Life's origins

Life originated about 3.8 billion years ago, arising most likely from organic molecules that formed in the environment of the early Earth. Many theories exist to explain the origin of the first organic molecules and the development of the first proto-cells from organic components. Researchers have also attempted to discover the most likely location of life's origin, suggesting potential locations that include deep-sea vents, frozen prehistoric oceans, and extraterrestrial planets.

Definition of Life

Science has no concise definition of what constitutes a living organism, relying instead on a list of characteristics that, when taken together, differentiate life from nonlife. Life is, therefore, understood as a biological system capable of growth, metabolism, reproduction, and a variety of other behaviors. The cell is the smallest unit of life, and all life-forms comprise one or more cells. The origin of life on Earth is that moment when the first cell came into being. The study of life's origins has, therefore, become the search for a theory of abiogenesis, or the generation of life from nonliving components.

Chronology of Life

Physicists and geologists estimate that Earth formed approximately 4.6 billion years ago from the dense cloud of gas that also gave rise to the sun and the other planets of the solar system. For hundreds of millions of years, Earth was an unstable, molten mass with no atmosphere; meteors and comets frequently impacted its surface. About 3.8 billion years ago, the Earth stabilized enough for clouds of gas to interact and form water vapor. Water began to accumulate into the first oceans, which helped to further stabilize the climate and form the first atmosphere. Scientists believe that Earth was then primed for the formation of the first organic molecules.

In 2011, physicist David Wacey and colleagues reported the discovery of microfossils from bacteria in samples of Australian sediment that were dated to 3.4 billion years ago or more. These microfossils are the metabolic remains of microorganisms that lived by metabolizing sulfur; some modern bacteria similarly thrive in extreme deep-sea environments. This discovery indicated that life may have taken four hundred million years or less to develop from inorganic molecules into living cells.

However, in 2015, scientists from the University of California published a controversial study suggesting that life may have formed on Earth three hundred million years earlier than previously thought. After examining zircons found in the hills of western Australia, researchers found small traces of graphite, a carbon mineral. In their study, published in the Proceedings of the National Academy of Sciences, they argue that the graphite contained in the 4.1-billion-year-old crystal had an isotopic pattern that resembled modern organic matter. While critics argue that a nonbiological explanation could exist for this finding, proponents of the idea that early Earth was more hospitable than originally believed remain hopeful that the study of older rocks on Mars could help support the theory.

Multidisciplinary research in the early 2020s provided many breakthroughs for scientists investigating the origin of life and the timeline of early life's development. Studies of zircon minerals, Earth's oldest-known material, newly discovered fossils, and samples retrieved from space missions like Japan's Hayabusa2 continue providing new, important findings. Scientists now assert that modern plate tectonics likely formed 3.6 billion years ago, beginning the foundation for the emergence of life.

Early Theories on the Origin of Life

The theory of spontaneous generation, in which life-forms allegedly emerge from inanimate or dead matter, dates back at least to the time of Aristotle, if not before. In the fourth century BCE, Aristotle claimed in his History of Animals that while some animals reproduce sexually, others, particularly insects, grow spontaneously from rotting food or vegetable matter. Like many of Aristotle's erroneous theories of biology, this idea went unchallenged for centuries, in large part because it was deemed to be consistent with biblical teachings.

The seventeenth century, however, saw the emergence of the Scientific Revolution across Europe. This period, a precursor to the European Enlightenment, was marked by the embrace of empiricism and the development of the experimental or scientific method, rather than the previously accepted Aristotelian approach of simple observation and reasoning. Accordingly, in 1668, Italian physician and poet Francesco Redi conducted the first recorded experiment intended to refute the theory of spontaneous generation. He observed that when he left the meat in open jars, maggots appeared on the meat, but when he covered the jars with gauze, maggots appeared on the outside of the gauze but not within the jar. Experiments by subsequent scientists expanded on Redi's observations and tentative conclusions. Among the most notable was eighteenth-century Italian priest and biologist Lazzaro Spallanzani, who demonstrated that boiling broth and then sealing it prevented the growth of microorganisms, suggesting that these microscopic life-forms did not emerge from the broth spontaneously but rather were carried in the air; however, Spallanzani's results were disputed because his sealed containers did not permit the entrance of air, which critics argued was necessary for spontaneous generation.

Finally, in 1859, French chemist Louis Pasteur conducted what is now regarded as the definitive experiment disproving spontaneous generation. Using swan-neck flasks that permitted the entrance of air but trapped dust particles in the neck before they could reach the boiled liquid in the flasks, Pasteur demonstrated that no microorganisms grew in the liquid until it was contaminated with the dust particles. He concluded that a sterile sample of organic material will not generate life unless it has been contaminated by living cells from the environment. Insects appear to grow within rotten meat or spoiled fruit because the sample has been contaminated with insect eggs or larvae. With spontaneous generation having been disproved, scientists then began searching for a new hypothesis of abiogenesis that could be understood given the known scientific laws and an understanding of Charles Darwin's evolutionary theory.

To understand the process of abiogenesis, scientists first must determine how nonliving matter combined for the first time to produce the molecules necessary for life, namely carbohydrates, proteins, enzymes, nucleic acids, and lipids. From there, they must determine how these molecules formed into the first cells.

Origin of Organic Molecules

The environment of early Earth was very different from its environment in the twenty-first century. The atmosphere had a different gaseous composition and the climate was affected by regular disturbances, including volcanic activity and electrical storms. The early oceans were relatively shallow and because there were no living cells, they were filled with high concentrations of dissolved molecules. This early marine environment became known as the primordial soup. In 1929, British-born evolutionary biologist and geneticist John B. S. Haldane proposed that ultraviolet (UV) radiation from the sun provided the energy to drive chemical reactions in the seas, thereby giving rise to the first organic molecules. Haldane's critics argued that UV radiation does not provide sufficient continuous energy to account for the necessary chemical reactions.

In the 1950s, University of Chicago biologists Harold Urey and Stanley Miller conducted an experiment in which they attempted to replicate the chemical composition of the primordial soup, then subjected samples of their soup to electrical energy, reasoning that electrical storms might have energized the early ocean. The Miller-Urey experiment, as it came to be called, succeeded in producing some types of organic molecules, though not a sufficient variety of molecules to assume that cellular formation would have followed.

Carnegie Mellon University chemist Jim Cleaves revisited the Miller-Urey experiment in 2011 using different gas concentrations from the original experiment, based on contemporary refinements regarding the atmosphere of early Earth. Cleaves believed that the products of volcanic eruptions, including hydrogen, methane, and hydrogen sulfide, were critical components in the formation of organic molecules. The experiment produced twenty-three types of organic molecules, many of which are essential for the formation of living cells, giving credence to the theory that the early environment facilitated the onset of abiogenesis.

In 2024, scientists in Australia and Canada found 1.75-billion-year-old (the oldest-known) oxygenic photosynthetic structures, which pushed the date of the first evidence of photosynthesis back 1.2 billion years. This finding supports the theory that organic materials and the earliest building blocks of life originated much earlier than previously thought.

Formation of Pre-cellular Life

Most biologists believe that it would have been impossible for living cells to form directly from inorganic materials and that there must have been one or more stages at which life existed in a pre-cellular form. Biologists postulate that proto-cells had formed from the organic molecules produced in the atmosphere of early Earth. These proto-cells, they believe, then underwent further waves of evolutionary refinement to give rise to modern cells.

For a cell to form, molecules must be present and capable of facilitating chemical reactions, thereby constituting a basic metabolism that fuels maintenance, growth, and replication. Protein molecules serve as the engines of metabolism in cells, facilitating and directing chemical reactions. Second, a cell requires a method of replicating and storing information to be passed to the cell's offspring, a requirement fulfilled by the cell's nucleotide sequences in its DNA and RNA. Cells also require a membrane, which establishes the cell's boundaries with respect to the environment and governs the passage of material to and from the cell. Researchers who study abiogenesis are divided into those who believe that metabolism came first, those who believe that reproductive structures arose first, and those who believe that cellular “containers” were the first to evolve.

Scientists who prefer a metabolism-first approach argue that proteins formed first, creating a disembodied system of metabolic reactions that were later associated with reproductive molecules and cellular containers. Proteins have a variety of functions, including functions as enzymes, which are molecules that catalyze or increase the speed or likeliness of certain chemical reactions.

The containers-first approach focuses on certain fatty acids forming naturally into spherical globules that scientists call vesicles. These vesicles can form around proteins and other molecules that occur in solution, thereby providing a contained environment for chemical reactions. When associated with certain proteins, vesicles will develop structural abnormalities that make it more likely for the vesicle to split apart, giving rise to two vesicles, each containing some portion of the associated proteins and other molecules.

Other researchers have argued for a gene-first theory, in which rudimentary DNA or RNA molecules were the first to evolve, creating self-replicating nucleotide structures that were later associated with fatty acid vesicles and proteins to facilitate the movement of energy.

RNA molecules have dual functions in cells: they can transmit genetic information, like DNA molecules, and they can also catalyze chemical reactions, like proteins. In a 1986 article, American molecular biologist and Nobel laureate Walter Gilbert was the first to use the term “RNA world” in theorizing that the early Earth may have been populated by RNA-based organisms in which RNA fueled both the metabolism and replication functions of early cells. While many biologists support the RNA world hypothesis, researchers have been unable to replicate the spontaneous synthesis of RNA molecules in laboratory conditions. Biologists and chemists are, therefore, uncertain whether it would have been possible for RNA to form in the early environment. In addition, no theory can explain how the first RNA molecules functioned in forming the earliest proto-cells.

Environmental Theories of Abiogenesis

Some biochemists have suggested that the key to understanding abiogenesis lies in determining the exact microenvironment in which the first cells formed. Since research in abiogenesis began, microbiologists have discovered that living cells populate nearly every environment on Earth. Cells have evolved to survive in highly acidic and basic environments and to endure extremes of temperature and pressure that would be fatal for most cells.

One of the most promising environmental theories of abiogenesis involves deep-sea formations known as hydrothermal vents, where gases and heated water from Earth's core spout into the oceans to create superheated and chemically rich undersea environments. University College London researcher Nick Lane and colleagues proposed in a 2010 article that life began in a unique type of hydrothermal vent, one with a highly specific complement of basic minerals, gases, and other molecules and with abundant, naturally occurring crevices and chambers on its surface. The combination of geochemical forces surrounding the vents provides a proton gradient, in which the electrical properties of the outer area are different from the electrical properties within the various channels on the surface of the vent. This difference in electrical potential provides the energy needed for chemical reactions and powers the production of energy-storing molecules that can be used to fuel additional reactions.

Lane and other scientists who support the hydrothermal vent theory propose that lipid vesicles formed inside the inorganic chambers around the vent and acted as containers for other organic molecules produced in this chemically unstable environment. Eventually, the early proto-cells housed in the chambers of the vent evolved an independent proton gradient similar to the one maintained at the vent's surface. Modern cells maintain a similar proton gradient with respect to the environment by utilizing a system of chemical reactions that transfer electrons between donor and recipient molecules.

In the late 1980s, German chemist Günter Wächtershäuser proposed another theory of abiogenesis associated with the hydrothermal vent environment. Wächtershäuser proposed that the earliest cells formed with naturally occurring structures composed of iron sulfide and associated metals around the surface of volcanically active vents in the deep ocean. Wächtershäuser theorized that these composite structures may have acted to filter chemical reactions in the superheated environment of the vents. The internal metals reacted with chemicals entering the system, forming a basic metabolic system that further facilitated the rate of chemical reactions. In time, organic molecules associated with the iron sulfide structures and eventually played a role in the chemical reactions inside the structures. Over millions of years, proto-cells formed within these structures, which gradually evolved the capacity to maintain their metabolic activity without the iron sulfide structure.

While hydrothermal vent hypotheses are considered to be among the most promising theories regarding life's origins, other researchers have proposed alternative locations for life's initial genesis. Biochemists Stanley Miller and Jeffrey Bada, from the Scripps Institution of Oceanography and the University of California, proposed a theory in the mid-1990s that life may have begun during a time when the world's oceans were frozen, a condition that paleontologists believe may have occurred when the Earth first formed because the sun was less luminous.

The formation of organic molecules may, therefore, have occurred in an ocean shielded by a layer of ice that may have been hundreds of feet thick in some areas. Organic molecules that formed under this sheet of ice may have lasted longer, because of the effect of cold temperatures in slowing decay and the protection that the ice layer provided against ultraviolet radiation and other disturbances. This “frozen soup” may, therefore, have allowed more time for molecules to interact and, potentially, to form into proto-cells.

Both the hydrothermal vent and the frozen ocean hypotheses have interesting implications for the search for life on other planets. Among the planets known to science, many have extreme environments characterized by intense heat or cold. If life first arose in one of Earth's extreme environments, it might mean that simple forms of life could exist on planets where conditions make it impossible for more complex forms of life to evolve.

Extraterrestrial Origins

The theory of panspermia, which was first developed in the mid-nineteenth century, holds that life was carried to Earth, and that this life may have been carried to numerous other planets on meteors, comets, and asteroids originating from elsewhere in the galaxy. Panspermia argues that when these natural projectiles impacted a planet, bacteria were deposited and began evolving to change the environment. The reasoning behind the theory is that it seems unlikely that life would have been able to evolve from nonliving systems in the four hundred million years that passed between the formation of the Earth and the appearance of the first cells.

One version of the panspermia theory holds that life developed on a distant planet where the conditions were different from Earth in such a way that biological life was more likely to develop. This “origin” planet may later have been obliterated, seeding the galaxy with meteors that carried dormant bacterium or proto-cells. Another version of the theory, more extreme and generally less respected, holds that life was deposited on Earth by a more advanced extraterrestrial race with an interest in seeding the galaxy. Research has demonstrated that bacteria can survive in a dormant state for millions of years and that some types of bacteria can survive the type of radiation, heat, and cold that might kill most cellular life on Earth.

Ultimately, the panspermia hypothesis does little to solve the problem of how life originated from nonlife. If panspermia were proven, the question of how, why, and where the first life appeared would simply shift to an extraterrestrial stage. Scientists theorize that humans may eventually use bacteria to initiate the process needed to colonize other planets. If this comes to fruition, there may one day be planets for which panspermia was the avenue for the generation of life.

An analysis of samples retrieved by Japan's Hayabusa2 mission to asteroid Ryugu found more than twenty types of amino acids, indicating an increased potential for life on early Earth. Similarly, freshwater sources, the first lifeforms, and a livable atmosphere may have been established on Earth much earlier than previously hypothesized, only 600 million years after Earth's formation. Another study of the fossil record using genomes of living organisms indicated an ecosystem likely developed rapidly on Earth when it was still in its infancy, making Earth-like biospheres possible in other parts of the universe.

Principal Terms

abiogenesis: a process involving the generation of living systems from nonliving components

enzyme: a substance that increases the rate of a certain biochemical reaction

hydrothermal vent: a structure on the ocean floor through which superheated water and gases spout from Earth's core

inorganic compound: a chemical compound that lacks carbon

microfossil: a fossil or fossil fragment that cannot be seen by the unaided human eye

nucleic acid: a molecule, such as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), used to store and transmit information about the formation of proteins; essential to reproduction in living systems

organic compound: a compound containing carbon that constitutes the basic building blocks of any living system

panspermia: a theory that life can be carried between planets in the form of bacteria integrated into the mineral components of meteorites

primordial soup: a combination of molecules and gases that were present in the seas at the origin of life and may have contributed to the formation of the first organic compounds

proto-cell: a theoretical structure representing a midpoint between nonliving chemical systems and the earliest true cells

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