Abiogenesis

Abiogenesis, or biopoiesis, describes the natural process where life emerges from non-living matter, like simple organic compounds. Scientists studying it aim to understand how chemical reactions before life began produced living things under conditions very different from modern Earth, drawing on biology and chemistry. The leading hypothesis holds that the shift from non-living to living entities was not a single event but a gradual increase in complexity. This involved forming a planet that could support life, synthesizing organic molecules without biology, and developing molecular self-replication, self-assembly, autocatalysis, and cell membranes. While no one has observed the full transition from non-life to life in a lab, many proposals exist for its various stages.

Life operates through the chemistry of carbon and water, relying on four key chemical families: lipids for cell membranes, carbohydrates for energy storage and structure, amino acids for proteins, and nucleic acids (DNA and RNA) for heredity. Any theory of abiogenesis must explain where these molecules came from and how they interacted. Many researchers focus on how self-replicating molecules first appeared, with the RNA world being a common idea, though other self-replicating or self-catalyzing molecules might have come before RNA. Other "metabolism-first" hypotheses suggest that early catalysis provided the building blocks for self-replication. The 1952 Miller–Urey experiment showed that amino acids could form from inorganic compounds under conditions like early Earth's, and amino acids have since been found in meteorites, comets, asteroids, and star-forming regions in space.

The last universal common ancestor (LUCA) of all modern life lived millions of years after life's origin, but studying it guides research into early life's traits. By comparing genes shared by Archaea and Bacteria, scientists have identified 60 proteins common to all life and 355 prokaryotic genes tracing back to LUCA. These suggest LUCA was anaerobic, used the Wood–Ljungdahl pathway for energy, relied on chemiosmosis, and had DNA, a genetic code, and ribosomes. Earlier cells might have had leaky membranes and been powered by natural proton gradients near deep-sea hydrothermal vents, or life may have started inside the continental crust or in surface water.

Earth is the only known place with life, but astrobiologists assume similar processes could occur on other planets. Geochemical and fossil evidence guides most studies. Earth formed 4.54 billion years ago, with the earliest life evidence dating to 3.8 billion years ago in Western Australia. Fossil microorganisms may have lived in hydrothermal vent deposits from Quebec soon after the oceans formed during the Hadean, suggesting the process was relatively rapid in geological terms.

Life involves reproduction with heritable variation. NASA defines it as "a self-sustaining chemical system capable of Darwinian evolution." Such a system is complex; LUCA, a likely single-celled organism from about 4 billion years ago, already had hundreds of genes in the universal DNA genetic code, implying machinery like messenger RNA, transfer RNA, and ribosomes to translate code into proteins. Those proteins included enzymes for anaerobic respiration via the Wood–Ljungdahl pathway and a DNA polymerase to copy its DNA. The challenge is explaining how such an interconnected system evolved step by step, since all parts seem necessary for function—for instance, DNA polymerase is made by translating its own gene, yet neither enzyme nor DNA can be produced without the other. Evolution likely started with molecular self-replication, self-assembly (like cell membranes), and autocatalysis via RNA ribozymes in an RNA world. The transition has not been observed experimentally, and some scientists see life and its origin as parts of the same process.

The preconditions for a cell like LUCA are known, though debated: a habitable world with minerals, liquid water, and energy sources like sunlight and geothermal heat. Prebiotic synthesis creates simple organic compounds that assemble into polymers like proteins and RNA. After LUCA, biological evolution produced diverse species and biochemical capabilities, but how LUCA arose from simple components remains poorly understood. Astrobiology seeks evidence of life elsewhere, and NASA's 2015 origin-of-life strategy aimed to identify interactions, structures, functions, energy sources, and environmental factors that contributed to evolvable macromolecular systems, mapping the chemical landscape of potential primordial informational polymers. Such polymers were likely a critical step in prebiotic chemical evolution.