The earliest life on Earth may be as old as 4.1 billion years, based on graphite found inside a zircon grain in the Jack Hills area of Australia. The first evidence of life in rock layers, not just in a single mineral, comes from 3.7-billion-year-old sedimentary rocks in Greenland that contain graphite. The oldest direct proof of life is stromatolite fossils found in 3.48-billion-year-old geyserite from the Dresser Formation in Western Australia. Microfossils of tiny organisms have been discovered in 3.4-billion-year-old rocks, such as 3.465-billion-year-old chert from the same region in Australia and 3.42-billion-year-old hydrothermal vent deposits in South Africa. Later in Earth's history, around 1.73 billion years ago, preserved molecules from living things suggest the presence of aerobic life. Therefore, life on Earth began at least 3.5 billion years ago and could have started as early as 4.1 billion years ago, shortly after the oceans formed 4.5 billion years ago and after Earth itself formed 4.54 billion years ago.
Biospheres
Earth is the only known place in the universe where life exists, and it can be found in many different environments. Scientists believe life on Earth began at least 3.5 billion years ago. Some studies suggest it may have started as early as 3.8 to 4.1 billion years ago. Since life first appeared, it has survived in many types of environments. The Earth's biosphere reaches at least 10 kilometers (6.2 miles) below the seafloor, up to 41–77 kilometers (25–48 miles) into the atmosphere, and includes soil, hydrothermal vents, and rock. The biosphere also extends at least 914.4 meters (3,000 feet; 0.5682 miles) below the ice of Antarctica and includes the deepest parts of the ocean. In July 2020, marine biologists discovered aerobic microorganisms, mostly in a resting state, in sediment 76.2 meters (250 feet) below the seafloor in the South Pacific Gyre, a place known as "the deadest spot in the ocean." Microbes have been found in the Atacama Desert in Chile, one of the driest places on Earth. In February 2023, scientists reported the discovery of a "dark microbiome" made up of unfamiliar microorganisms in the Atacama Desert. Microbes have also been found in deep-sea hydrothermal vent environments, where temperatures can exceed 400 °C. Microbial communities can survive in cold permafrost conditions as low as -25 °C. Under certain test conditions, life forms have been observed to survive in the vacuum of outer space. More recently, studies on the International Space Station showed that bacteria can survive in outer space.
Geochemical evidence
The Earth is about 4.54 billion years old. The earliest clear signs of life on Earth are found in rocks called stromatolites, which are at least 3.5 billion years old. Some computer models suggest life might have started as early as 4.5 billion years ago. The oldest evidence of life is not direct but is found in the way certain elements, like carbon, are distributed in different forms. Microorganisms often use lighter forms of atoms to build their bodies because it requires less energy to break their bonds. Living materials usually have more of the lighter forms of carbon compared to the rocks around them. Scientists use carbon isotopes, measured as δC, to find evidence of life. Living things often use the lighter carbon isotope instead of the heavier one. This difference in carbon can be recorded in living materials.
The oldest debated evidence of life is found in a tiny grain of zircon from Jack Hills in Western Australia. This grain contains graphite with a δC signature similar to carbon from living things. Other early life evidence is found in rocks from the Akilia Sequence and the Isua Supracrustal Belt in Greenland. These rocks, about 3.7 billion years old, also contain graphite with carbon isotope patterns that suggest biological processes.
A challenge with using isotopes to find life is that non-living processes can also create similar patterns. Studies of the Akilia graphite show that changes from heat, chemical reactions in hot water, and volcanic activity might explain the lighter carbon isotopes. The graphite in the ISB rocks might have formed from non-living chemical reactions due to changes from hot fluids. However, further analysis of the ISB graphite suggests it is more likely from living things.
Rocks from the 3.5 billion-year-old Dresser Formation in Australia show better-preserved evidence. These rocks contain carbon and sulfur isotopes in barite, which are changed by microbial processes during sulfate reduction. These patterns match biological activity. However, the Dresser Formation was in an area with active volcanoes and hot water, so non-living processes might also explain these patterns. Many of these findings are supported by direct evidence, such as the presence of microfossils.
Fossil evidence
Fossils are direct proof that living things existed in the past. When scientists look for the earliest life, they often use both fossils and chemical clues from rocks. Fossils do not go back as far as chemical clues because natural processes that change rocks over time can destroy fossils in some rock layers.
Stromatolites are layered rock structures formed by tiny living things that grow on the surface of sediment. A key sign that stromatolites are made by living things is their curved-up shape and wavy layers, which show how these tiny organisms built structures toward sunlight. Some scientists reported finding stromatolites in rocks from the Isua metasediments that are 3.7 billion years old. These rocks have curved-up, cone-shaped, and dome-like shapes. However, further studies found that the layers inside these rocks do not match the features needed to identify stromatolites, suggesting the shapes might be caused by rock movement from tectonic forces instead.
The oldest direct evidence of life is found in stromatolites from 3.48 billion-year-old chert in the Dresser Formation of the Pilbara Craton in Western Australia. Features in these fossils are hard to explain without living things, such as thicker layers over curved areas that suggest more sunlight. Sulfur in the rocks also supports a living origin. While most scientists agree these are fossils, some abiotic (non-living) explanations are still possible because of the environment where these rocks formed and unclear chemical evidence.
Most stromatolites older than 3.0 billion years are found in Australia or South Africa. In the Pilbara Craton, stromatolites from 3.47 billion years ago are found in the Mount Ada Basalt. In South Africa, stromatolites from 3.46, 3.42, and 3.33 billion years ago are found in the Onverwacht Group. In Western Australia, stromatolites from 3.43 billion years ago in the Strelley Pool Formation show changes in layers that may show how microbes adapted to changing environments. This suggests photosynthesis (using sunlight to make energy) may have started as early as 3.43 billion years ago.
Some of the oldest possible life evidence comes from tiny fossilized microbes (microfossils) in rocks from the Nuvvuagittuq Belt in Quebec, Canada. These rocks may be 4.28 billion years old, which would make them the oldest life evidence on Earth, suggesting life appeared quickly after oceans formed 4.41 billion years ago. However, these shapes might also be made by non-living processes, such as silica-rich water, chemical reactions, and volcanic activity.
The 3.48 billion-year-old Dresser Formation contains microfossils of tiny, thread-like life forms in silica veins, which are the oldest known fossil evidence of life on Earth. However, some scientists think these might have formed from volcanic activity. In Australia, 3.465-billion-year-old Apex chert rocks may have once contained microbes, but this is debated. In South Africa, "possible" microfossils from 3.42 billion years ago in the Barberton greenstone belt may show ancient microbes living near hydrothermal vents. In the 3.43 billion-year-old Strelley Pool Formation, scientists found many types of microfossils, including round, lens-shaped, and thin, film-like shapes. These structures are more likely to be from living things because their chemical makeup has been preserved well.
Biomarkers are chemicals in rocks that come from living things and can show evidence of past life. These chemicals are not found until later in Earth’s history, but they help scientists study early photosynthesis. Lipids (a type of chemical found in living things) are especially useful because they can survive for a long time and help scientists understand past environments.
Scientists found fossilized lipids in 2.7 billion-year-old rocks in the Pilbara Craton and 2.67 billion-year-old rocks in South Africa. However, later studies showed these lipids were not from the rocks themselves but came from other sources. The oldest clearly natural biomarkers are from 1.64 billion-year-old rocks in the Barney Creek Formation in Australia. Older biomarkers from 1.73 billion-year-old rocks in the same area were also found but are less clear.
Other natural biomarkers are found from the Mesoproterozoic era (1.6 to 1.0 billion years ago). In China, 1.4 billion-year-old rocks contain hydrocarbons likely from prokaryotes (simple life forms). In Australia, 1.38 billion-year-old rocks have biomarkers in siltstone. In China, 1.37 billion-year-old rocks have hydrocarbons possibly from bacteria and algae. In Mauritania, 1.1 billion-year-old black shale contains biomarkers from living things.
Genomic evidence
By comparing the genetic material of modern organisms in the Bacteria and Archaea domains, scientists have found evidence of a last universal common ancestor, or LUCA. Another name for LUCA is cenancestor, which refers to a group of organisms, not just one individual. LUCA is not believed to be the first life on Earth, but it is the only type of organism from that time that has living descendants today. In 2016, M. C. Weiss and other scientists identified a basic set of genes found in at least two groups of Bacteria and two groups of Archaea. They believed that this pattern of genes was unlikely to result from gene sharing between different species, so these genes must have come from LUCA. A model called a molecular clock estimates that LUCA may have lived between 4.477 and 4.519 billion years ago, during the Hadean eon.
RNA replicators
Scientists studied models of geothermal areas similar to those on early Earth. These models showed that such environments could support the creation and copying of RNA molecules, which may have helped life begin. Rock structures with small holes and heated air-water surfaces were found to help ribozymes copy both strands of RNA. This process allowed the strands to separate, which made it possible to create, release, and shape active ribozymes.
Hypotheses for the origin of life on Earth
Current evidence suggests that life on Earth may have begun as early as 4.1 billion years ago, with fossils showing life existed 3.5 billion years ago. Some scientists believe life could have started nearly 4.5 billion years ago. Biologist Stephen Blair Hedges stated, "If life appeared quickly on Earth, it might be common throughout the universe." The idea that life on Earth could have come from space has been studied. In 2018, a study found that meteorites from Earth, which are 4.5 billion years old, contained liquid water and organic substances that might help create life.
Scientists have long thought that hydrothermal vents could be where life began. These vents have conditions, such as specific chemistry, pressure, and temperatures, that may help form organic molecules from non-living materials. In experiments by NASA, scientists showed that organic compounds like formate and methane could form from non-living materials under conditions similar to ancient hydrothermal vents. These molecules could then combine to create more complex ones, such as amino acids, which are needed to form RNA or DNA.
Charles Darwin is famous for his theory of evolution through natural selection. He also proposed that life might have started in a "warm little pond" with elements like ammonia and phosphoric salts, along with heat and electricity, which could form proteins. He noted that such an environment today would likely not last long enough for life to develop. This idea is often linked to the concept of spontaneous generation.
In 1924, Alexander Oparin suggested that Earth's early atmosphere had substances like ammonia, methane, water vapor, and hydrogen gas. This idea was supported after methane was found on other planets. In 1929, J. B. S. Haldane proposed similar conditions for early Earth. These ideas were later tested in the Miller–Urey experiment.
In 1953, Stanley Miller, a graduate student at the University of Chicago, conducted an experiment with his professor, Harold Urey. They used gases like methane, ammonia, water vapor, and hydrogen gas to mimic Earth's early atmosphere. A spark simulated lightning, and a device heated water to create a "primordial ocean." After one day, the experiment produced a brown sludge containing amino acids like glycine, alanine, aspartic acid, and aminobutyric acid. Scientists later repeated this experiment, which became a key method for studying how life might have started from non-living materials.
In 1966, Cairns-Smith proposed that processes involving crystals might have played a role in early biological development. In 1975, Hartman suggested that metabolism could have started in simple environments like clay. Clay can help form molecules like amino acids and nucleotides, which are building blocks for macromolecules such as RNA or DNA. This process could have led to the development of cells.