History › Earth & early life
First life
by 3.5 billion years ago; possibly 3.7–4.1 billion· Event· Life & early humans
Mostly agreedGood evidence; some details, dates or numbers uncertain.
Simple single-celled life, similar to bacteria, appeared on Earth by about 3.5 billion years ago, possibly much earlier.
See it on the History map →What happened
Somehow, chemistry in water became biology: molecules that copy themselves, wrapped in membranes. Candidate settings include deep-sea hot vents and shallow pools. The earliest firm evidence is layered mounds called stromatolites, built by microbes, about 3.5 billion years old; disputed traces reach back 3.7–4.1 billion years. For nearly two billion years, all life stayed single-celled. The Curious Life map covers this in depth.
Why it matters
Every living thing descends from these first cells. Life then reshaped the planet itself, from the air to the rocks.
How we know
Fossil stromatolites and microfossils in Western Australia; carbon chemistry in ancient rocks; and genetic comparisons pointing to one shared ancestor of all living things.
Still debated
How non-living chemistry became life is one of science's biggest unsolved questions. Where and exactly when it happened are also unknown.
Inside this (2)
Connections (5)
▸Forging of heavy elements in stars provided the carbon and oxygen for First lifeWell documented
The connection
Life is built mostly from carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. All except hydrogen were forged in stars and reached Earth in the dust that formed it. Carbon's ability to form long chains and rings makes it the backbone of every living molecule.
When and how
Elements made in stars across the Milky Way over billions of years, then built into the early Earth about 4.5 billion years ago.
How we know
Living cells use the elements that stars make most abundantly; organic molecules have been found in meteorites and in gas clouds in space.
Debates and open questions
Whether some of life's building blocks, such as amino acids, arrived ready-made on meteorites or formed on Earth is debated.
▸Birth of the Moon created tides that may have aided First lifeDebated
The connection
The young Moon was much closer, so tides were far stronger. Some scientists propose that tides repeatedly flooded and dried coastal pools, concentrating chemicals in cycles that could have helped build life's first molecules. The Moon also steadied Earth's tilt, stabilizing climate later on.
When and how
On early Earth's shorelines, roughly 4–3.5 billion years ago; the fast-tide idea was argued by biologist Richard Lathe in 2004.
How sure are we?
This is a hypothesis. Lab experiments show wet-dry cycles help link molecules into chains, but no direct record of tides' role exists.
Debates and open questions
Many researchers favor deep-sea hot vents, where tides matter little, so the Moon may not have been needed for life to begin.
▸Birth of the Sun and Earth provided water and chemistry for First lifeMostly agreed
The connection
Earth formed at a distance from the Sun where water stays liquid. Its water, perhaps from water-bearing rocks and comets, pooled into oceans. Volcanoes, minerals, lightning and sunlight supplied energy and raw chemicals. Within a few hundred million years of the surface cooling, life appeared in this setting.
When and how
On Earth, between about 4.4 billion years ago, when oceans may already have existed, and 3.5 billion years ago.
How sure are we?
Zircon crystals about 4.4 billion years old suggest liquid water existed early; fossil microbes about 3.5 billion years old show life followed.
Debates and open questions
Where Earth's water came from, and how quickly life followed once conditions allowed, are debated.
▸First life gave rise to Oxygen-making photosynthesis (cyanobacteria)Mostly agreed
The connection
Early microbes fed on chemicals or used sunlight without releasing oxygen. Over hundreds of millions of years, the ancestors of cyanobacteria combined two light-capturing systems into one that could split water. That gave them an almost unlimited fuel supply, with oxygen released as waste.
When and how
In oceans and shallow waters, by about 2.7–3 billion years ago, possibly earlier.
How sure are we?
Photosynthesis genes show family links between cyanobacteria and other light-using bacteria; ancient rocks contain chemical signs of early oxygen.
Debates and open questions
How and when the two light-capturing systems came together are actively debated.
▸First life evolved, by merging cells, into First complex cells (eukaryotes)Mostly agreed
The connection
Two kinds of simple cells merged. An archaeon, a microbe from a branch of life separate from bacteria, engulfed or partnered with a bacterium. Instead of digesting it, the host kept it, and the bacterium became the mitochondrion. The partnership produced a new, more complex kind of cell with a nucleus.
When and how
In ancient seas, about 2–1.6 billion years ago; Lynn Margulis championed the idea in 1967, building on suggestions from the early 1900s.
How sure are we?
Mitochondria have their own bacteria-like DNA and divide like bacteria; Asgard archaea, found in seafloor mud in 2015, share key genes with eukaryotes.
Debates and open questions
Whether the nucleus or the mitochondrion came first, and what the host cell looked like, are still debated.