abiogenesis

abiogenesis, the idea that life arose from nonlife more than 3.5 billion years ago on Earth. Abiogenesis proposes that the first life-forms generated were very simple and through a gradual process became increasingly complex. Biogenesis, in which life is derived from the reproduction of other life, was presumably preceded by abiogenesis, which became impossible once Earth’s atmosphere assumed its present composition.

Although many equate abiogenesis with the archaic theory of spontaneous generation, the two ideas are quite different. According to the latter, complex life (e.g., a maggot or mouse) was thought to arise spontaneously and continually from nonliving matter. While the hypothetical process of spontaneous generation was disproved as early as the 17th century and decisively rejected in the 19th century, abiogenesis has been neither proved nor disproved.

The Oparin-Haldane theory

In the 1920s British scientist J.B.S. Haldane and Russian biochemist Aleksandr Oparin independently set forth similar ideas concerning the conditions required for the origin of life on Earth. Both believed that organic molecules could be formed from abiogenic materials in the presence of an external energy source (e.g., ultraviolet radiation) and that the primitive atmosphere was reducing (having very low amounts of free oxygen) and contained ammonia and water vapour, among other gases. Both also suspected that the first life-forms appeared in the warm, primitive ocean and were heterotrophic (obtaining preformed nutrients from the compounds in existence on early Earth) rather than autotrophic (generating food and nutrients from sunlight or inorganic materials).

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Oparin believed that life developed from coacervates, microscopic spontaneously formed spherical aggregates of lipid molecules that are held together by electrostatic forces and that may have been precursors of cells. Oparin’s work with coacervates confirmed that enzymes fundamental for the biochemical reactions of metabolism functioned more efficiently when contained within membrane-bound spheres than when free in aqueous solutions. Haldane, unfamiliar with Oparin’s coacervates, believed that simple organic molecules formed first and in the presence of ultraviolet light became increasingly complex, ultimately forming cells. Haldane and Oparin’s ideas formed the foundation for much of the research on abiogenesis that took place in later decades.

The Miller-Urey experiment

In 1953 American chemists Harold C. Urey and Stanley Miller tested the Oparin-Haldane theory and successfully produced organic molecules from some of the inorganic components thought to have been present on prebiotic Earth. In what became known as the Miller-Urey experiment, the two scientists combined warm water with a mixture of four gases—water vapour, methane, ammonia, and molecular hydrogen—and pulsed the “atmosphere” with electrical discharges. The different components were meant to simulate the primitive ocean, the prebiotic atmosphere, and heat (in the form of lightning), respectively. One week later Miller and Urey found that simple organic molecules, including amino acids (the building blocks of proteins), had formed under the simulated conditions of early Earth.

Modern conceptions of abiogenesis

Modern abiogenesis hypotheses are based largely on the same principles as the Oparin-Haldane theory and the Miller-Urey experiment. There are, however, subtle differences between the several models that have been set forth to explain the progression from abiogenic molecule to living organism, and explanations differ as to whether complex organic molecules first became self-replicating entities lacking metabolic functions or first became metabolizing protocells that then developed the ability to self-replicate.

The habitat for abiogenesis has also been debated. While some evidence suggests that life may have originated from nonlife in hydrothermal vents on the ocean floor, it is possible that abiogenesis occurred elsewhere, such as deep below Earth’s surface, where newly arisen protocells could have subsisted on methane or hydrogen, or even on ocean shores, where proteinoids may have emerged from the reaction of amino acids with heat and then entered the water as cell-like protein droplets.

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Some scientists have proposed that abiogenesis occurred more than once. In one example of this hypothetical scenario, different types of life arose, each with distinct biochemical architectures reflecting the nature of the abiogenic materials from which they developed. Ultimately, however, phosphate-based life (“standard” life, having a biochemical architecture requiring phosphorus) gained an evolutionary advantage over all non-phosphate-based life (“nonstandard” life) and thereby became the most widely distributed type of life on Earth. This notion led scientists to infer the existence of a shadow biosphere, a life-supporting system consisting of microorganisms of unique or unusual biochemical structure that may have once existed, or possibly still exists, on Earth.

As the Miller-Urey experiment demonstrated, organic molecules can form from abiogenic materials under the constraints of Earth’s prebiotic atmosphere. Since the 1950s, researchers have found that amino acids can spontaneously form peptides (small proteins) and that key intermediates in the synthesis of RNA nucleotides (nitrogen-containing compounds [bases] linked to sugar and phosphate groups) can form from prebiotic starting materials. The latter evidence may support the RNA world hypothesis, the idea that on early Earth there existed an abundance of RNA life produced through prebiotic chemical reactions. In fact, in addition to carrying and translating genetic information, RNA is a catalyst, a molecule that increases the rate of a reaction without itself being consumed, meaning that a single RNA catalyst could have produced multiple living forms, which would have been advantageous during the rise of life on Earth. The RNA world hypothesis is one of the leading self-replication-first conceptions of abiogenesis.

Some modern metabolism-based models of abiogenesis incorporate Oparin’s enzyme-containing coacervates but suggest a steady progression from simple organic molecules to coacervates, specifically protobionts, aggregates of organic molecules that display some characteristics of life. Protobionts presumably then gave rise to prokaryotes, single-celled organisms lacking a distinct nucleus and other organelles because of the absence of internal membranes but capable of metabolism and self-replication and susceptible to natural selection. Examples of primitive prokaryotes still found on Earth today include archaea, which often inhabit extreme environments with conditions similar to those that may have existed billions of years ago, and cyanobacteria (blue-green algae), which also flourish in inhospitable environments and are of particular interest in understanding the origin of life, given their photosynthetic abilities. Stromatolites, deposits formed by the growth of blue-green algae, are the world’s oldest fossils, dating to 3.5 billion years ago.

There remain many unanswered questions concerning abiogenesis. Experiments have yet to demonstrate the complete transition of inorganic materials to structures like protobionts and protocells and, in the case of the proposed RNA world, have yet to reconcile important differences in mechanisms in the synthesis of purine and pyrimidine bases necessary to form complete RNA nucleotides. In addition, some scientists contend that abiogenesis was unnecessary, suggesting instead that life was introduced on Earth via collision with an extraterrestrial object harbouring living organisms, such as a meteorite carrying single-celled organisms; the hypothetical migration of life to Earth is known as panspermia.

Research on abiogenesis has benefited significantly from astrobiology, the field of study concerned with the search for extraterrestrial life (life beyond Earth) and with understanding the conditions required for life to form. Astrobiological investigations of the moon Titan, for example, which has an atmosphere lacking free oxygen, have revealed that complex organic molecules are present there, offering scientists a glimpse into the formation of biological materials in a prebiotic habitat resembling that of early Earth.