How did the first life emerge on Earth? At some point, abiogenesis — the transition from chemicals to living cells — occurred. Since there is no direct evidence of this first life, scientists must rely on experiments that try to recreate the conditions on the early Earth to see how easily the building blocks of life can assemble.

Our search for the origin of life is also informed by our knowledge of the range of conditions under which life can survive.  At a chemical level, life consists of many types of molecules that interact with one another to carry out the processes of life. Life also needs an environment in which those complicated molecules are stable (don’t break down before they can do their jobs) and their interactions are possible. Your own biochemistry works properly only within a very narrow range of about 10°C in body temperature and two-tenths of a unit in blood pH (pH is a numerical measure of acidity, or the amount of free hydrogen ions). Life overall must also have limits to the conditions in which it can properly work but, as we will see, they are much broader than human limits.

The Building Blocks of Life

What were the conditions on the early Earth like? After the Earth formed and the period of heavy bombardment ended, chemical reactions occurred between simple molecules, leading to more complex molecular structures. How far can natural processes go toward the formation of the first biological cell? How can the non-living suddenly become alive? The study of prebiotic chemistry has yielded some important insight into these questions.

The Miller-Urey Experiment

Once biochemists understood some of the important chemistry in living organisms, a new field of synthetic biology was born. Biochemists tried to replicate the pathway for the evolution – or the complexification – of simple chemistry. Could chemicals on the early Earth provide a pathway to the formation of the first living cells?

In 1953, Stanley Miller and Harold Urey published the results of their now famous experiment: they spontaneously produced amino acids from simple elements under conditions that they believe emulated early Earth. Given the importance of amino acids and proteins for life, this experiment was viewed as an important step toward understanding the origin of life.

The experimental setup, depicted in Figure 1 above, consisted of two large flasks. One flask contained gases thought to be present in the primitive atmosphere (methane, ammonia, and hydrogen) and the other contained water, representing the ocean. Electrodes were inserted into the atmosphere flask to produce electrical sparks that would emulate lightning.

The ocean flask was heated, producing water vapor that traveled throughout the setup and interacted with gases in the atmosphere and simulated lightning. The enriched vapor then traveled through a condenser and cooled to a liquid state in a u-bend at the bottom of the setup where samples could be collected. Within a day of running the experiment, the liquid at the bottom of this u-bend had turned pink. The inaugural run of the experiment lasted for a week. From these results, Miller was able to identify five different amino acids that had been created from this simple set up of atmospheric gases, lightning, and water alone. As expected, the chirality of these amino acids included both left- and right-handed molecules, even though only left-handed amino acids are incorporated into biology as we know it today. Recent reanalysis of sealed vials have detected more than 20 different amino acids that were actually formed during that one-week experiment.

Since 1953, there have been many additional experiments that aim to synthesize the building blocks of life. Longer chains of amino acids and polypeptides have been synthesized. An impressive experiment was carried out by John Sutherland’s lab in 2009. Sutherland began with a compound called 2-amino-oxazole, and using phosphorus as a catalyst, he was able to synthesize some of the nucleotides used in RNA and DNA.

Want to know more: The Murchison Meteorite – amino acids in space

On September 28, 1969, two months after Apollo 11 landed on the moon, a huge meteorite soared through the sky and landed near Murchison, Victoria, in Australia. The Murchison Meteorite is one of the most valuable and well-studied meteorites. Importantly, the incoming meteorite was highly visible. It flew through the atmosphere as a fireball and caused a noticeable tremor when it landed. This allowed for rapid recovery of the meteorite, preventing terrestrial contamination. A piece of the Murchison meteorite is shown below.

In total, more than 100 kg (the average weight of an NFL linebacker) of the Murchison Meteorite was recovered over an area of 13 square kilometers. This meteorite contained more than 70 amino acids, many of which are not known to exist naturally on Earth. There is a slight over-abundance of left-handed amino acids, which suggests that there may be a cosmic origin to the dominance of left-handed amino acids that are used by life on Earth. Every amino acid discovered on the Murchison Meteorite has since been successively synthesized using a Miller-Urey-like setup with gases, water, and simulated lightning, demonstrating that these amino acids can spontaneously form under fairly simple conditions. The Murchison Meteorite is proof that amino acids can easily form in space. Amino acids: so easy to synthesize that a rock can do it!

Even more complex organic molecules have been detected in interstellar space, a testament to how easily complex, carbon-based structures can form and remain stable. Some of these molecules have carbon ring-structures called polycyclic aromatic hydrocarbons and are similar to the organic molecules found in biological organisms. This confirms that carbon is a good base for forming complex molecules structures, including amino acids. While this gives a hint that carbon-based life may also exist on other planets, there are still many steps we don’t yet understand about how to go from prebiotic chemistry to life.

Abiogenesis

Keep in mind that just because we discover processes that can create the chemicals we have identified as vital to living things does not answer the question of how life emerges. The transition from such chemicals to lifeforms and biotic systems is a process generally called abiogenesis. Many mysteries remain in the study of this question.

Once amino acids are formed, they can link together through peptide bonds to form chains (see the section on “amino acids to proteins” for a review). These polypeptide chains can fold into proteins, which perform functions essential for life. Thus, we see a natural increase in the chemical complexity of simple molecules, leading from monomers to polymers. Just as amino acids polymerize to form proteins, another crucial step in the origin of life involves the polymerization of nucleotides to form RNA (see “nucleic acids to genes“). Let’s explore how the RNA world hypothesis provides insights into the early development of self-replicating systems and the foundation of genetic information.

RNA World

Proteins are needed to catalyze chemical reactions critical to the survival of cells. But, it is difficult to imagine how proteins could have been the precursor to living cells because DNA is required to manufacture proteins, presenting a “chicken-and-egg” problem. A breakthrough came when Sidney Altman and Thomas Cech discovered a class of RNA molecules called ribozymes that could catalyze their own replication. Ribozymes show that RNA, which can encode genetic information, can also act as an enzyme. This discovery was awarded the Nobel prize in Chemistry in 1989 and supported a hypothesis called the Early RNA world, where ancient life used RNA for storing genetic information and catalyzing chemical reactions. This hypothesis had been suggested in the 1960’s by Carl Woese, Frances Crick and Leslie Orgel.

According to this hypothesis, the instability of RNA promoted mutations and natural selection eventually evolved a more stable, double-stranded DNA molecule as ribozymes were phased out. A fascinating “smoking gun” for this hypothesis is the fact that the ribosome, which assembles proteins in cells today, is a ribozyme! While the current day ribosome incorporates some proteins, none of the proteins are anywhere near the active site where chemical reactions take place. They appear to exist largely for structural support for the ribosome.

While the capabilities of RNA seem to make it the perfect candidate to explain the origin of life, RNA is a far more complicated structure and not as easy to make as amino acids. Miller-Urey experiments have been capable of synthesizing a series of smaller, very reactive molecules. When enough of these molecules are made, detectable amounts of purine and pyrimidine bases, which are essential components of nucleotides needed for RNA, can be detected. Components of nucleotides have also been discovered on meteorites like the Murchison Meteorite. However, no complete extraterrestrial nucleotides or nucleic acid chains have been discovered yet.

A breakthrough came in 2009 from British chemist John Sutherland along with Matthew Powner and Beatrice Gerland (read more here about their experiment and results). Sutherland’s group determined a chemically efficient pathway for nucleotides to form that is plausible in a prebiotic environment. Rather than form each component of the nucleotide individually, which would require separate and unlikely chemical environments, they proposed a method that formed and attached a purine and a ribose sugar in the same reaction. Phosphate is used to help catalyze the reaction and incorporated into the nucleotide later. This work showed that the building blocks of RNA could form naturally on the early Earth.

Knowing that RNA nucleotides can form under prebiotic conditions, how can these nucleotides assemble into self-replicating RNA molecules? In 2009, Drs. Tracey Lincoln and Gerald Joyce created an RNA enzyme that was capable of self-sustained replication indefinitely (read their paper in Science).

Understanding how nucleotides can assemble into self-replicating RNA molecules explains part of the puzzle of how biological life began. But how did these complex molecules organize into structures resembling the first living cells? Next, we examine the spontaneous formation of protocells, or vesicles, from fatty acids, providing a protective environment where these RNA molecules could further evolve and function.

Encapsulation and Protocells

Now that we have complex RNA molecules that can makes copies of themselves, what’s the next step for forming life? Encapsulation, the action of capturing and surrounding something within a container or membrane (which is a wall made out of chemicals) is suggested by astrobiologists as being an important process in forming primitive life. Cells today are bound and regulated by membranes, which are composed of of phospholipids. It is not clear how the first phospholipids formed, but it is possible that the earliest membranes in protocells were first composed of a less complicated lipid connected to hydrocarbon chains.

Protocells can be thought of as simple cell-like structures that form spontaneously from molecules such as fatty acids that were present in the prebiotic environment. For a protocell to form, it requires that it can encapsulate all the molecules needed for that cell to function. However, if you’ve ever heard the old saying “oil and water” don’t mix, you’ll know that certain molecules don’t generally dissolve in water, making them hard to encapsulate within a protocell. So, how do we get different types of molecules to all cooperate together in one protocell?

A type of molecule called an amphiphile may be the solution to our “non-mixing molecules” problem. Amphiphiles are molecules that have a water “loving” head (i.e. part of the molecule that can dissolve in water) and an oil “loving” tail (i.e. part of the molecule can dissolve in oil). This allows it to interact with both oils and waters, helping them co-exist to form protocells. Detergents such as dish soaps are amphiphilic. The hydrophilic head of dish soap will bond with water molecules and the hydrophobic tail will bond with the oils (for example, grease on a pan), therefore allowing the grease to mix with the water and be removed from the pan.

The first protocells developed from even simpler compartments which are composed of a type of molecule called lipids. An early form of protocells, called vesicles, can form naturally and can randomly encapsulate different molecules at different concentrations. Vesicles can be made in a lab!

Vesicles can form in a natural environment due to what is known as wet/dry cycling (Figure 3). When an environment is dry, the lipids form sheets (Step 1), however when they get wet again (like after it rains) the sheets can slowly bud off (Steps 2-4) and eventually form vesicles (Step 5) and encapsulate whatever material was between the lipid sheets. This material can include molecules important to the origin of the first life!

Clays like montmorillonite can also catalyze the formation of longer RNA chains by providing a surface upon which lipid molecules can become concentrated, react and polymerize. Chains of up to 50-60 nucleotides have been formed experimentally in this way.

Extreme Conditions

Extremophiles are organisms that live in habitats that seem extreme to humans (the suffix -phile means “lover of”). The ability of organisms on Earth to live in a wide range of environments is the secret to the survival of life on this planet and extremophiles help us to understand the limits to life. On the early Earth, the large swings in climate would have cycled between hot and freezing conditions. Before microbes invented photosynthesis, life adapted to take advantage of many different metabolic pathways. There is evidence in the geologic record that oxygen levels in the atmosphere fluctuated wildly before a precipitous rise 2.4 billion years ago. Through every mass extinction, there were some niches of life that survived. Even our flammable, oxygen-rich atmosphere constitutes an extreme environment.

Being “extreme” is in the eye of the beholder. Most of the organisms that humans identify as extremophiles are small prokaryotes in the archaea or bacteria domain on the Tree of Life. They are known to survive in a wide range of extraordinary environments.

Temperature

Both high and low temperatures can cause a problem for life. As a large organism, you are able to maintain an almost constant body temperature whether it is colder or warmer in the environment around you. But this is not possible at the tiny size of microorganisms; whatever the temperature in the outside world is also the temperature of the microbe, and its biochemistry must be able to function at that temperature. High temperatures are the enemy of complexity—increasing thermal energy tends to break apart big molecules into smaller and smaller bits, and life needs to stabilize the molecules with stronger bonds and special proteins. But this approach has its limits.

Thermophiles

Thermophiles (temperature-loving organisms) live in high temperature environments, some at temperatures of 235°F (113°C). High-temperature environments like hydrothermal vents on the deep seafloor (Figure 4) and hot springs on land surfaces (Figure 5) often offer abundant sources of chemical energy and therefore drive the evolution of organisms that can tolerate high temperatures. Bacteria feeding on this chemical energy form the base of a food chain that can support thriving communities of animals—in the case shown in Figure 1, a dense patch of red and white tubeworms growing around the base of the vent. What appears to be black smoke is actually superheated water filled with minerals of metal sulfide.

High temperatures are challenging for life because the energy causes proteins to denature, or unfold. Chemical reactions proceed more quickly, and temperatures above 100°C can denature nucleic acids, causing DNA to lose its helical shape or break apart the structure of biomolecules. The fluidity and permeability of cell membranes are affected by heat. High temperatures decrease the solubility of carbon dioxide and oxygen in water, a problem for aquatic aerobic life. Thermophiles have metabolisms that take advantage of the higher chemical reactivity enabled by high temperatures. They make use of special temperature-resistant proteins and protective mechanisms for shielding DNA. Thermophile coping mechanisms include an altered ratio of saturated hydrocarbons in cell membranes.

The young Earth, with a tremendous amount of internal energy from accretion and differentiation, may have harbored many hot springs and ocean floor hydrothermal vents. Young Earth would have been a paradise for thermophiles, and the archea and bacteria that are most deeply rooted in the phylogenetic tree of life are thermophiles. The range of metabolisms for thermophiles may be the result of the diversity of chemistry at deep sea hydrothermal vents.

Currently, the high temperature record holder is a methane-producing microorganism that can grow at 122°C, where the pressure also is so high that water still does not boil. That’s amazing when you think about it. We cook our food—meaning, we alter the chemistry and structure of its biomolecules—by boiling it at a temperature of 100°C. In fact, food begins to cook at much lower temperatures than this. And yet, there are organisms whose biochemistry remains intact and operates just fine at temperatures 20 degrees higher.

Psychrophiles

Cold can also be a problem, in part because it slows down metabolism to very low levels, but also because it can cause physical changes in biomolecules. Psychrophiles are organisms that can withstand extreme cold. They are found in liquid brine inclusions in ice cores, or living under rocks in the extreme deserts of the world. This class of extremophiles includes bacteria and eukaryotes.

Cells must be resilient to freezing, which can form ice shards that would pierce and destroy ordinary cells. Cell membranes—the molecular envelopes that surround cells and allow their exchange of chemicals with the world outside—are basically made of fatlike molecules. And just as fat congeals when it cools, membranes crystallize, changing how they function in the exchange of materials in and out of the cell. One defense of a psychrophile is to lower the freezing point with solutes in cytoplasm that essentially act as an antifreeze. To increase fluidity of cell membranes, psychrophiles have evolved a different ratio of unsaturated to saturated fats. At cold temperatures, proteins are more rigid, so enzymes are used to lower the activation energy for biochemical reactions. Thus far, the coldest temperature at which any microbe has been shown to reproduce is about –25 ºC in Arctic permafrost.

pH

Acidophiles and alkaliphiles thrive in environments with very low or high pH, respectively. Conditions that are very acidic or alkaline can be problematic for life because many of our important molecules, like proteins and DNA, are broken down under such conditions. The most acid-tolerant organisms (acidophiles) are capable of living at pH values near zero—about ten million times more acidic than your blood. Acidophiles have been found in conditions as acidic as battery acid. They have been able to adapt mechanisms for keeping the acid out so that the cell cytoplasm can have a neutral pH. Figure 6 shows Rio Tinto in Spain where acidophiles thrive; the rusty red color that gives the river its name comes from high levels of iron dissolved in the waters

At the other extreme, some alkaliphiles can grow at pH levels of about 13, which is almost a million times more alkaline than your blood. This is also comparable to the pH of household drain cleaner, which does its job by breaking down the chemical structure of things like hair clogs.

Salt

High Radiation

Life has even been found next to nuclear waste storage sites in the presence of enough radiation to grant super powers. Radiation can be very damaging to DNA and cause cancerous cells or tumors. These radioresistant organisms have evolved mechanisms to protect their DNA even in the presence of over 1000 times the radiation a typical organism can withstand. There is even an organism, Deinococcus radiodurans, that can tolerate ionizing radiation (such as that released by radioactive elements) a thousand times more intense than you would be able to withstand. It is also very good at surviving extreme desiccation (drying out) and a variety of metals that would be toxic to humans.

Life is everywhere on Earth

Many other adaptions to environmental “extremes” are also known. Endoliths are extremophiles that live inside of rocks. On Earth, endoliths have been found living inside rocks in deserts in Antarctica, Chile, and Namibia, as well as regions deep below the Earth’s surface, including inside a gold mine in South Africa that is more than 2 miles underground. Barophiles thrive in high-pressure environments. Very high pressures can literally squeeze life’s biomolecules, causing them to adopt more compact forms that do not work very well. But we still find life—not just microbial, but even animal life—at the bottoms of our ocean trenches, where pressures are more than 1000 times atmospheric pressure. There also still exist anaerobes that do not require oxygen and live deep underground. Xerophiles require very little water and are found in the soil of the world’s largest deserts.

From many such examples, we can conclude that life is capable of tolerating a wide range of environmental extremes—so much so that we have to work hard to identify places where life can’t exist. A few such places are known—for example, the waters of hydrothermal vents at over 300°C appear too hot to support any life—and finding these places helps define the possibility for life elsewhere. The study of extremophiles over the last few decades has expanded our sense of the range of conditions life can survive and, in doing so, has made many scientists more optimistic about the possibility that life might exist beyond Earth. Extremophiles are a great boon to astrobiology both in demonstrating the wide variety of life that is possible, and by expanding the definition of what is “habitable.”

Earth Analogs in the Solar System

There is no other place in our solar system that today has the same hospitable conditions as Earth, with liquid water oceans on its surface today. However, we can learn from the extreme conditions on Earth that host life and identify similar environments on other planets or moons. Regions similar to frozen lakes and dusty deserts where we find extremophiles on Earth can be pinpointed elsewhere in the solar system, where a search for similar extremophiles in these regions holds promise.

Mars

Today, Mars is a barren world with a thin atmosphere dominated by CO₂. Photos of the surface Mars taken by the rovers show a landscape that looks similar to some deserts on Earth (Figure 8). Can you tell which picture is of Earth and which is of Mars (before you read the caption)?

The Atacama desert is the driest place on Earth so represents a reasonable analog for some places on Mars, such as the Jezero Crater. Life has been found within the extreme location of the Atacama desert, including halophiles, endoliths and radioresistant bacteria.

Europa and Enceladus

Jupiter’s moon Europa and Saturn’s moon Enceladus are both believed to harbor liquid water oceans beneath their icy surfaces. Several places on Earth serve as analogs for these cold, dark, high-pressure environments. Lakes buried beneath Earth’s surface, such as Lake Vostok in Antarctica, have similar conditions to what we would expect for the oceans of Europa and Enceladus; searches for life within the overlaying ice or in the water of Lake Vostok and other subglacial lakes provide some clues as to what we could reasonably expect to find there. A rich variety of Hydrothermal vent communities, such as the “Lost City” at the Mid-Atlantic Ridge on the floor of the Atlantic Ocean, including alkaliphiles and microbes that use chemosynthesis to generate energy.

Tardigrades

Tardigrades, also known as water bears or moss piglets, are not technically extremophiles but are nevertheless extreme in their own right as well as strangely adorable. They are a type of micro-animal, only 0.5 mm in length on average, with eight-legs, typically found in water, and among the most resilient creatures known. They are different from extremophiles, which are adapted to live in terrifyingly harsh conditions, because they do best in average conditions. However, they are capable of surviving the most extreme conditions on this world, including low pressure environments in space.

The video below nicely describes how tardigrades can exist in a state known as anhydrobiosis (can you guess what this word means just based on the roots hydro and biosis?).

In 2007, samples of tardigrades were taken into low Earth orbit and exposed to the vacuum and radiation of space for 10 days. After the reanimation of the tardigrades back on Earth, most of the tardigrades began living as normal after 30 minutes (though most of the sample did suffer later health effects). Their incredible resilience allowed water bears to survive through five great extinction events on Earth. Their capability to survive in space gives new life to the theory of panspermia. Though not technically extremophiles, tardigrades certainly win a prize for resilience.

In April 2019, an Israeli spacecraft called Beresheet almost made it to the moon. The privately funded mission was the first stage of a privately funded project to transfer DNA to the moon to build a repository for rebuilding life in the event of catastrophic mass extinction. Among the passengers on the spacecraft: dehydrated tardigrades. It is unlikely that these creatures could survive on the moon without liquid water.

Review Questions

Summary Questions

1. What was the Miller-Urey experiment? How was it set up?
2. What did the results of the Miller-Urey experiment imply about the formation of the building blocks of life on the early Earth?
3. What does abiogenesis mean? Is there direct evidence for abiogenesis on Earth? Explain your answer.
4. What are ribozymes?
5. How does the RNA World hypothesis explain how ribozymes could be a pathway to the first life on Earth?
6. How has experimental work on synthetic self-replicating systems informed our ideas on the first life on Earth?
7. How can protocells naturally form on the early Earth, without the need for proteins?
8. How can clay minerals, like montmorillonite, facilitate the process of forming protocells?
9. Which environments on Earth represent “extreme” conditions for life to survive in?
10. What types of organisms can thrice in these extreme environments?
11. For each of the following extremophiles, explain what extreme condition they can survive in and where they are found on Earth: thermophiles, psychrophiles, acidophiles, alkaliphiles, halophiles, radioresistant, endoliths, xerophiles and barophiles.
12. What are some other places in our solar system with extreme conditions that could support extremophiles?
13. What are tardigrades? Which extreme conditions can they survive in?

Exercises

1. Check out the Exploring Origins Project, created in collaboration with Jack Szostak’s lab in 2006.