Abiogenesis
In 1676, Antonie van Leeuwenhoek drew and described microorganisms that he called animalcules. He used a simple microscope to observe protozoa and bacteria in pond water. This observation challenged the ancient belief that life could arise spontaneously from decaying matter. Aristotle had taught that insects generated themselves from rotting organic substances. By the middle of the 19th century, scientists considered spontaneous generation disproven after Francesco Redi showed no maggots appeared when flies were prevented from laying eggs on meat.
Charles Darwin wrote about these ideas in a letter dated the 1st of February 1871. He speculated that the original spark of life may have been in a warm little pond containing ammonia and phosphoric salts. He noted that light, heat, and electricity were present during this process. Darwin explained that such matter would be instantly devoured by living creatures today but not before life existed. Alexander Oparin proposed in 1924 that early cells slowly self-organized from a primordial soup. J.B.S. Haldane suggested Earth's prebiotic oceans consisted of a hot dilute soup where organic compounds formed.
Stanley Miller and Harold Urey carried out an experiment in 1952 to demonstrate how organic molecules formed spontaneously. They used a mixture of methane, ammonia, hydrogen, and water vapor under electrical discharges. The experiment produced amino acids from inorganic precursors. Current scientific consensus describes the primitive atmosphere as weakly reducing or neutral. This diminishes the amount and variety of amino acids that could be produced compared to their initial results.
The Big Bang occurred roughly 14 billion years ago with only hydrogen, helium, and lithium present. These elements gradually accreted into disks of gas and dust orbiting protoplanetary centers. Gravitational accretion at hot and dense centers formed stars through hydrogen fusion. Early massive stars produced heavier elements by stellar nucleosynthesis until reaching iron-56. Heavier elements formed during supernovae at the end of a star's lifecycle.
Carbon became the fourth most abundant element in the universe after white dwarf stars ejected it. The Solar System began forming 4.6 billion years ago when part of a giant molecular cloud collapsed gravitationally. Most collapsing mass collected in the center to form the Sun while the rest flattened into a protoplanetary disk. Earth formed 4.54 billion years ago according to radiometric dating of calcium-aluminium-rich inclusions in carbonaceous chondrite meteorites.
During its formation, Earth lost much initial mass and lacked gravity to hold molecular hydrogen. A collision with Theia created debris that eventually formed the Moon around 4.48 billion years ago. This impact removed Earth's primary atmosphere leaving behind clouds of viscous silicates and carbon dioxide. The unstable atmosphere soon condensed to form bulk silicate Earth with water vapor, nitrogen, and carbon dioxide dominating. Carbon dioxide solution in water made seas slightly acidic with pH about 5.5.
Condensation to form liquid oceans occurred as early as the Moon-forming impact. Zircon crystals dated 4.404 billion years old from Mount Narryer support this scenario. The Hadean atmosphere functioned like a gigantic productive outdoor chemical laboratory similar to volcanic gases today. Life likely evolved at ocean depths exceeding ten meters to shield against impacts and ultraviolet radiation.
Organic compounds containing carbon exist abundantly in space within molecular clouds and circumstellar envelopes. Purine and pyrimidine nucleobases including guanine, adenine, cytosine, uracil, and thymine have been found in meteorites. Sugars also appear in extraterrestrial material. These substances could provide materials for DNA and RNA formation on early Earth.
The amino acid glycine was detected in material ejected from comet Wild 2. Comets carry dark tar-like organic substance formed from simple carbon compounds under ionizing radiation. During the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements annually. Currently forty thousand tons of cosmic dust falls to Earth each year.
Alexander Butlerov showed in 1861 that heating formaldehyde under basic conditions with divalent metal ions created sugars. R. Breslow proposed the reaction was autocatalytic in 1959. Nucleobases such as guanine and adenine synthesize from hydrogen cyanide and ammonia. Formamide produces all four ribonucleotides when warmed with terrestrial minerals.
Freezing temperatures assist purine synthesis by concentrating key precursors like HCN. Seven amino acids and eleven types of nucleobases formed in ice when ammonia and cyanide remained in a freezer for twenty-five years. S-triazines, pyrimidines including cytosine and uracil, and adenine synthesize through freeze-thaw cycles under reductive atmosphere with spark discharges.
Phospholipids form lipid bilayers in water while under agitation. These molecules were not present on early Earth but other membrane-forming amphiphilic long-chain molecules existed. Lipid bodies expand by insertion of additional lipids and spontaneously split into two offspring vesicles. Such structures may have provided sheltering envelopes for information-storing polymers like RNA.
Irene Chen and Jack W. Szostak suggest elementary protocells give rise to cellular behaviors including primitive differential reproduction. Competition for membrane molecules favors stabilized membranes suggesting selective advantage for cross-linked fatty acids. Micro-encapsulation allows metabolism within the membrane while retaining large biomolecules inside.
Fatty acid vesicles in alkaline hydrothermal vent conditions stabilize via isoprenoids synthesized by the formose reaction. Vesicles undergo evolutionary processes under pressure cycling conditions simulating tectonic fault zones. Pressure cycling forms vesicles periodically alongside random peptide chains selected for integration ability. Further selection leads to functional peptide structures increasing vesicle survival rates.
Alexander Rich proposed the RNA world hypothesis in 1962 while Walter Gilbert coined the term in 1986. Many researchers concur that an RNA world preceded modern DNA-based life. Small RNAs catalyze all chemical groups and information transfers required for life. The ribosome structure contains a central core of RNA with no amino acid side chains within eighteen angstroms of the active site.
RNA replicase codes and catalyzes further RNA replication making it autocatalytic. Some catalytic RNAs link smaller sequences enabling self-replication. Natural selection favors proliferation of such autocatalytic sets. Self-assembly of RNA may occur spontaneously in hydrothermal vents. A preliminary transfer RNA could assemble into a replicator molecule possessing heritability, variation, and differential reproduction mechanisms.
Aminoacylation ribozymes charge transfer RNAs with cognate amino acids during in vitro experimentation. Di- and tripeptides synthesize using only aminoacyl phosphate adaptors and RNA guides. Evidence shows approximately one in ten thousand random sequences had ATP binding function. Significant study explores how early functional proteins arose from random sequences without fully-fledged biosynthesis systems.
William Martin and Michael Russell suggested life began at submarine hydrothermal vents where hydrogen-rich fluids emerge below sea floor. These form through serpentinization of ultra-mafic olivine reacting with seawater creating pH interfaces with carbon dioxide-rich ocean water. Alkaline vents create abiogenic proton motive force chemiosmotic gradients ideal for abiogenesis.
Microscopic compartments provide natural means concentrating organic molecules composed of iron-sulfur minerals like mackinawite. Mineral surfaces inside deep-ocean hydrothermal vents possess catalytic properties similar to enzymes. They create simple organic molecules including methanol, formic acid, acetic acid, and pyruvic acid out of dissolved carbon dioxide if driven by applied voltage or reaction with hydrogen sulfide.
Arguments against vent settings state hyperthermophily resulted from convergent evolution rather than inherited traits. Production of prebiotic organic compounds at hydrothermal vents estimated far lower than Miller-Urey experiment environments. Methane concentrations reach two to four orders of magnitude lower than expected. Submarine hydrothermal vents not conducive to condensation reactions needed for macromolecule polymerization due to strong dilution by seawater.
Surface bodies of water provide wet-dry cycles concentrating prebiotic compounds enabling condensation reactions. Lakes receive detrital input from weathering continental apatite-containing rocks providing phosphates. Hot springs allow influxes from deep penetrating waters and surface runoff transporting eroded sediments. Mulkidjanian argues marine environments lack ionic balance found in cells while hot springs contain suitable pH and precipitates absorbing harmful ultraviolet radiation.
Carl Woese began genomics studies placing the last universal common ancestor between Bacteria and Archaea in 1977. A set of three hundred fifty-five genes likely present in LUCA identified in 2016 among six point one million prokaryotic genes sequenced. Results suggest LUCA anaerobic with Wood-Ljungdahl pathway nitrogen- and carbon-fixing thermophilic.
LUCA inhabited an anaerobic hydrothermal vent setting in geochemically active environment requiring hydrogen, carbon dioxide, iron, and transition metals. Its genetic material probably DNA requiring four-nucleotide code messenger RNA transfer RNA and ribosomes translating code into proteins. Sixty proteins common to all life previously identified though metabolic reactions remain disputed.
Molecular clock analysis suggests LUCA emerged prior to 3.9 billion years ago. A 2024 study inferred LUCA age around 4.2 billion years analyzing pre-LUCA gene duplicates calibrated from fossil microorganisms. Phylogenomic analysis shows intracellular fluid ionic composition identical to hot springs suggesting dependence on synthesized organic matter for growth. Experiments demonstrate RNA-like polymers synthesize in wet-dry cycling and ultraviolet light exposure encapsulated in vesicles after condensation.
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Common questions
When did Antonie van Leeuwenhoek observe microorganisms called animalcules?
Antonie van Leeuwenhoek observed and described microorganisms in 1676. He used a simple microscope to view protozoa and bacteria found in pond water.
What date did Charles Darwin speculate about life arising from a warm little pond?
Charles Darwin wrote about these ideas in a letter dated the 1st of February 1871. He speculated that the original spark of life may have been in a warm little pond containing ammonia and phosphoric salts.
How old are the zircon crystals from Mount Narryer that support early ocean formation?
Zircon crystals dated 4.404 billion years old from Mount Narryer support the scenario of liquid oceans forming as early as the Moon-forming impact. These crystals provide evidence for condensation to form liquid oceans during the Hadean eon.
Who proposed the RNA world hypothesis and when was the term coined?
Alexander Rich proposed the RNA world hypothesis in 1962 while Walter Gilbert coined the term in 1986. Many researchers concur that an RNA world preceded modern DNA-based life.
When did Stanley Miller and Harold Urey carry out their experiment on organic molecule formation?
Stanley Miller and Harold Urey carried out an experiment in 1952 to demonstrate how organic molecules formed spontaneously. They used a mixture of methane, ammonia, hydrogen, and water vapor under electrical discharges to produce amino acids from inorganic precursors.