Life › Origin of life
Origin of life
sometime c. 4.4–3.7 billion years ago (unknown)· Event· Microbes & cells
DebatedScientists disagree about when, how or why.
Sometime between about 4.4 and 3.7 billion years ago, non-living chemistry became living cells; nobody knows exactly how.
See it on the Life map →What happened
Life needs three things working together: molecules that store and copy instructions, a membrane bag to hold them, and a way to capture energy. Leading ideas place the start in a 'soup' of organic molecules, at deep-sea hot springs, or in drying pools on volcanic land, with RNA as an early molecule that both carried information and did chemical work. Labs have made many building blocks, but no one has made life from scratch.
Why it matters
Every living thing descends from this step. How easily it happened also bears on whether life is common elsewhere in the universe.
The evidence (fossils, genes, rocks)
All indirect: timing from rocks and early fossils, lab chemistry since the Miller–Urey experiment (1953), building blocks found in meteorites, and gene comparisons that reconstruct LUCA.
Still debated
Where it happened, whether genes or metabolism came first, how long it took, and whether it happened once or several times.
Inside this (5)
Connections (10)
▸Giant impact forms the Moon steadied Earth's tilt, possibly aiding Origin of lifeDebated
The connection
The Moon's pull keeps Earth's tilt near 23 degrees, damping wild climate swings over millions of years. The early Moon, much closer, also raised strong tides that may have washed shorelines wet and dry, concentrating chemicals. Both are possible aids to life, not proven requirements.
When and how
The Moon formed about 4.5 billion years ago; Jacques Laskar and colleagues modeled its steadying effect on Earth's tilt in 1993; tidal-pool origin ideas followed, for example Richard Lathe's in 2004.
How sure are we?
Computer models show Earth's tilt would wander more without the Moon, and rocks show tides were stronger in the past. The link to life itself is inference.
Debates and open questions
Later studies suggest a moonless Earth's tilt might wander less than first thought; tides matter only if life began on shores rather than at deep vents.
▸Late Heavy Bombardment may have delayed or disrupted Origin of lifeDebated
The connection
Large impacts could have boiled the top of the oceans and sterilized the surface, perhaps wiping out any early life. But impacts also delivered organic molecules and created hot, mineral-rich crater systems that some see as good places for life to start. Models suggest microbes underground or deep at sea could have survived.
When and how
About 4.1–3.8 billion years ago, if the spike happened; survival models by Oleg Abramov and Stephen Mojzsis at the University of Colorado, 2009.
How sure are we?
Mostly modeling. Crater counts and moon rock ages show heavy bombardment, but there is no direct record of life before or during it.
Debates and open questions
Whether the bombardment was a short spike or a long, declining tail, and whether life began once or was wiped out and restarted.
▸Water & the first oceans provided the setting for Origin of lifeStrong evidence
The connection
All known life is built of watery cells, so liquid water was the stage. Oceans dissolved minerals, carried chemicals from volcanoes and vents, and shielded molecules from harsh ultraviolet light. Whether life began in the open sea, at the seafloor or in pools on land fed by rain is unresolved.
When and how
Oceans existed by about 4.4 billion years ago; the earliest likely traces of life follow by about 3.7–3.5 billion years ago.
How sure are we?
The link rests on biochemistry: every known cell needs liquid water. The exact setting is inferred, not observed.
Debates and open questions
Water also breaks long molecules apart, which is why some scientists favor drying pools on land over the open ocean.
▸Primordial soup (Oparin–Haldane hypothesis) offers one explanation of Origin of lifeHypothesis
The connection
The soup idea says energy from lightning and ultraviolet light turned simple gases in oxygen-free air into organic molecules that collected in seas or ponds. Over long periods they combined into larger and larger molecules until some could copy themselves, the first step toward cells.
When and how
Proposed independently by Alexander Oparin (Russia, 1924) and J.B.S. Haldane (Britain, 1929); tested by Stanley Miller and Harold Urey at the University of Chicago, published 1953.
How sure are we?
Lab experiments make amino acids and other building blocks, and meteorites such as Murchison carry dozens of amino acids. No experiment has produced life itself.
Debates and open questions
Early air may have been mostly carbon dioxide and nitrogen, which yields fewer building blocks, and open seas may have been too dilute for molecules to join.
▸Deep-sea hydrothermal vent origin offers one explanation of Origin of lifeHypothesis
The connection
The vent idea says life began inside porous mineral chimneys on the seafloor. Hydrogen-rich vent fluid met carbon dioxide-rich seawater across thin mineral walls, creating natural chemical and acidity differences. These could have powered reactions making organic molecules, with tiny pores serving as ready-made compartments before cells had membranes.
When and how
Proposed by John Corliss and colleagues after the 1977 vent discovery; developed from the late 1980s by Michael Russell and later William Martin around alkaline vents.
How sure are we?
Lab reactors mimicking vents make simple organic molecules, and all cells still run on acidity differences across membranes. There is no direct fossil evidence.
Debates and open questions
Whether vent chemistry could go beyond simple molecules to RNA, and how early cells left the vents, which required evolving their own membranes and pumps.
▸RNA world proposes a first step in Origin of lifeHypothesis
The connection
Today DNA stores genes and proteins do the work, but each is needed to make the other, a chicken-and-egg problem. RNA can do both jobs. So the first living systems may have been self-copying RNA molecules, with DNA and proteins added later.
When and how
Suggested by Carl Woese, Francis Crick and Leslie Orgel in the 1960s; RNA enzymes found by Thomas Cech and Sidney Altman in the early 1980s; named by Walter Gilbert in 1986.
How sure are we?
RNA enzymes exist; the core of the ribosome, every cell's protein factory, is RNA; lab-made RNA enzymes can copy short RNA strands.
Debates and open questions
Whether RNA was truly the first genetic molecule or replaced a simpler, now-vanished one, and where on early Earth its fragile building blocks could have gathered.
▸Panspermia: life seeded from space would only relocate the problem of Origin of lifeHypothesis
The connection
Panspermia says life reached Earth from elsewhere: on meteorites from Mars, on comets, or from other star systems. Even if true, life still had to begin somewhere through some chemistry. So it moves the question rather than answering it, and adds the challenge of surviving space travel.
When and how
Promoted by Swedish chemist Svante Arrhenius in 1903; later by Fred Hoyle and Chandra Wickramasinghe, and as 'directed panspermia' by Francis Crick and Leslie Orgel in 1973.
How sure are we?
The point is logical rather than experimental: life arriving from elsewhere would still need a chemical origin there, by processes like those proposed for Earth.
Debates and open questions
Mars is the usual candidate source. Finding Martian life, and learning whether it shares Earth's DNA-based chemistry, would test whether the two planets' life is related.
▸Origin of life eventually produced LUCA, the last universal common ancestorStrong evidence
The connection
The first living systems were probably crude and leaky. Over a long time, natural selection refined them into true cells with DNA, proteins and membranes. Many early lineages likely died out; one population, LUCA, left the descendants that include everything alive today. LUCA was already complex, so much evolution came before it.
When and how
Sometime between about 4.4 and 3.8 billion years ago; how long the gap between life's start and LUCA lasted is unknown.
How sure are we?
Gene comparisons show LUCA already had hundreds to thousands of genes, so a long history must lie before it.
Debates and open questions
How long the gap was, and whether LUCA was a single species or a community of cells swapping genes.
▸Earliest claimed traces of life would push back the date of Origin of lifeDebated
The connection
If carbon traces in 4.1-billion-year-old crystals or 3.7-billion-year-old Greenland rocks were made by life, life began within a few hundred million years of Earth having oceans. That would suggest life starts readily when conditions allow. If the claims fail, the firm record starts about 3.5 billion years ago.
When and how
Isua, Greenland, claims from 1996 to 2016; Jack Hills zircon carbon, Western Australia, reported by Elizabeth Bell and colleagues at UCLA in 2015; Nuvvuagittuq, Quebec, 2017.
How sure are we?
Each claim rests on light carbon (life prefers carbon-12) or on shapes; both can form without life, so none is fully accepted.
Debates and open questions
Whether such heated, squeezed rocks can ever settle the date; molecular-clock estimates placing LUCA near 4.2 billion years ago suggest life is older than any firm trace.
▸Miller–Urey experiment simulated early chemistry for Origin of lifeWell established
The connection
By showing that protein building blocks form readily from simple gases and energy, the experiment made life's origin a lab problem. It launched prebiotic chemistry: in 1961 Joan Oró made adenine, one of DNA's letters, from hydrogen cyanide, and later work made sugars and RNA pieces under plausible early-Earth conditions.
When and how
Chicago, 1953 onward; followed by labs worldwide, from Oró in Houston in 1961 to John Sutherland's group in England, which made RNA building blocks in 2009.
The evidence
Prebiotic experiments repeatedly produce amino acids, sugars and pieces of nucleotides, and meteorites carry many of the same molecules, so this chemistry clearly happens naturally.
Debates and open questions
No experiment has yet produced a self-copying system from scratch, and researchers disagree whether life started in ponds, at deep-sea vents or somewhere else.