How did life begin?
Somewhere on the young Earth, chemistry became biology. We still don't know how.
The question
Every living thing we know comes from another living thing. Yet about four billion years ago there was no life on Earth, and then there was. At some point, molecules that were simply following the laws of chemistry turned into something that could copy itself, use energy and evolve.
In the 1860s Louis Pasteur showed that life does not arise spontaneously in a flask of broth. That settled the question for today's world but sharpened it for the past: if life only comes from life, how did the first life come about? In 1924 Alexander Oparin, and soon after J. B. S. Haldane, proposed that it arose step by step from simple chemistry in the early oceans. A century later we can name many of the steps, but not the whole path.
What we know
- Earth formed about 4.5 billion years ago. Fairly convincing traces of life, layered structures built by microbes, are about 3.5 billion years old, and some researchers argue for older signs. Life seems to have appeared relatively soon after the planet became habitable.
- All life today shares one genetic code, the same basic molecules and the same core chemistry. It descends from a last universal common ancestor, which was already a complex cell. Some recent analyses place it more than four billion years ago.
- The building blocks form easily. In 1953 Stanley Miller passed sparks through a mix of gases and water and obtained amino acids. Amino acids and other organic molecules have been found in meteorites and in samples brought back from asteroids.
- RNA can both store information, like DNA, and speed up chemical reactions, like proteins. In 2009 John Sutherland's group showed how some RNA building blocks could form under plausible early-Earth conditions, by a route nobody had expected.
- Simple fatty molecules spontaneously form membranes and bubbles that can grow and divide. Researchers have built such "protocells" that hold RNA inside.
Main hypotheses
The RNA world
Before DNA and proteins, life may have relied on RNA alone: molecules that copied themselves and catalysed their own chemistry. This is the most widely held framework. Its problem is that nobody has yet made an RNA molecule that copies itself well enough to evolve.
Metabolism first
Perhaps self-sustaining cycles of chemical reactions came first and genetic molecules later. Deep-sea hydrothermal vents, with natural gradients of energy and minerals that act as catalysts, are a favourite candidate setting.
Warm little ponds
Darwin's old idea in a modern form: shallow pools on land where repeated drying and wetting concentrates molecules and helps them join into chains, something hard to achieve in the open ocean.
Systems chemistry
Perhaps no single class of molecules came first. Nucleic acids, proteins and membranes may have formed together, from shared chemistry, and supported each other from the start.
Where understanding ends
We can make many of the parts. What we can't do is get from parts to a system that begins to evolve by itself. The gap lies between a chemical mixture and the first thing to which Darwinian evolution applies — and we don't know where that threshold was or what crossed it first.
There are smaller puzzles inside it. Life uses molecules of only one "handedness", left or right, while chemistry produces both equally; how the choice was made is unclear. The chain of reactions that leads to RNA needs different conditions at different steps, and it isn't clear how one place on the early Earth could provide them all.
And the history is almost erased. Rocks older than about four billion years barely survive, so we may never find direct traces of the transition. With only one example of life, we also can't tell whether its origin was nearly inevitable or an extraordinary accident.
What would settle it
- A chemical system made in the laboratory, from plausible early-Earth materials, that copies itself with variation and begins to evolve.
- Geological evidence of the conditions on the early Earth and of the first stages of life.
- Life found elsewhere, on Mars, the moons of Jupiter or Saturn, or beyond. A second, independent origin would show whether life arises easily, and comparing the two would show what is necessary and what is chance.
Sources
- Miller, S. L. (1953). A production of amino acids under possible primitive Earth conditions. Science, 117(3046), 528–529.
- Gilbert, W. (1986). Origin of life: The RNA world. Nature, 319(6055), 618.
- Szostak, J. W., Bartel, D. P., & Luisi, P. L. (2001). Synthesizing life. Nature, 409(6818), 387–390.
- Martin, W., & Russell, M. J. (2003). On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 358(1429), 59–85.
- Powner, M. W., Gerland, B., & Sutherland, J. D. (2009). Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions. Nature, 459(7244), 239–242.
- Weiss, M. C., et al. (2016). The physiology and habitat of the last universal common ancestor. Nature Microbiology, 1(9), 16116.
- Moody, E. R. R., et al. (2024). The nature of the last universal common ancestor and its impact on the early Earth system. Nature Ecology & Evolution, 8(9), 1654–1666.