Where Is Life Suggested to Have Begun?

The origin of life, or abiogenesis, explores how non-living matter first assembled into self-replicating systems on the early Earth. Approximately 4 billion years ago, the planet lacked free oxygen, and the surface was subjected to high levels of ultraviolet (UV) radiation. Widespread volcanic activity created an environment rich in heat, gases, and mineral compounds that fueled chemical reactions. Investigating life’s origin requires identifying a primitive location that could concentrate organic molecules and provide the energy needed to transition from simple building blocks to the first living cells.

The Primordial Soup Hypothesis

The classic “primordial soup” theory, proposed independently by Alexander Oparin and J. B. S. Haldane in the 1920s, posits that life began in shallow, surface water bodies like oceans or ponds. This hypothesis suggested that the early Earth’s atmosphere, rich in reducing gases such as methane, ammonia, and hydrogen, served as the initial chemical factory. Energy from intense sources like lightning and solar UV radiation would have driven reactions between these atmospheric gases.

In 1953, the Miller-Urey experiment provided experimental support for this concept. Stanley Miller and Harold Urey constructed a closed apparatus that simulated the proposed early Earth conditions, including a “sea” of boiling water and an “atmosphere” of reducing gases. They introduced electrical sparks to mimic lightning and successfully produced several organic monomers, including multiple amino acids, the building blocks of proteins.

However, a significant challenge to this theory is the instability of fragile organic molecules when exposed to the intense UV radiation that permeated the atmosphere due to the lack of an ozone layer. Furthermore, the concentration of these molecules in a vast ocean would have been too low to easily form the longer chains, or polymers, necessary for life.

Deep-Sea Hydrothermal Vents

In response to the challenges of the surface-based soup theory, deep-sea hydrothermal vents have emerged as an alternative location for abiogenesis. These underwater fissures offer a continuous, geothermally driven source of energy and chemical compounds, completely shielded from destructive UV radiation. The vents create environments where the necessary building blocks could be synthesized and protected.

There are two main types of vents, but the “alkaline” vents, such as the Lost City Hydrothermal Field, are often favored for the origin of life. Unlike the hot, acidic “black smoker” vents, alkaline vents produce cooler (around 40–90°C), chemically gentle fluids with a high pH (9–11). These systems form elaborate, porous rock structures through serpentinization, which generates hydrogen gas and methane.

The microscopic pores within these chimney structures could have acted as natural compartments, concentrating organic molecules and metal catalysts. This environment provides continuous energy in the form of chemical gradients, created by the mixing of the vent’s alkaline fluid with the slightly acidic ocean water. This gradient is thought to be a precursor to the energy-harnessing mechanisms in modern cells. Mineral surfaces within the pores may have also facilitated the synthesis of complex organic molecules like nucleobases and RNA polymers.

Geothermal Land Environments

A recent hypothesis suggests that life may have begun in terrestrial geothermal environments, such as hot springs, geysers, or volcanic pools on land. This scenario shares the advantages of heat and chemical energy found in deep-sea vents but introduces a mechanism crucial for building large biological molecules: the wet/dry cycle. The first cells may have emerged in pools of condensed and cooled geothermal vapor, which are often enriched in compounds like phosphate and boron.

The unique benefit of these land-based pools is that they would periodically evaporate and then rehydrate, driven by the thermal springs or daily temperature oscillations. This cycle of dehydration and rehydration is necessary for polymerization, the process of linking small monomers like amino acids and nucleotides into long chains like proteins and RNA. When water is present, these long chains tend to break down, or hydrolyze.

During the drying phase, the water activity is reduced, which favors the formation of chemical bonds and allows organic molecules to concentrate on mineral surfaces. If lipids are present, the subsequent rehydration phase can cause these newly formed polymers to become encapsulated within membranous compartments, forming protocells. This continuous cycling of hydration and desiccation is a mechanism for driving chemical evolution toward self-replicating systems, an advantage not easily replicated in the constant aqueous environment of the deep ocean.

Extraterrestrial Contributions to Early Life

The question of life’s origin involves considering the source of the initial organic building blocks, a concept often referred to as exogenesis. While life itself may have originated on Earth, a significant portion of the raw materials for abiogenesis likely arrived from space via meteorites and comets. During the early history of the solar system, Earth was subjected to a greater influx of extraterrestrial material, which may have delivered millions of tons of prebiotic elements annually.

Analysis of carbonaceous chondrite meteorites, such as the Murchison meteorite, confirms this contribution. These space rocks contain a vast array of organic molecules, including over 70 types of amino acids, as well as sugars and nucleobases—the structural components of DNA and RNA. The presence of these complex molecules, which are non-biological in origin, suggests that the ingredients for life were already being synthesized in the interstellar medium.

This theory proposes that space delivered a ready-made supply of complex chemical components to Earth’s surface. The subsequent assembly of these imported molecules into a living, replicating system would still need to have occurred in one of the terrestrial environments—the primordial soup, a deep-sea vent, or a geothermal hot spring—where the necessary conditions for concentration and polymerization were met.