Life's origin: what we know and what we don't

How did chemistry become biology? This is one of the few big questions where honest scientists answer "we do not know". But that answer hides a lot of structure. Some parts are known (measured, repeated), some are hypotheses (coherent stories with partial support) and some are unknown. In this post every claim carries one of those three labels.

In The universe in one year: a cosmic calendar the Earth forms on 2 September and clear traces of life exist by 30 September. That is about 28 calendar days, or a billion years. This post zooms into that month.

A common working definition, used by NASA, is that life is a self-sustaining chemical system capable of Darwinian evolution. Every living cell has three parts: a boundary (a membrane), a metabolism (energy flowing through chemistry) and heredity (information that can be copied). Evolution, as described in Five ideas that built modern science, starts to work once heredity exists. The origin of life is the earlier question of how those three parts came together. Pasteur's flask experiments of the 1860s showed that life today comes only from life, so the first cell was a one-off event in the deep past.

Known. The Earth formed 4.54 billion years ago. Tiny zircon crystals from Jack Hills in Western Australia are up to 4.4 billion years old, and their oxygen isotopes suggest they formed in contact with liquid water (an interpretation that is still debated).

Known, with debate. The firmest traces of life are stromatolites, layered mounds built by microbial mats, in the Pilbara of Western Australia: the Dresser Formation (about 3.48 billion years) and Strelley Pool (3.43). Most specialists accept them, though layered rock can form without life, and the 1993 claim of 3.46-billion-year-old "microfossils" in the Apex chert was later challenged as mineral artefacts.

Contested. Carbon with a life-like isotope signature and structures claimed to be stromatolites in Isua, Greenland (3.7 to 3.8 billion years; the structures were disputed in 2018), and a carbon flake inside a 4.1-billion-year-old zircon (2015) that may be contamination or not biological.

So: life by about 3.5 billion years ago is solid, 3.7 to 3.8 is possible, and anything older is speculation. The window between the first likely oceans and firm evidence is up to 900 million years, but it says nothing about how long the origin itself took. It could have been quick.

In 1952 a 22-year-old graduate student, Stanley Miller, worked with Harold Urey at the University of Chicago to test an idea from Oparin and Haldane in the 1920s: that sunlight and lightning on a primitive atmosphere could make the molecules of life. Water, methane, ammonia and hydrogen went in; sparks stood for lightning. After a week the water was red-brown and held amino acids (the paper appeared in Science on 15 May 1953). In 2008, a reanalysis of samples Miller had stored from a 1958 version of the experiment, run with hydrogen sulfide, found more than 20.

Known: organic molecules central to life form easily from simple ingredients plus energy. Revised: the early atmosphere was probably less rich in methane and ammonia (more carbon dioxide and nitrogen) than Miller assumed, which lowers yields, so his exact recipe is doubtful. The conclusion survives because nature makes them elsewhere. The Murchison meteorite, which fell in Australia in 1969, holds more than 70 amino acids. Samples from the asteroids Ryugu and Bennu, returned to Earth in 2020 and 2023, contain amino acids and nucleobases, the "letters" of RNA and DNA; the 2025 Bennu results report 14 of the 20 protein amino acids and all five bases.

Unknown: why life uses only left-handed amino acids. Laboratory syntheses make a 50-50 mix, and some meteorites show only a small excess of one hand. And building blocks are not life: a heap of bricks is not a cathedral.

The problem. In cells, DNA stores instructions and proteins do the work, but copying DNA needs proteins and making proteins needs DNA. Which came first? RNA can do both jobs. In the early 1980s Thomas Cech and Sidney Altman found RNA molecules that act as enzymes (Nobel Prize 1989), and the core of the ribosome, the machine that builds every protein, turned out to be RNA (structures solved around 2000, Nobel Prize 2009). Walter Gilbert named the idea the RNA world in 1986.

Hypothesis, with real pieces of support. In 2009 John Sutherland's group made the activated building blocks of RNA from simple compounds by a route that avoids older dead ends, and in 2015 a network driven by ultraviolet light produced precursors of RNA, proteins and fats together. Evolved ribozymes can copy short stretches of RNA and, in a 2009 experiment by Lincoln and Joyce, a pair of them assembled each other from ready-made pieces. But none has copied itself from single letters.

Problems. RNA is fragile in water, strands stick to their copies, and copying errors pile up. On the compartment side, Jack Szostak's group showed that bubbles of fatty acids grow and divide by themselves, and that RNA trapped inside can be copied a little without enzymes. Alternatives treat metabolism first (energy-releasing chemistry, perhaps on mineral surfaces, before genes) or compartments first, and many researchers expect the truth to be a mixture.

In 1871 Darwin wrote to the botanist Joseph Hooker about a "warm little pond" with ammonia, phosphates, light, heat and electricity in which a protein might form. Ocean-floor vents offered a rival after 1977, when scientists found life thriving around them, and in 1979 hot "black smokers" at about 350 degrees Celsius.

The vent case rests on Lost City, found in 2000 near the Mid-Atlantic Ridge: warm, alkaline towers up to 60 m tall, powered by water reacting with mantle rock (the ridge machinery of Plate tectonics: the moving ground). Hydrogen and methane flow through porous walls, creating a natural chemical gradient that is strikingly like the one cells use to make energy. The pond case rests on wet-dry cycles that concentrate and join molecules, and on ultraviolet chemistry that makes RNA precursors. Both are hypotheses; neither has produced anything alive. Which is more likely is unknown, and a mixture of sites is possible.

LUCA is the Last Universal Common Ancestor: the population from which all living cells descend. It is not the first life. Its relatives left no descendants. We infer it because every cell shares the same genetic code, the same ribosome and the same energy currency. In 2016 Madeline Weiss and colleagues traced 355 gene families back to it and pictured a microbe living on hydrogen and carbon dioxide without oxygen, possibly near vents. In 2024 Edmund Moody and colleagues, using molecular clocks, put LUCA at about 4.2 billion years ago, with roughly 2,600 proteins and even a primitive immune system. This is a hypothesis and many experts doubt it, as it would mean a complex cell only some 300 million years after the Moon formed. Estimates of this kind carry wide error bars.

That last question is limited by having only one example. Every living thing we know shares one ancestor. If we find a second origin, on Mars, under the ice of Europa or Enceladus, or in a lab, the problem changes completely. Until then, the best summary is an honest one. The parts list is largely known, the assembly instructions are not, and the next 3.5 billion years, which produced everything in Nature as engineer: what nature teaches us, are the easy part by comparison.