EARTH SCIENCE

Did life on Earth begin in the "little warm pond" ?

Viewpoint: Yes, the theory that life began in the "little warm pond" has supporting evidence from a number of experiments, and competing theories are more problematic.

Viewpoint: No, either life began on the surface during the period known as the late heavy bombardment, or it began in a protected environment away from the surface and the devastating impacts.

The origin of life on Earth is one of the most important, and elusive, problems in science. Efforts to understand the origin of life have been frustrated by lack of evidence. In the face of this fundamental difficulty, the search for a scientific explanation for the origin of life has relied upon speculative hypotheses, observations from present conditions on Earth and elsewhere in our solar system, and laboratory experiments that seek to simulate the conditions of the earliest period of Earth's history.

While earlier natural philosophers considered the problem of the origin of life, it was Charles Darwin who first posed an explanation for life's origin that is consonant with the larger picture of the evolution of life on Earth. Darwin suggested that simple chemicals in small or shallow bodies of water might spontaneously form organic compounds in the presence of energy from heat, light, or electricity from lightning strikes. These organic compounds could then have replicated and evolved to create more complex forms.

Darwin's "little warm pond" remains one of the most suggestive explanations for the origin of life. A classic experiment performed by Harold Urey and Stanley Miller in the 1950s brought the problem of the origin of life, and the "little warm pond," into the laboratory. Urey and Miller filled a flask with the gases they believed were present in the atmosphere of the ancient Earth, and suspended it over a small pool of water. They applied electrical sparks to the system, and observed that complex organic compounds, including amino acids, formed abundantly in the water. Amino acids are the most basic components of life on Earth. Their production in this experiment suggested that the beginnings of life could indeed have formed in appropriate settings on ancient Earth.

The Urey-Miller experiments have remained the paradigmatic explanation for the origin of life in school textbooks, but they leave many issues unresolved, and scientists have raised serious questions about nearly every aspect of the "little warm pond" model. For example, the atmosphere of ancient Earth may well have been made primarily of carbon dioxide and nitrogen—which do not produce amino acids—rather than the more hospitable mixture of methane, hydrogen, and ammonia used by Urey and Miller. And even if amino acids were abundant in the ancient seas, it is not understood how they could have evolved into more complex forms, including the proteins that make up the genetic code of DNA that is found in all life forms today.

Scientists who believe that ancient Earth did not provide conditions similar to those of the Urey-Miller experiments have looked elsewhere to try to find alternative scenarios to explain the origin of life. Observations of the Moon and other planets in the solar system have suggested that during the first billion years or so of Earth's history, the surface of our planet was extremely volatile and even hostile, bombarded by meteorites and intense solar radiation. These conditions make the formation of stable, warm, shallow ponds unlikely, but they suggest other interesting possibilities to explain the origin of life on Earth.

For example, the constant bombardment of ancient Earth by meteorites and comets may itself explain the source of amino acids—perhaps they formed elsewhere in the solar system and were carried here by these interplanetary travelers. Hypothetically, amino acids and other organic compounds could survive a trip from Mars or another planet to Earth, and although no undisputed evidence of such a transfer has been identified, scientists continue to study meteorites and comets to better understand the effect of these impacts on the formation of life.

Another promising focus of study is on those areas of Earth that come closest to the harsh conditions likely to have been present on early Earth. By investigating regions near volcanic rifts in the ocean floor and in deep underground wells, scientists have found microorganisms capable of surviving at the very high temperatures, which probably characterized the surface of ancient Earth.

Scientists have also tried to understand what the earliest life on Earth might have been like by studying genetic differences between organisms. These differences can be used to construct a genetic "tree of life," showing how groups of organisms are related and how they might have evolved from common, simpler ancestors. The oldest branch of life appears to include the thermophilic organisms that thrive in the hot environments on today's Earth.

Although evidence about the conditions of ancient Earth remains elusive, by bringing together observations from elsewhere in the solar system, genetic studies, and contemporary life in the strange micro-environments close to the heat of Earth's crust, scientists have begun to put together a picture of what the first billion tumultuous years of Earth's history might have been like. While this evidence challenges the simple idea of life emerging from a "little warm pond," primarily by suggesting that ancient Earth's surface lacked the stability to support such ponds, the fundamental suggestion that some combination of water, energy, and atmospheric chemicals produced simple organic compounds that ultimately gave rise to complex life remains a central idea in the efforts to explain the origin of life.

—LOREN BUTLER FEFFER

Viewpoint: Yes, the theory that life began in the "little warm pond" has supporting evidence from a number of experiments, and competing theories are more problematic.

How Did Life Begin?

The best theory we currently have regarding the origin of life on Earth is that it first originated as the accumulation of organic compounds in a warm body of water. This hypothetical "warm little pond" has supporting evidence from a number of experiments. However, the origin of life is clouded in uncertainty, and the precise mechanisms by which basic chemicals came together to form complex organisms is not known. The lack of evidence from ancient Earth means we may never know precisely how life began. Nevertheless, of all the speculative theories, the warm little pond remains the most promising.

In some ways the nature of this question means that a simple "yes and no" debate is of little value. To begin with, arriving at a satisfactory definition of "life" has proved difficult. While a number of attempts have been made, some definitions are so broad as to include fire and minerals, while others are so narrow they exclude mules (which are sterile).

Another major problem with determining how life began on Earth is the lack of evidence. The fossil record is limited by the fact that almost all rocks over three billion years old have been deformed or destroyed by geological processes. In addition to debating the issue of when life emerged, scientists also debate the conditions of ancient Earth. Some theories posit that early conditions on our planet was extremely cold, while other theories suggest that it was warm and temperate, and even boiling hot. Computer models have suggested that a variety of temperature ranges are possible, but without further evidence there is little consensus.

Life on Earth may have had a number of false starts. Early Earth was subjected to massive geological upheavals, as well as numerous impacts from space. Some impacts could have boiled the ancient oceans, or vaporized them completely, and huge dust clouds could have blocked out sunlight. Life may have begun several times, only to be wiped out by terrestrial or extra-terrestrial catastrophes.

Any theory on the origin of life must contain a great deal of speculation. What scientists can agree upon are the general characteristics that define life from non-life. Early life must have had the ability to self-replicate, in order to propagate itself and survive. Self-replication is a tricky process, implying a genetic memory, energy management and internal stability within the organism, and molecular cooperation. Just how the ingredients of the "primordial ooze" managed to go from simple chemical process to complex self-replication is not understood. Moreover, the process of replication could not have been exact, in order for natural selection to occur. Occasional "mistakes" in the replication process must have given rise to organisms with new characteristics.

Life from a Chemical Soup

The modern debate on the origin of life was inaugurated by Charles Darwin. In a letter to a fellow scientist he conjectured that life originated when chemicals, stimulated by heat, light, or electricity, began to react with each other, thereby generating organic compounds. Over time these compounds became more complex, eventually becoming life. Darwin imagined that this process might occur in shallow seas, tidal pools, or even a "little warm pond." Later theorists have suggested variations on this theme, such as a primordial ocean of soup-like consistency, teeming with the basic chemical ingredients needed for life. While Darwin and his contemporaries saw life as a sudden spontaneous creation from a chemical soup, modern theories tend to regard the process as occurring in a series of small steps.

In the early 1950s the "little warm pond" theory of life was given strong experimental support by the work of Harold Urey and Stanley Miller. Miller, a student of Urey, filled a glass flask with methane (natural gas), hydrogen, and ammonia. In a lower flask he placed a small pool of water. He then applied electric shocks to mimic lightning. The results were more than either scientist had hoped for—within a week Miller had a rich reddish broth of amino acids. Amino acids are used by all life on Earth as the building blocks for protein, so Miller's experiment suggested that the building blocks of life were easy to make, and would have been abundant on early Earth.

Further experiments by Sidney W. Fox showed that amino acids could coagulate into short protein strands (which Fox called proteinoids). It seemed that scientists were on the verge of creating life from scratch in a test tube. However, Fox's work now appears to be something of a dead end, as there is no further step to take after proteinoids. Proteins and proteinoids are not self-replicating, and so either there are missing steps in the process, or something altogether different occurred. Miller's work, too, has lost some of its shine, as there are now strong doubts that the atmosphere of ancient Earth contained the gases he used in his experiment. It is possible that rather than methane, hydrogen,

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and ammonia the early atmosphere was rich in carbon dioxide and nitrogen.

Even if amino acids were common on early Earth there is still the question of how these simple compounds gave rise to the complexity of life, and to DNA, the double helix that contains the genetic code. DNA cannot replicate without catalytic proteins, or enzymes, but the problem is that the DNA forms those proteins. This creates something of a chicken-and-egg paradox. One possible explanation is that before the world of DNA there was an intermediate stage, and some scientists have suggested that RNA is the missing gap. RNA is similar to DNA, but is made of a different sugar (ribose), and is single-stranded. RNA copies instruction from DNA and ferries them to the chemical factories that produce proteins in the cell. RNA may have come first, before DNA. The RNA world may have provided a bridge to the complexity of DNA. However, RNA is very difficult to make in the probable conditions of early Earth, and RNA only replicates with a great deal of help from scientists. Some theorists think there was another, more simple, stage before RNA, but again, no evidence has been found.

Other Theories on the Origin of Life

Because of the difficulties with the warm little pond theory and its variants a number of new theories have recently emerged to challenge it. Many of these theories are interesting, intriguing, and even possible. However, they all have unanswered questions, making them even more problematic than the idea of the "little warm pond."

Several decades ago scientists were amazed to discover organisms that live in very hot conditions. Dubbed thermophiles, these hot-living bacteria have been found in spring waters with temperatures of 144°F (80°C), and some species near undersea volcanic vents at the boiling point of water. There is even some evidence of under-ground microbes at even higher temperatures (336°F [169°C]). The discovery of such hardy organisms has led some to speculate that life originated not in a warm pond, but in a very hot one. Perhaps ancient Earth was peppered with meteor and comet impacts, raising temperatures and boiling oceans, but also providing the necessary chemical compounds to create life. Or possibly hot magma from volcanic sources provided the vital gases and compounds, and the energy, to assemble the first living organisms.

A variant of this theory considers the undersea volcanic vents as the birthplace of life, with the chemical ingredients literally cooked into life. There are even those who champion a deeper, hotter, underground origin for life. Underwater and underground origins have some advantages over other theories. Such depth might make early life safe from the heavy bombardment of material from space the planet received, depending on the size of the object striking Earth and the depth of the water. They would also be safe from other surface dangers, such as intense ultraviolet radiation. There is even some genetic evidence to support these hot theories, as thermophiles do seem to date back to near the beginnings of the tree of life. However, whether they were the trunk of the tree, or merely an early branch, is not known. There is also the question of how these hot organisms could have moved into cooler areas. Some theorists argue that it is easier to go from cool to hot, not the other way around. Also, environments such as undersea volcanic vents are notoriously unstable, and have fluctuations that can cause local temperature variation that would destroy rather than create complex organic compounds.

Some theorists have gone to the other extreme of the temperature scale, and envision life beginning on a cold, freezing ancient Earth. Just as hot microbes have been discovered, so have organisms capable of surviving the Antarctic cold. Some suggest these as our common ancestors. Again, there are some advantages to such a theory. Compounds are more stable at colder temperatures, and so would survive longer once formed. However, the cold would inhibit the synthesis of compounds, and the mobility of any early life. Also, the premise that ancient Earth was a cold place is not widely accepted.

Others have looked to the heavens for the origins of life. The early solar system was swarming with meteors and comets, many of which plummeted to Earth. Surprisingly there are many organic compounds in space. One theory suggests that the compounds needed to form the primordial soup may have arrived from space, either from collisions, or just from near misses from comet clouds. Even today a constant rain of microscopic dust containing organic compounds still falls from the heavens. Could the contents of the little warm pond have come from space?

There are also suggestions that life may have arrived from space already formed. Living cells could possibly make the journey from other worlds, perhaps covered by a thin layer of protective ice. The recent uncovering of a meteorite that originated on Mars has leant support to this theory. There is some suggestion that the meteorite contains fossilized microorganisms, but most scientists doubt this claim. However, the collision of comets and meteors is far more likely to have hindered the development of life than help create it. Objects that would have been large enough to supply a good amount of organic material would have been very destructive when they hit. It seems probable that life began on Earth, rather than in space somewhere. Also, the idea that life may have traveled to Earth does not help explain its origin; it merely transposes the problem to some distant "little warm pond" on another world.

There are a number of other theories proposing various origins of life that have appeared in recent years. Gunter Wachtershauser, a German patent lawyer with a doctorate in organic chemistry, has suggested that life began as a film on the surface of fools gold (pyrite). Some small experiments have given it some credence, but the idea is still at the extreme speculative stage. Sulfur is the key ingredient in some other theories, such as the Thioester theory of Christian R. de Duve. Thioesters are sulfur-based compounds that Duve speculates may have been a source of energy in primitive cells. In the primal ooze thioesters could have triggered chemical reactions resembling those in modern cellular metabolism, eventually giving rise to RNA. However, again there is a lack of supporting experimental evidence.

All of these new theories suffer from the same problems that beset the standard interpretation. That is, the difficulty of going from simple chemical process to self-replicating organisms. Many of these new theories are merely new twists on the original warm little pond concept. Some are boiling ponds, others are cold, but only a few offer completely different ways of viewing the origin of life. While some of these theories have some strong points, they have yet to provide the hard evidence to support the speculation. None of them has gained enough support to topple the "little warm pond" from its place as the most likely theory we have. There is much supporting evidence for the standard theory, in the form of Miller's experiments and the work on RNA. Darwin's throw-away comment in a letter may have led to more than he bargained for, but his theory on the origin of life still remains the best and most useful theory we currently have.

—DAVID TULLOCH

Viewpoint: No, either life began on the surface during the period known as the late heavy bombardment, or it began in a protected environment away from the surface and the devastating impacts.

Scientists know little about the origin of life—except that it happened. Earth formed 4.5 billion years ago from innumerable collisions of smaller rocks. The leftover debris from planet formation careening through the solar system bombarded Earth even after it achieved its full size. During this period, known as the late heavy bombardment, the surface was routinely devastated by large impacts. This period ended about 3.8 billion years ago. Oddly enough, the oldest known microfossils date back 3.5 billion years ago, and there is tantalizing evidence of life as early as 3.8 billion years ago. It seems that, as soon as life could survive on the surface of Earth, it began—in a very short amount of time. But many scientists believe the origin of the first life forms from non-life forms must have taken more than the few million years between the end of the late heavy bombardment and the first evidence of life. Therefore, either life began on the surface during the late heavy bombardment, or it began in a protected environment away from the surface and the devastating impacts.

The Miller-Urey Experiment

In 1953, Stanley Miller and Harold Urey performed a radical new experiment. They generated a spark in a mixture of water, methane, ammonia, and hydrogen, a composition believed to contain the major components of Earth's early atmosphere. After only one week, Miller and Urey succeeded in forming complex organic carbon compounds, including over 20 different types of amino acids, the building blocks of life. The Miller-Urey experiment, which has been replicated thousands of times in high schools and colleges around the world, stands as a proof of the concept that, in the "warm little ponds" believed to pepper the surface of early Earth, the beginnings of life could form.

This perhaps occurred in a "warm little pond" on Earth 4 billion years ago. But long before the amino acids collected themselves into the organized structures known as life, one of the frequent large impacts of leftover solar system debris smacked into Earth, devastating the surface—boiling seas, heating the atmosphere enough to melt silica—and effectively sterilizing the "warm little pond," along with the remainder of Earth's exposed surface. The Miller-Urey experiment proved that the beginnings of life could assemble in a simple environment comprised of basic molecules. But it failed to insure that such an idyllic environment existed on early Earth. Amino acids are but one prerequisite to life. The others are a source of food or energy and—more importantly—a location stable enough to allow the organization of molecules into life.

Much of what scientists know about the conditions on early Earth comes from the Moon. Lacking any atmosphere or oceans, events recorded on the surface do not erode away over time. The Moon, in essence, acts as a geological recording device, writing its history on its surface for all to see. The lunar record can be read on any clear night on the face of the full Moon: the dark areas, or maria, are large basins excavated by massive impacts and later filled with lava. A simple pair of binoculars reveals even more craters, created by smaller impactors. Overall, the oldest features on the Moon, dating back 3.9 to 4.0 billion years ago, tell a story of frequent, massive impacts in the Earth-Moon system. Four billion years ago, the surface of the Moon—and, by extension, Earth—was a hostile environment.

Another factor rendered the surface of Earth inhospitable to life. Four billion years ago, Earth's atmosphere was largely made of nitrogen and carbon dioxide (oxygen, a product of photosynthesis, didn't arise in appreciable quantities until after microbial life flourished on Earth for over 2 billion years). An oxygen atmosphere, illuminated by radiation from the Sun, produces O 3, or ozone. Therefore, without oxygen, Earth lacked an ozone layer capable of protecting life from harmful and even fatal ultraviolet radiation. To make matters worse, the Sun was far more active in the past, producing tremendous solar flares and incredible bursts of ultraviolet radiation. Any organism on the surface of the Earth would be damaged by the intense radiation—a fate ill-suited to further reproduction.

Lastly, the surface of Earth was much different four billion years ago. The total amount of exposed land area, or continents not covered by water, was much less than today. Therefore, any potential microbial environments were most likely covered in a layer of water, perhaps 3,300 ft (1 km) thick or more. This reduces the chance of the proper compounds being confined to a "warm little pond," as in the Miller-Urey experiment.

But scientists know one thing for certain—life did begin. Somehow, the proper building blocks such as amino acids were formed on, or delivered to, Earth. Other than the "warm little pond," there are several other sources of amino acids in the solar system. The oldest meteorites, known as carbonaceous chondrites, represent the pristine material from which the planets were formed. These objects routinely rain down on Earth even today, and represent a potential source of amino acids for life. In addition to meteorites, comets deliver a large amount of material to Earth's surface. While no samples of comets have been examined to date, NASA's Stardust mission plans to return a sample of comet Wild-2 in 2006, and scientists expect to find complex carbon compounds, including amino acids. Even if the surface of Earth did not allow for the formation of amino acids, and later life, ample sources of raw materials exist.

Life Found in Extreme Heat

The surface of Earth is not the only place where life could have started. Today, two unique environments support extensive microbial ecosystems, in what many would consider inhospitable locations. Near volcanically active rifts on the sea floor, known as hydrothermal vent systems, microorganisms not only exist, but also thrive and support a dynamic ecosystem of macroscopic life forms as well. Volcanically heated water leaves the hottest vents, known as black smokers, with a temperature in excess of 750°F (400°C). This water interacts with the seawater at 28°F (-2°C). Some microorganisms found in this environment thrive at temperatures in excess of 212°F (100°C), or the boiling point of water. The organisms get the energy by chemical reactions with the materials released by the black smokers. On early Earth, where submarine volcanism was widespread, these environments were abundant. They have the advantage of being located thousands of feet underwater, and are well protected from both smaller impacts and ultraviolet radiation.

Still, some of the largest impactors to hit Earth during the heavy bombardment had enough energy to boil the oceans completely, destroying even the submarine environments. Therefore, scientists have looked deep underground for habitats capable of supporting life. Recently, sediments retrieved from deep wells near the Hanford Nuclear Reservation in Washington State show evidence of microbial activity. Some of these organisms are from samples originally at depths of almost 10,000 ft (about 3 km). The organisms thrive on water from deep aquifers and chemical reactions in the subsurface. Due to their depth, these environments would also be protected from impacts and UV radiation.

Both of these high-temperature environments exist—over 212°F (100°C) in the hydrothermal vents, and as high as 167°F (75°C) in the subterranean environments. Organisms whose optimum growth temperature is above 113°F (45°C) are known as thermophiles, and organisms with an optimum growth temperature above 176°F (80°C) are known as hyperthermophiles. Clearly, the first organisms to live in such environments would have to be thermophiles or hyperthermophiles. Therefore, if the organisms living on early Earth were tolerant to heat, chances are they originated in one of these environments.

Scientists have no samples of life from early Earth, but the genetic code of all organisms contains remnants from the first organisms. By studying the differences in the genes between organisms, scientists can determine how one species relates to one another—which organism evolved from which, and how closely they are related. By charting the genetic differences between organisms, scientists construct a genetic "tree of life" relating all the species to one another. Study of all the life on Earth reveals several major classes of organisms. First of all, there are the eukarya, containing all the microorganisms whose cells have a nucleus, as well as the familiar macroscopic life like fungi, oak trees, and humans. Next are the bacteria (monara), single-cell organisms without cell nuclei. Lastly, the oldest organisms are the archaea, which are closest to the "root" of the tree, and presumably most closely related to the common ancestor that first evolved on early Earth. Study of the archaea reveal a large number of thermophilic and hyperthermophilic organisms, indicating the common ancestor was also likely to be thermophilic, making the subterranean and hydrothermal vents appealing locations for the evolution of life.

Did Life Begin in Space?

Another intriguing idea is that life did not begin on Earth at all, but was later brought to Earth during the heavy bombardment by interplanetary meteorites. To date, there are at least a dozen known Martian meteorites, launched from the surface of Mars after a large impact and later retrieved on Earth's surface. In the summer of 1996, David McKay and other researchers at NASA released preliminary evidence of microbial activity in Martian meteorite ALH84001. Since that time, much of their evidence has been discounted as a misinterpretation or contamination by terrestrial sources. However, the discovery opened up the question of interplanetary "seeding" of biological material. Amino acids and other compounds can survive the trip from Earth to Mars, and it may be possible that microorganisms can as well. It is possible that life swapped worlds several times, as sterilizing impacts rendered one planet inhospitable for a time, and the other planet acting as a temporary safe haven until the next large impact. If this is the case, the question "where did life begin?" becomes even more complex.

Scientists have learned, from studying the Moon, that Earth's surface was far too hostile to support "warm little ponds" stable enough for life to form unless life began frequently—and very rapidly—between impacts, or was delivered to another world for safe keeping. It appears as if sterilizing impacts forced life's origin to sites deep underground or underwater. In the near future, the new science of astrobiology may be able to answer the question "where did life begin?" Two locations in the solar system, other than Earth, are likely places to look for life today. Our neighboring planet Mars may have subterranean environments similar to those found on Earth. Jupiter's moon Europa is composed of a rocky core surrounded by a shell of liquid water and ice. Hydrothermal vents may exist below the surface of the kilometer-thick ice sheet on the ocean floor. If life is found in either of these locations, it is likely that life began in hydrothermal or subterranean environments on Earth as well.

—JOHN ARMSTRONG

KEY TERMS

AMINO ACIDS:

Amino acids are complex carbon compounds that make up proteins and other molecules important to life.

CARBONACEOUS CHONDRITES:

The oldest meteorites in the solar system, believed to be remnants of the pristine building blocks of planets. They also can be rich in amino acids.

HYDROTHERMAL VENTS:

Locations on the sea floor near spreading centers where magma beneath the surface heats water to high temperature and jets to the surface. They support thriving ecosystems on the otherwise barren deep sea floor.

MARIA:

The dark areas on the Moon that are large crater basins filled with cooled lava. Comes from the Latin for "Sea."

TREE OF LIFE:

Tool used by microbiologists to study the genetic relationships between organisms. It is composed of three branches—the eukarya, bacteria, and archaea.

Did life on Earth begin in the "little warm pond"

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