water (100 °C [212 °F] at sea level). Rain may have fallen in a deluge, drenching the planet. On the other hand, rain falling at the rate of 1 centimeter (0.39 inch) per year could have created an ocean 10 kilometers (6 miles) deep in only 1 million years. Many researchers believe that the first permanent ocean was likely in place from 4.3 billion to 3.8 billion years ago. This early sea may have covered the entire planet to an unknown depth as rainwater collected everywhere on the nearly featureless landscape.
The chemical composition of the earliest oceans was probably different from that of modern seawater. For example, the earliest oceans were probably not salty. Instead, they may have had high levels of other elements—including calcium, iron, and magnesium—that dissolved out of rocks by weathering. Studies by rovers on Mars have found water-soluble minerals (able to be dissolved in water) containing iron that may have formed in a similar but short-lived ocean on Mars.
If Earth's early atmosphere included high levels of carbon dioxide, the early oceans may have been highly acidic. The sulfur and hydrochloric acid that erupted from volcanoes also may have contributed to the acidity of the oceans. In this acidic and oxygen-poor setting, rocks would have weathered more quickly and many metals, such as iron, would have been much more soluble in seawater than they are now.
Seawater may have become salty because volcanoes released large amounts of chlorine in the form of hydrochloric acid, as well as water vapor, into the early atmosphere. The hydrochloric acid fell to the ground with rainwater. As the water flowed over the surface, the acid ate away the rocks, stripping them of their salts, particularly sodium. Some of the sodium atoms combined with chlorine atoms in the water to form sodium chloride—more commonly known as table salt—the primary salt in the sea.
Seawater is salty today because of billions of years of chemical processes in which some materials were added and others removed. For example, many processes remove such elements as calcium from seawater, but few remove sodium or chlorine. As a result, these elements tend to remain in seawater for a very long time. On the average, a sodium or chlorine atom remains in the ocean for about 100 million years.
The Earliest Continents
The discovery of the zircon grains in Australia indicates that crust had begun forming on Earth by 4.4 billion years ago. These grains were not found in the rock in which they formed but in younger sedimentary rocks that are at least 3 billion years old. The Acasta Gneiss in Canada, the oldest known rock, is actually metamorphic rock—that is, rock that forms when heat or pressure or both cause changes in “parent” rock. That means that the Acasta Gneiss formed from even older rock, probably granite. Granite, in turn, usually forms by the slow cooling and crystallization of magma (molten rock) in which such light elements as potassium, silicon, and sodium become concentrated. Some crust also could have formed as the light minerals found in granite floated upward in Earth's magma ocean and in the pools of molten rock created by large meteor impacts. Geologists believe that, by about 3 billion years ago, at least 10 percent of the present continental crust had formed because rocks that old are found over about 5 percent of the present continents.
At first, Earth probably did not have large tectonic plates that, like modern plates, collided, moved apart, or slid past one another. Most of the earliest crust on Earth probably consisted of mosaics of small pieces of crust joined by collisions. In some places, magma from Earth's interior may have risen to the surface, where it cooled and was added to the crust. In other places, parts of the crust may have sunk into the interior. But probably none of these moving pieces of crust would have been wider than a few hundred kilometers. Because the inner Earth was more dynamic then than it is today, these mini-plates would also have moved much faster than modern plates, which travel at the rate of about 10 centimeters (4 inches) per year.
The Appearance of Life
The oldest known evidence of life on Earth consists of the 3.5-billion-year-old fossils of primitive, plantlike organisms called cyanobacteria. Sometimes called blue-green algae, cyanobacteria are simple, one-celled organisms that are related to bacteria. The organic compounds that combined to produce Earth's first life forms, however, are certainly much older than these primitive organisms—perhaps as old as 4.2 billion years.
Scientists have developed two main theories explaining the origin of life—the theory of chemical evolution and the theory of panspermia. The more widely accepted theory of chemical evolution was developed independently during the 1920's by Soviet biochemist Alexander I. Oparin and by British biologist J. B. S. Haldane. Oparin and Haldane theorized that because hydrogen is the most abundant element in the universe, Earth's early atmosphere had large quantities of this gas. Under such conditions, the hydrogen-containing compounds ammonia, formaldehyde, hydrogen cyanide, methane, and water would also have been abundant. According to this theory, sunlight, lightning, and volcanoes powered reactions among these compounds that produced simple biological molecules, such as sugars and amino acids.
Two American chemists, Stanley L. Miller and Harold C. Urey, in 1953 provided the first experimental evidence in support of the theory of chemical evolution. They subjected a mixture of ammonia, hydrogen, methane, and water to the energy of high-voltage sparks for one week. After that time, amino acids and other simple biochemical compounds had formed. Scientists have repeated this experiment under various conditions. For example, some researchers have assumed that the early atmosphere contained little hydrogen but large quantities of carbon dioxide. They have replaced the hydrogen-rich “atmosphere” of the Miller-Urey experiment with various mixtures high in carbon dioxide and relatively low in hydrogen. These mixtures also have yielded biochemical compounds when exposed to sparks of energy. Other organic compounds on Early Earth may have come from carbon-rich meteorites known as carbonaceous chondrites. Scientists have found simple organic compound in several of this type of meteorite.
Most scientific research supports—or at least does not contradict—the idea that life arose through chemical evolution. For example, the surface of Earth experiences a continuous flow of energy as it gets light from the sun and radiates heat into outer space. Physics research has demonstrated that such an energy flow increases molecular organization. Thus, the evolution of complex biochemical molecules may be viewed as part of this natural process.
From Molecule to Cell
Scientists have developed three major theories to explain the transition from early organic molecules to living cells. All three theories are based on the idea that the simple organic compounds formed more complex ones, which then gave rise to the structures that make up cells. The oldest of these theories states that chemical reactions in the ocean or in lakes led to the formation of large molecules. These molecules then acted as catalysts (substances that speed up chemical reactions) to cause the formation of complex organic compounds. A second view holds that chemical reactions producing the first complex organic compound took place on the surfaces of clays or of minerals called pyrites. In this view, the clays or pyrites acted as catalysts.
A third theory is based on the knowledge that cell-like structures with membranes will form spontaneously in mixtures of certain lipids (fatlike substances) and water and that such structures fold into shells the size of small cells. Supporters of this theory argue that the chemical reactions leading to the formation of complex organic compounds took place inside and on the surface of these shells.
The earliest ancestors of living organisms were molecules that acquired the ability to make copies of themselves. Those self-replicating molecules may have been early forms of ribonucleic acid (RNA). Found in all living cells, RNA is important in making proteins and in the transmission of genetic information from one generation to the next. American chemist Julius Rebek of the Massachusetts Institute of Technology in Cambridge created the first self-replicating molecules in the laboratory in 1953. By 2006, scientists had found a number of these molecules. Very simple molecules are capable only of reproducing exact copies, and any variation in their structure prevents reproduction. More complex molecules, however, can reproduce even if the resulting copies are not exact. Scientists do not know which molecules gave rise to DNA, however. DNA (deoxyribonucleic acid) is a thin, chainlike molecule found in every living cell on Earth. It directs the formation, growth, and reproduction of cells and organisms.
The Theory of Panspermia
Because the leap from nonliving to living has not been replicated, some scientists suggest that the building blocks of life actually fell to Earth in meteorites and comets, a theory known as panspermia. For example, in 1969, investigators found amino acids, organic proteins found in living things, in the core of a meteorite that had drifted in space for 4.5 billion years. Scientists determined that the amino acids had been in the meteorite before it hit Earth. In 2004, geochemist Jennifer Blank of the Lawrence Livermore National Laboratory in Livermore, California, showed that amino acids could survive the extreme pressures and temperatures produced by a collision with Earth. Under these conditions, the amino acids also became more complex molecules called peptides. But protein is only one element found in living cells.
Scientists using the National Aeronautics and Space Agency's (NASA) Spitzer Space Telescope have found other evidence for the theory that life originated in space. In 2005, astronomer Fred Lahuis of Leiden Observatory in the Netherlands spotted forms of DNA and protein in the dust surrounding a young star called IRS 46. However, the theory of panspermia does not solve any of the important problems connected with the origin of life; it merely places the origin of life somewhere else. Wherever life appeared, it must have developed through chemical processes, something more likely to happen on Earth than in space.
Life In the Extreme
Scientists can gain clues to the possible nature of the earliest life by looking at present-day life. The most primitive organisms on Earth, called archaea, look like conventional bacteria under the microscope. However, the way in which their DNA reproduces and the structure of their cell walls are different. Many archaea are extremophiles. Scientists have found these hardy organisms in boiling hot springs, in Antarctic lakes with permanently frozen surfaces, in desert lakes saturated with salt, and in extremely acidic runoff from mine wastes. One of the most interesting extremophile habitats is the highly polluted Rio Tinto in Spain. For many years, sulfide mineral ores in the rocks have been weathering out and releasing sulfuric acid into the river. Although the river is too acidic and toxic for most life forms, it is rich in extremophiles.
Extremophiles have shown researchers that life could have arisen on Earth even if conditions were different from those that exist today. Extremophiles could have survived an extremely hot or immensely cold Early Earth. They could have lived in highly acidic early oceans or flourished deep below Earth's surface.
In their efforts to understand Early Earth, geologists continue to look for older and older rocks. Although no Earth rocks older than 4 billion years have yet been found, geologists remain hopeful. Perhaps the original rocks that contained the zircons from Australia still exist.