Origin of Life on Earth: How and When Life First Began

The origin of life on Earth is the process by which non-living chemistry gave rise to the first living cells, probably between about 4.0 and 3.5 billion years ago. Fossils show that microbes were thriving by about 3.5 billion years ago, yet how the first life emerged remains one of the great unsolved problems in science. Researchers have proposed several settings, from a “primordial soup” to deep-sea hydrothermal vents, and have recreated some of the key chemical steps in the laboratory.

Background: from spontaneous generation to chemistry

For centuries many people believed that simple life arose regularly from non-living matter: maggots from meat, or mice from grain. This idea, called spontaneous generation, was tested by experiments over two centuries. Around 1859 to 1862 the French chemist Louis Pasteur used flasks with long, curved “swan necks” to show that boiled broth stayed free of microbes unless dust from the air could reach it.

Pasteur’s work showed that life today comes from existing life. But it left open the question of how the very first life began. In a private letter to the botanist Joseph Hooker in 1871, Charles Darwin speculated that life might have begun in some “warm little pond” rich in chemicals, although he regarded this as pure speculation.

In the 1920s the Soviet biochemist Alexander Oparin and the British scientist J. B. S. Haldane independently proposed that the early Earth had an atmosphere without free oxygen, and that energy from sunlight and lightning could have turned simple gases into organic molecules. These would have accumulated in the oceans, forming what Haldane called a “hot dilute soup”. The idea became known as the Oparin–Haldane hypothesis or the primordial soup theory.

The early Earth as a setting

Life arose on a planet very different from today’s. After the formation of the Earth about 4.54 billion years ago, the surface was at first molten. Evidence from ancient zircon crystals suggests that crust and liquid water existed by about 4.4 billion years ago, so conditions suitable for life may have appeared surprisingly early.

The early atmosphere contained little or no free oxygen, which would otherwise have destroyed many fragile organic molecules. The planet was also struck frequently by asteroids and comets, which could have both delivered organic material and disrupted any early ecosystems.

The Miller–Urey experiment

The first famous test of the primordial soup idea took place at the University of Chicago in 1952. The graduate student Stanley Miller, working with the Nobel laureate Harold Urey, sealed water, methane, ammonia and hydrogen in glass apparatus. He boiled the water and passed electric sparks through the gases to imitate lightning.

Within days the water turned brown and contained several amino acids, the building blocks of proteins. Miller published the results in the journal Science in 1953. The Miller–Urey experiment became one of the most famous in biology, showing that molecules of life can form naturally.

Later work refined the picture. Geologists came to think that the early atmosphere was probably less rich in methane and ammonia than Miller assumed, which reduces the yield of such experiments. However, a 2008 re-analysis of samples Miller had stored found that some of his variant experiments had produced more than 20 amino acids. In 1960 the Spanish-American chemist Joan Oró showed that adenine, one of the bases in DNA and RNA, could form from hydrogen cyanide and ammonia.

Molecules from space

Some of life’s ingredients may also have come from space. The Murchison meteorite, which fell in Victoria, Australia, on 28 September 1969, contains dozens of different amino acids and many other organic compounds. Such meteorites show that the chemistry of life’s building blocks is common in the Solar System.

The RNA world

Living cells depend on a chicken-and-egg partnership. DNA stores genetic information, but proteins are needed to copy DNA, and DNA is needed to make proteins. Which came first?

In the early 1980s Thomas Cech and Sidney Altman independently discovered that some RNA molecules can act as catalysts, speeding up chemical reactions in the way enzymes do. They shared the 1989 Nobel Prize in Chemistry. RNA can therefore both carry information and do chemical work.

In 1986 the American biologist Walter Gilbert coined the term “RNA world” for a stage in early evolution when RNA performed both jobs. Strong support came when the structure of the ribosome, the cell’s protein-making machine, was solved around 2000: its catalytic core is made of RNA, not protein, suggesting it is a relic of an RNA world.

A major difficulty was explaining how RNA’s building blocks, nucleotides, could form without enzymes. In 2009 a team led by John Sutherland at the University of Manchester showed a plausible chemical route to two of them under conditions that might have existed on the early Earth. Laboratories have since produced RNA molecules that can copy short stretches of other RNA, although a fully self-replicating RNA has not yet been made.

Where did life begin?

Deep-sea hydrothermal vents

In 1977 scientists aboard the submersible Alvin discovered hydrothermal vents on the Galápagos Rift, surrounded by rich communities of animals living without sunlight. Black smokers, vents pouring out mineral-laden water at more than 350 degrees Celsius, were found on the East Pacific Rise in 1979.

These discoveries led researchers such as Michael Russell and William Martin to propose that life began at hydrothermal vents. They argue especially for alkaline vents like the Lost City field in the mid-Atlantic, discovered in 2000. Such vents have warm, mineral-lined pores and natural differences in acidity and electrical charge across thin mineral walls, which resemble the energy gradients that all living cells use today.

Shallow pools and hot springs

Other researchers, including David Deamer and John Sutherland, favour land-based settings such as volcanic hot springs or shallow pools. There, repeated cycles of wetting and drying could concentrate molecules and help them link into longer chains. Ultraviolet light from the Sun, absent in the deep sea, also drives some of the key reactions in Sutherland’s chemistry.

Other ideas

Some scientists have suggested that clay minerals helped organise early molecules, or that life began in ice. The idea of panspermia, promoted by the Swedish chemist Svante Arrhenius in the early 20th century, proposes that life arrived from elsewhere in space. There is no direct evidence for it, and it does not explain how life originated in the first place.

The earliest evidence

Detecting the first traces of life in ancient rocks is extremely difficult, because those rocks have been heated and deformed. Claims are therefore often disputed.

• About 3.48–3.43 billion years ago: The Pilbara stromatolites of Western Australia, layered structures built by microbial mats, are widely accepted as among the oldest convincing evidence of life.
• About 3.7 billion years ago: Carbon enriched in the light isotope carbon-12, a common sign of biological activity, has been reported in rocks of the Isua Greenstone Belt in Greenland. A 2016 claim of 3.7-billion-year-old stromatolites there was challenged in 2018.
• About 3.77 billion years or more: Tiny tube-like structures from the Nuvvuagittuq belt in Quebec, reported in 2017, have been interpreted as fossils of vent microbes, but many researchers doubt they are biological.
• About 4.1 billion years ago: Carbon inside a Jack Hills zircon from Australia has an isotope signature consistent with life, but non-biological origins cannot be ruled out.

The last universal common ancestor

All known living things share the same genetic code and many of the same molecules, which indicates that they descend from a single population known as LUCA, the last universal common ancestor. Studies comparing genes across living organisms suggest LUCA was already a fairly complex single-celled organism, possibly living in a hot environment. A 2024 study estimated that LUCA lived about 4.2 billion years ago, although such estimates carry large uncertainties.

Myths and facts

• Myth: the Miller–Urey experiment created life. It produced amino acids, not living cells. Making a living system from scratch has not been achieved.
• Myth: scientists think life arose in a single lucky event. Most researchers see it as a long series of chemical steps, each of which can in principle be studied and reproduced.
• Fact: life appeared early. Life seems to have emerged within the first billion years of Earth’s history, perhaps soon after the surface became habitable.

Timeline

• c. 4,400 million years ago: Zircons suggest liquid water on Earth’s surface.
• c. 4,100 million years ago: Possible biological carbon is preserved in a Jack Hills zircon.
• c. 3,700 million years ago: Isotopically light carbon in Greenland rocks hints at life.
• c. 3,480–3,430 million years ago: Stromatolites in the Pilbara record microbial communities.
• AD 1859–1862: Pasteur’s experiments undermine spontaneous generation.
• AD 1924 and 1929: Oparin and Haldane propose the primordial soup theory.
• AD 1952: Stanley Miller carries out his spark-discharge experiment, published in 1953.
• AD 1977: Hydrothermal vent communities are discovered on the Galápagos Rift.
• AD 1986: Walter Gilbert names the “RNA world”.
• AD 2009: A plausible prebiotic route to RNA nucleotides is reported.

Why it matters

The origin of life connects chemistry, geology and biology, and it bears on one of humanity’s oldest questions: are we alone? If life arose quickly and easily on the early Earth, it may be common on other worlds. Every later development, from the Great Oxidation Event to the evolution of animals and humans, traces back to those first microscopic cells.