Cedric and Winnie have finally saved enough money to buy a house. They dream about owning a Victorian cottage in a serene older neighborhood. They hope to find their home near Winnie’s workplace and in a community with outstanding schools for their two young children.
Cedric and Winnie want to live in the comfort zone. Likewise, most organisms on Earth exist in “comfortable” environments.1 Most life exists at temperatures between 5 and 40 °C (41-102 °F). Surface life generally demands near constant atmospheric composition (80% nitrogen and 20% oxygen) and pressure (1 bar). Aquatic life typically needs high levels of dissolved oxygen, moderate salt levels, and a near neutral pH. For the most part, life requires stringent, yet unspectacular, conditions. However, some creatures prefer the extreme.
Unlike Cedric and Winnie, Hans likes the hot humid hills of Haiti. Electricity and indoor plumbing may be necessities for Cedric and Winnie, but for Hans, they represent frivolous amenities. He loves the lack of modern conveniences that make life tolerable for most. Though Cedric and Winnie might find Haiti a tough place to live, Hans thrives in the extreme.
Over the past few decades, scientists probing some of the harshest environments on Earth have discovered organisms more like Hans than like Cedric and Winnie.2 For example, researchers have recovered single-cell microorganisms at thermal vents on the ocean floor. These vents spew out geysers of water at 350 °C. The hydrostatic pressure on the ocean floor is nearly 300 bars. (Water boils at about 400 °C at this pressure.) The single-cell microorganisms around these vents use chemical energy to anchor the food chain for more complex creatures that make up vent communities.
Scientists have also discovered single-cell microorganisms in the four-kilometer frozen ice sheet above Lake Vostok in Antarctica. Investigators studying the Dead Sea and Great Salt Lake have isolated single-cell organisms thriving at high-salt levels. Scientists have even harvested single-cell organisms from the hot, highly acidic soil of regions with fading volcanic activity. This volcanically active soil literally boils with temperatures up to 100 °C.
Scientists call these remarkable hardy creatures extremophiles, which literally means “extreme loving” (see sidebar, “Introducing Extremophiles”). Some extremophiles are more complex, but the majority are single-cell microorganisms. Although they superficially resemble bacteria, scientists group extremophiles separately (see Archaea sidebar).
In contrast, organisms that require more “comfortable” conditions for life (like Cedric and Winnie) are termed mesophiles. Many bacteria are mesophiles, organisms needing moderate conditions, between 20 and 40 °C. However, a few bacteria are extremophiles living at temperatures between 60 and 80 °C.
Extremophiles fascinate researchers. Like Hans, they challenge the current perception of life’s limits for habitability. As scientists learn about the physiology and biochemistry that allow extremophiles to thrive in harsh environments, basic biology advances.
Extremophiles hold much commercial potential as well. Molecules isolated from extremophiles are being evaluated for use in industrial processes that involve high temperatures, pressures, and salinity, and pH (high-low) extremes.3 Extremophiles that degrade oil may one day be used to clean up oil spills, and those resistant to ionization radiation may help with bioremediation at nuclear waste sites.4
Extremophiles and the Origin of Life
Beyond their basic scientific interest and commercial use, extremophiles assume an increasingly important role in explanations of life’s origin. Some origin-of-life researchers committed to the evolutionary paradigm hope that extremophiles will provide a way around the many intractable problems confronting naturalistic explanations for life’s beginning.5
Research advances during the last decade increasingly challenge abiogenesis (life from nonlife).6 Geochemical evidence places the presence of life on Earth at 3.86 billion years ago. The oldest rocks date to 3.9 billion years ago. Prior to this time, Earth existed largely in a molten state, unsuitable for life. Origin-of-life researchers begrudgingly acknowledge that early conditions were not conducive to the formation of prebiotic molecules and, consistent with this conclusion, the geochemical record yields no evidence for a prebiotic soup. Geochemical and fossil records highlight the chemical complexity of Earth’s first life. Taken together, these observations indicate that as soon as the planet could remotely support life, chemically complex life appears.
The discovery of single-cell organisms thriving in hostile regions suggests to some researchers that life could have arisen under the extreme conditions of Earth prior to 3.9 billion years ago. This idea provides a loophole that keeps the naturalistic explanation for life’s origin alive.7
Cedric and Winnie's ideal neighborhood wasn’t always serene and comfortable. Quite likely a time existed in the past when an extremist like Hans inhabited a much more hostile version of the same area. Likewise, some origin-of-life researchers suggest that extremophiles came first and later gave rise to mesophiles.
Proponents of such an origin-of-life scenario claim support from evolutionary analysis of DNA sequences. This work places extremophiles at the base of the hypothetical evolutionary tree of life.8 Extremophiles appear to be the oldest and most primitive organisms on Earth. Laboratory experiments simulating a hot chemically harsh environment, modeled after deep sea hydrothermal events, indicate that amino acids, peptides, and other molecules can form under harsh conditions.9 These reactions represent some of the necessary early steps in an extremophilic origin-of-life pathway.
Related to the origin-of-life question, extremophiles fuel the search for life on other solar system bodies. Researchers reason that extremophiles’ capacity to flourish in Earth’s extreme environments makes life possible in the hostile conditions found in interplanetary and interstellar space.10 For example, the discovery of microorganisms in the subzero ice of Antarctica prompts scientists to search for life on Europa, a frozen-water world orbiting Jupiter.11 The discovery of extremophilic microbial communities as deep as 2.8 kilometers below Earth’s surface also suggests to some investigators that life may be present below the hostile Martian surface.12
In spite of this circumstantial evidence apparently favoring an extremophilic beginning to life, other scientific data challenge that explanation. In light of these recent studies, the much-touted discovery of new extremophiles does not necessarily represent new and mounting evidence for a naturalistic origin of life. Significant hurdles still stand in the way—including barriers to psychrophilic (cold-loving microorganisms) and deep biosphere origin scenarios. These scenarios must wait for discussion in future articles. For now the focus rests on the challenges specific to a thermophilic origin of life.
The thermophilic (heat-loving organisms) origin-of-life scenario faces daunting difficulty on three fronts: (1) natural history, (2) biochemistry, and (3) chemistry.
The Natural History of Life’s Origin
Courthouse records can reveal much to Cedric and Winnie about their prospective neighborhood’s history. They indicate who first built and occupied the house. In the same vein, the history of the early Earth provides researchers with insight into Earth’s first occupants. Geological, geochemical, and fossil records contain clues researchers can use to determine whether Earth’s first life was extremophilic or mesophilic.
Origins-of-life investigators hope to extend the time available for life’s start-up phase by pushing the events leading to life’s beginning back farther than 3.9 billion years ago, to a time when Earth’s environment was hot. A thermophilic or hot origin of life would provide some additional time desperately needed for natural process hypotheses. Scientists from Stanford University and NASA’s Ames Center examined this possibility by estimating when (and how long), prior to 3.9 billion years ago, Earth’s surface temperature resided in the vicinity of 100 °C-the temperature thermophiles require.13
The most significant event contributing to Earth’s hot, largely molten state came from the impact of an object roughly the size of Mars, just after Earth formed.14 When it crashed into Earth, the core of the impactor fused with Earth’s core. Lighter elements from the impact spewed into Earth’s orbit and quickly coalesced to form the moon.15 Immediately after this collision, Earth’s surface temperature was hot enough to vaporize silica (sand). The surface eventually cooled to temperatures conducive for liquid water.
The Stanford University and NASA Ames researchers determined that as Earth cooled from the moon-forming impact, its surface temperature persisted within a thermophilic window for only 100,000 to 10,000,000 years—bad news for the thermophilic hypothesis—a time too short for life to emerge through natural processes. And, this timeframe likely overestimates the time available for a thermophilic origin-of-life scenario. The moon-forming impactor stands as only one of a large number of objects striking Earth between 4.5 and 3.9 billion years ago.16 Each collision returned Earth to high temperatures, melting rock on the surface and subsurface and elevating the surface temperature above the maximum temperature survivable by thermophiles.
If, somehow, thermophilic life did emerge prior to 3.9 billion years ago, it could not have persisted beyond that time. At 3.9 billion years ago, Earth experienced an event astronomers call the Late Heavy Bombardment.17 Gravitational disturbance in the solar system caused a massive number of comets and asteroids to pelt Earth and inner solar system planets. This bombardment, like previous impact events, would have vaporized any oceans and melted surface rock, sterilizing any life present.
Early Earth’s history simply doesn’t allow sufficient time for a thermophilic beginning to life. Researchers looking to extend the time for a natural process origin of life beyond 3.9 billion years meet with failure.
Geochemical and Fossil Records
Geochemical and fossil records of Earth’s earliest life, like the geological record, fail to support a thermophilic origin of life. Thread-like filaments discovered in 3.2-billion-year-old rocks formed at ancient sea-floor thermal vents represent the oldest remains of thermophilic microorganisms.18 Presumably, these microbes used sulfur as an energy source to form the organic compounds needed for life. In spite of the early occurrence of these creatures, geochemical and fossil evidence unequivocally indicate that mesophilic microorganisms predated them.
Scientists from Denmark and Australia have recovered ancient sulfide deposits from rocks dated at around 3.5 billion years.19 Chemical analysis indicates that sulfate-reducing microorganisms produced the sulfide deposits. The geological features of the rocks containing the sulfide deposits indicate that the microbes producing these deposits lived at moderate temperatures—below 60 °C. These sulfate-reducing microbes were mesophiles, not thermophiles.
Origin-of-life researchers recognize that photosynthetic bacteria existed as early as 3.5 billion years ago and possibly as far back as 3.86 billion years ago.20 Bacteria resembling photosynthetic cyanobacteria left fossil remains in 3.5 billion-year-old rocks. Origin-of-life investigators have isolated carbonaceous materials—compounds made up of carbon—that represent biological activity in rocks 3.86 billion years old. The carbonaceous compounds’ chemical profile strongly suggests that photosynthetic microbes produced these substances.21 Chemical studies and comparison of gene sequences, likewise, place the appearance of photosynthesis well before 3.5 billion years ago.22 Photosynthetic bacteria, like the early sulfate-reducing bacteria, thrived at moderate surface conditions.
The geochemical and fossil records fail to validate a thermophilic origin of life. Instead, the historical record demonstrates that mesophiles were well established long before thermophiles appeared. Nature’s historical record contradicts thermophilic origin-of-life scenarios.
Just as natural history calls into question a hot origin-of-life scenario, constraints imposed by thermophilic biochemistry make a high-temperature beginning for life unlikely. Based on a series of studies, a team of French scientists questions the traditional evolutionary tree and the placement of thermophilic microorganisms at the tree’s root.23 These scientists, working from an evolutionary point of view, compared DNA sequences from a large collection of organisms in an attempt to define the deepest branches of the purported tree of life. Applying improved methods, they concluded that mesophiles, not thermophiles, form the root for the tree of life. While still somewhat controversial within the evolutionary community, these results indicate that extremophiles’ long-held placement at the base of the evolutionary tree lacks unequivocal support. If extremophiles do not root the tree then scientists cannot consider them life’s first organisms.
In the early research stages of their research, investigators have made significant progress toward understanding the biochemical differences that allow extremophiles to thrive in unfriendly environments.24 One such biochemical difference—the increased guanosine and cytidine (GC) content of transfer RNA (tRNA) and ribosomal RNA (rRNA) molecules—allows microorganisms to grow at high temperatures (see “RNA Structure and Growth Temperature” sidebar).
Though their work has not met with universal acceptance by the origin-of-life community, another group of researchers from France exploited the correlation between GC content and optimal growth temperature to estimate the growth temperature of the last universal common ancestor (LUCA).25 These investigators, assuming that all life evolved from LUCA, compared rRNA sequences from life’s major lineages working backwards to find the GC content of LUCA. Their analysis indicates that the GC content of rRNAs for the hypothetical LUCA would have been too low to support a thermophilic lifestyle. In other words, according to the evolutionary paradigm, LUCA must have been a mesophile—not a thermophile.
Viewed from an evolutionary perspective, the biochemical evidence indicates that Earth’s first life was mesophilic, not thermophilic. This finding stands in agreement with the geological, geochemical, and fossil records.
Scientists investigating the chemical aspects of the origin-of-life question have also uncovered significant problems for a hot origin-of-life scenario. Though researchers performing laboratory experiments simulating deep-sea hydrothermal vents have successfully produced relevant life molecules, they do acknowledge that these conditions also readily destroy most organic molecules.
Renowned chemist Stanley Miller and his team conducted multiple experiments to examine the stability of biomolecules under both deep-sea hydrothermal vent conditions and ideal thermophilic temperatures.26 The team determined that an aqueous environment at 350 °C allows an amino acid half-life of only a few minutes. At 250 °C, the half-life of sugars measures in seconds. A polypeptide’s half-life measured anywhere from a few minutes to a few hours. Likewise, RNA hydrolyzed within minutes at 250 °C and within seconds at 350 °C. In other words, life’s building blocks fall apart rapidly—when they do come together—under hot conditions.
Miller’s group also determined the stability of the RNA components A, G, U, and C at the more moderate, though still thermophilic, temperature of 100 °C. The half-life of A and G measured at 1 year, U at about 12 years, and C at 19 days. Miller’s work indicates that the compounds needed to launch the chemical pathways leading to life would be destroyed shortly after forming. These compounds, therefore, could not accumulate at deep-sea hydrothermal vents or in environments possessing more moderate, but thermophilic, temperatures. And this accumulation is crucial to the thermophilic hypothesis.
Scientists from New Zealand also examined the likelihood of a thermophilic origin of life by characterizing the stability of RNA three-dimensional structures at high temperatures.27 These researchers chose to focus attention on RNA molecules because of the wide acceptance of the “RNA World” hypothesis for the origin of life. This scenario presents RNA as the first life molecules. The RNA World later gave rise to life chemistry, say origin-of-life investigators, developing DNA and proteins through a chemical evolutionary process.
The New Zealand scientists showed that RNA molecules unfold at thermophilic temperatures, losing their three-dimensional structure. Without a stable three-dimensional structure, RNA molecules lack functional capacity. The team observed that those RNA molecules capable of retaining some folded character at high temperatures lose structural specificity and rapidly transition to a variety of three-dimensional shapes. These scientists noted yet another problem: the magnesium ion (Mg2+), which stabilizes RNA three-dimensional structure promotes the chemical breakdown of RNA.28
Extremophiles challenge the traditional view of what constitutes a habitable environment. Origin-of-life researchers exploit these expanded domains of habitability to buttress naturalistic explanations for life’s start. These investigators argue that the existence of organisms thriving in seemingly hostile environments indicate that life could have emerged in the hot unfriendly conditions of early Earth.
Growing scientific evidence, however, fails to support thermophilic origin-of-life scenarios. The conditions needed for a hot start to life persisted too briefly and were repeatedly disrupted by sterilizing impact events. Geochemical and fossil evidences indicate that mesophilic surface microorganisms existed long before thermophilic bacteria. Likewise, biochemical characterization of DNA sequences challenges the long-held tradition that thermophiles form the root of the so-called tree of life. Chemical studies corroborate both the biochemical data and the geological and fossil records. Biomolecules readily decompose in a hot aqueous environment, and the complex three-dimensional structures of proteins, DNA, and RNA prove unstable near water’s boiling point.
Appealing to an extremophilic origin of life offers no help in researchers’ attempts to bypass the mounting problems associated with the textbook description of life’s genesis. As it turns out, mesophiles (like Cedric and Winnie) became Earth’s first occupants, as soon as the neighborhood was ready for them. Extremists like Hans came later.
Sidebar: Introducing Extremophiles
Scientists refer to organisms that make their living in hostile environments as extremophiles. Researchers have identified the following types:
- Thermophiles are “heat lovers,” typically growing at temperatures between 50 and 70 °C. Thermophiles thrive in or near hot springs and undersea vents.
- Hyperthermophiles are “extreme heat lovers,” growing at temperatures between 80 and 113 °C. (Water boils at 100 °C at normal atmospheric pressure.) Hyperthermophiles typically cannot be cultivated at temperatures below 80 °C.
- Psychrophiles love cold temperatures. Polaromonas vacuolata, recovered from the Antarctic Ocean, grows best at 4 °C and can’t survive at temperatures above 12 °C. (Water freezes at 0 °C at normal atmospheric pressure.)
- Acidophiles, found in volcanic pools and hot sea vents, thrive in acidic conditions at pH values less than 2. (Blood has a pH of 7.4. The pH of vinegar is approximately 3.) Remarkably, Picrophilus oshimae and Picrophilus torridus can survive at a pH of 0.
- Alkalophiles flourish in alkaline conditions. Recovered in alkaline lakes and deserts these bacteria grow at pH values greater than 10. (Household bleach has a pH of 10.)
- Halophiles are found in salt lakes and mines. These microbes inhabit environments consisting of 20 to 30% salt.
- Barophiles need high pressures to grow. Recovered at great ocean depths, some of these organisms require pressures hundreds of times greater than that on Earth’s surface (1 bar) to survive. The first barophile discovered, named MT41, grows best at 300 to 700 bar.
- Some extremophiles live dual lives, existing under doubly harsh conditions. For example, many thermophiles double as acidophiles and many alkalophiles are also halophiles. Sulfolobus acidocaldarius, a thermoacidophile, resides in the hot acidic waters of thermal springs. Natronobacterium pharaonis, a haloalkalophile, lives in alkaline lakes with high sodium carbonate (salt) levels.
Prior to 1977, scientists viewed bacteria as a homogeneous group. In 1977, Carl Woese, a microbiologist at the University of Illinois, demonstrated that bacteria actually comprise two distinct groups with fundamental biochemical differences. Woese grouped bacteria into two separate domains: Archaea and Eubacteria. Scientists assign most extremophiles to the domain Archaea. Most bacteria belong to the domain Eubacteria.
Domain refers to a relatively new level in the biological classification hierarchy. A domain consists of a grouping of kingdoms. Three kingdoms comprise Archaea: euyarchaeota, crenarchaeota, and korarchaeota. All domain members share fundamental biochemical features. Woese also suggested a third domain, Eukarya, which would include the protozoan, fungi, plant, and animal kingdoms.
Sidebar: RNA Structure and Growth Temperature
tRNA and rRNA are two specialized types of RNA (ribonucleic acid) molecules. RNA are chain-like molecules that form when cellular machinery links together four subunit molecules: adenosine (A), uridine (U), guanosine (G), and cytidine (C).
rRNA’s and tRNA’s three-dimensional structure is critical for their function. Weak chemical interactions stabilize their three-dimensional structure. The four RNA components G, C, A, and U mediate one type of stabilizing interaction, hydrogen bonding. G and C always pair, and A and U always pair to impart integrity to tRNA and rRNA structures. When G and C pair, three hydrogen bonds form; A and U pairs form two hydrogen bonds. Therefore, microorganisms growing in a high temperature environment utilize tRNAs and rRNAs with a higher GC content and a diminished AU content. This increases the number of stabilizing interactions. The tRNAs and rRNAs would unfold in thermophilic surroundings without these additional stabilizing interactions.