Nobody knows how life started. That is the honest first sentence, and most articles about this topic work hard to avoid it.
This one puts it first, because the interesting part is not the mystery. It is that scientists have built two serious, detailed, and genuinely different accounts of how life could have begun. They can also say exactly what evidence would knock each one down.
Two Accounts That Are Not Rivals
A 2020 review by Longo and Damer argues that the two leading ideas have been compared as competitors when they are not. They describe different stages of the same problem.
The first is the submarine hydrothermal vent hypothesis. Life begins on the seafloor, at vents where seawater is heated by magma and pushed back out.
The second is the hot spring hypothesis. Life begins in warm freshwater pools on land, built from organic molecules delivered from space and from the ground.
The vents account is mostly about making the raw material. Its job is to get simple organic molecules to form and concentrate. The hot springs account is mostly about assembly. Its job is to get those molecules into compartments that copy and compete.
That is why they can both be right and still look like opposites. The environments are so different that moving the chemistry of one into the other would probably break it.
The Vent Account, Step by Step
The building material comes from a reaction called serpentinisation, which happens when hot, alkaline vent fluid meets the rocky seafloor.
The vent fluid carries dissolved carbon dioxide, hydrogen sulfide, nitrogen, and hydrogen. Together with water, those supply five of the six elements biochemistry needs. The sixth, phosphorus, arrives more slowly, from rock weathering on land.
This chemistry can produce ammonia, methane, pyruvate, and other simple organic molecules. Those are the ingredients life is later assembled from.
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| A black smoker hydrothermal vent. Seawater circulates, is heated by magma, and rises through the chimney at 360 to 400 degrees Celsius. Image: W.R. Normark, NOAA, public domain, via Wikimedia Commons |
Vents Come in Two Types
Black smokers are the dramatic ones. They form where seawater meets a magma chamber, and vent sulfur-rich fluid at 360 to 400 degrees Celsius.
White smokers sit off to the side of the ridge and vent much cooler fluid, between 40 and 90 degrees. They are also highly alkaline, with a pH between 9 and 11.
Longo and Damer note that white smokers are generally considered the more promising of the two. The temperature and pH there are closer to what these reactions need.
Where the Vents Version Is Weakest
The hardest step is joining small molecules into long chains. Joining them means losing water, and in salty water that reaction fights you the whole way.
This is described in the review as the largest remaining gap in the timeline. Vents may solve the supply problem and still have trouble with the assembly problem.
The Hot Spring Account
The hot spring version starts from organics that arrive from outside Earth. Longo and Damer rule out falling meteorites as the main supply, because those organics would be diluted to useless concentrations in the ocean.
Instead the account focuses on ponds on land. Ponds that repeatedly fill, dry out, and refill give molecules a rhythm. Wet and dry cycling concentrates some of them and washes others away.
The idea is that compartments containing partially copied polymers would differ from each other. The ones that copied better would tend to survive and reproduce more. Selection would do the rest.
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| Microbial mats color the runoff at Pendant Spring, Yellowstone. Modern hot springs are studied as possible analogues for the early Earth. Image: James St. John, CC BY 2.0, via Wikimedia Commons |
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| The geyserite rim and microbial mats at Morning Glory Pool. Warm water flows out, cools, and moves through wet and dry phases. Image: James St. John, CC BY 2.0, via Wikimedia Commons |
How Each One Could Be Proved Wrong
This is the part worth remembering, and it is the part most often left out.
The vent hypothesis could be ruled out by showing that it cannot work thermodynamically. If carbon cannot be locked into high enough concentrations of the starting materials in real vent conditions, the account fails.
The hot spring hypothesis could be ruled out by showing that the available molecules never assemble into compartments in real pools. If meteoritic, atmospheric, and geothermal supply cannot produce compartments that undergo selection, the account fails.
Both of those are experiments someone could run. A hypothesis you cannot break is not science, and these two can be broken.
What the Rocks Actually Show
The evidence for early life comes from rock, and it is thinner than you might expect.
A 2016 review by Knoll and Bergmann reports sedimentary and chemical evidence for life at 3500 million years ago, and possibly earlier. Beyond that, phylogenetic and functional detail is limited.
The reason is straightforward. Few rocks that old survive, and those that do have usually been heated or altered beyond holding traces.
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| A stromatolite from the Dresser Formation, 3.48 billion years old. These are among the oldest known traces of life on Earth. Image: James St. John, CC BY 2.0, via Wikimedia Commons |
What a Stromatolite Is
A stromatolite is a layered rock built by microbes. Knoll and Bergmann describe modern examples where the process is still linked to the pattern in the rock. Mats of cyanobacteria trapped fine sediment and precipitated carbonate, layer on layer.
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| Stromatolite layers from the Strelley Pool Formation. Microbes built these by trapping sediment and precipitating minerals layer on layer. Image: James St. John, CC BY 2.0, via Wikimedia Commons |
Evidence That Is Not a Fossil
Some of the strongest evidence is chemical rather than visible. Organic carbon in Archean rock has a carbon isotope ratio of about -30 to -40 parts per thousand, sitting next to carbonate at zero.
That offset is what biology leaves behind. Photosynthetic organisms prefer carbon-12, so the carbon that gets locked into organic matter is lighter than the carbon in the carbonate around it.
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| Microscopic fossils in rock. These are among the few traces that survive from the earliest part of Earth history. Image: Hannes Grobe, CC BY 3.0, via Wikimedia Commons |
The Change That Transformed Everything
Life spent most of its early history without oxygen. Knoll and Bergmann report that the permanent oxygenation of the atmosphere and surface ocean happened around 2400 million years ago.
Before that, Archean rock records what they call the first iron age. Reduced iron was the main electron donor for photosynthesis. Iron was the main electron acceptor for respiration, and it was a key part of proteins.
Almost every metabolic strategy early life used was built around iron and oxygen-poor conditions. When oxygen finally became permanent, half of Earth history was already over. Modern marine ecosystems only then began to take shape.
Why the Answer Is Not Only About Earth
NASA frames astrobiology as understanding how we get from star stuff to life. It notes that water is the second most common molecule in the universe.
That matters for how seriously these questions are taken. If life is a natural result of chemistry in the right conditions, then finding life on Europa or Enceladus would not be a curiosity. It would be a measurement, telling us how common the process is.
It is also why the two hypotheses are site-specific. Hot springs do not exist under the ice of the outer moons. And the review notes that vents have never been found in the northern ocean Mars is thought to have once had.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means here |
| Abiogenesis | Life arising from non-living matter. |
| Prebiotic chemistry | The chemistry that runs before anything alive is using it. |
| Serpentinisation | A reaction between hot alkaline vent fluid and rock that produces simple organic molecules. |
| Protocell | A membrane-bounded compartment containing polymers, before anything self-reproducing. |
| Stromatolite | A layered rock built by microbial mats, trapping sediment. |
| Microfossil | A microscopic fossil, usually of a single cell. |
| Isotope ratio | The relative amounts of isotopes, used to detect biological activity in ancient rock. |
| Black smoker | A hydrothermal vent venting fluid at 360 to 400 degrees Celsius. |
| Falsification | The evidence that would prove a hypothesis wrong. |
The words in this article that would otherwise send you to a dictionary.
Key Takeaways
- Nobody knows how life began. Saying so is more useful than pretending otherwise.
- The vent and hot-spring accounts are often called rivals but answer different halves of the same problem.
- Both have stated conditions that would disprove them. That is what makes them testable rather than story-shaped.
- The evidence for early life is chemical as much as fossil. Rocks from 3500 million years ago record life, with little detail beyond that.
- Life spent most of its early history without oxygen, using iron where we would use oxygen.
- Water is common in space, so the question is not only about Earth.
Frequently Asked Questions
Which hypothesis do scientists think is right?
There is no settled answer. Researchers often favor one locally, but the review this article is built on argues the two may not be answering the same question.
How old is the oldest evidence of life?
Sedimentary and chemical evidence reaches 3500 million years ago, and possibly older. Knoll and Bergmann report that detail beyond that point is limited.
Do we know what the first living thing looked like?
Not directly. A 2016 review notes that animals are evolutionary latecomers occupying distant branches of a tree mostly made of microbes. The earliest forms left almost nothing behind.
Could life have started more than once?
Nothing in the evidence rules it out, and nothing requires it. If life arose independently more than once, the second origin would probably share little with the first, and might not be recognised.
Why did the Miller-Urey experiment not settle this?
It did show that organic molecules can form from simple gases under the right conditions, which mattered. But building amino acids is not the same as building a self-copying system, and that is where the hard problem still sits.
Key references
• Longo and Damer 2020: origin-of-life hypotheses
• Knoll and Bergmann 2016: life, the first two billion years
• NASA: Astrobiology research
Images: Wikimedia Commons, with the author and license named in each caption. No hypothesis is presented as settled here, because none is. The two leading ones are described as answering different parts of the question rather than competing.
