The origin of life

Life runs on a few molecules. DNA stores the instructions. Proteins do the work. But to copy DNA you need proteins, and to build proteins you need DNA. So how did either one get started, when each seems to need the other already in place?

The old picture is a soup. On the early Earth, simple gases and water, lit by sunlight and lightning, slowly brewed a broth of organic molecules, and somewhere in it the first living thing assembled itself. The famous version is a 1950s experiment that sparked a flask of basic gases and produced amino acids, the parts of proteins, out of nothing living at all.

The real story is messier, and the soup was probably not the whole of it. The leading idea today is that neither DNA nor protein came first. A third molecule, RNA, did both jobs at once, storing information and speeding up reactions, and DNA and proteins were added later. DNA, the thing the headline credits, was almost certainly the last piece to arrive, not the first.

Nobody has built life from scratch, so the origin of life is still open. What we do know is that the basic ingredients form easily, that membranes assemble themselves, and that it all happened remarkably fast once the planet had cooled.

The chicken-and-egg problem is real. In every cell alive today, DNA is read and copied by proteins, and those proteins are built from instructions held in the DNA. Each depends on the other, so neither is a plausible first step on its own.

The soup idea came from Alexander Oparin and J.B.S. Haldane in the 1920s: an early Earth where organic molecules built up in the oceans, fed by sunlight and lightning, until the mixture grew rich enough for life. In 1953 Stanley Miller put it to the test. He sparked a sealed flask of methane, ammonia, hydrogen and water, and within days it held amino acids. Building blocks of life, made from plain chemistry. One caveat has aged into the result: the early atmosphere was probably not as rich in those gases as Miller assumed, though volcanic settings could have supplied them locally, and later reruns of his experiment turned up an even wider haul of molecules.

The way out of the chicken-and-egg trap is a molecule that does both jobs. That is RNA. It can carry information like DNA, and it can fold up and catalyze reactions like a protein. This stopped being speculation when Thomas Cech and Sidney Altman found ribozymes, RNA molecules that act as enzymes. The clincher is that the ribosome, the machine every cell uses to build proteins, has RNA at its catalytic heart. The protein factory is itself run by RNA, exactly what you would expect if RNA came first. This is the RNA world, named by Walter Gilbert in 1986.

Making RNA from scratch was the sticking point for decades, because its parts do not join easily under natural conditions. In 2009 John Sutherland's group found a route that sidesteps the problem, building RNA's units from simple feedstocks such as hydrogen cyanide, with sunlight as the energy source, the same chemistry that can also yield precursors of amino acids and lipids.

Two questions stay wide open. Where did it happen? Some favor warm little ponds, with cycles of wetting and drying that push small molecules to link into chains. Others favor hydrothermal vents on the sea floor, where natural chemical gradients across mineral walls could have powered an early metabolism. And which came first, the genes or the metabolism? One camp says a self-copying molecule like RNA started everything. The other says a self-sustaining network of reactions came first and genetics was bolted on later. Through all of it, fatty molecules help. Drop them in water and they assemble themselves into hollow bubbles, ready-made compartments. Jack Szostak has shown such bubbles can grow, split, and hold RNA inside: the rough beginnings of a cell.

Why DNA came last. DNA is RNA with two upgrades for stability: the sugar loses one oxygen (deoxyribose), and the base uracil is replaced by thymine, which makes a common kind of chemical damage easier to spot and repair. On top of that the molecule pairs into a double helix that shields the information inside. Those are the features you want in a long-term archive and the wrong ones for a reactive, do-everything molecule. The natural reading is that life first ran on RNA, then handed catalysis to proteins, which are better at it, and finally copied its master record onto DNA for safekeeping. The enzyme reverse transcriptase, which writes DNA from RNA, and the fact that cells still make DNA's building blocks by modifying RNA's, are fossils of that handover.

Energy first. A cell is not only information, it is a controlled flow of energy. Every living thing powers itself with ion gradients across membranes, a mechanism called chemiosmosis. Mike Russell and Nick Lane have argued that alkaline hydrothermal vents come with natural proton gradients across thin mineral compartments, so the energy machinery could predate any genetics, the first life essentially plugging into a geological battery. This is the strongest version of the metabolism-first view, and it ties the origin of life to the way cells still run today.

LUCA is not the start. Comparing the genomes of all living things lets us reconstruct their Last Universal Common Ancestor. The picture that emerges is of an organism that was already complex, with a genetic code, protein synthesis, and a carbon-fixing metabolism using hydrogen, plausibly living in a hydrothermal setting. LUCA already had DNA and the full DNA, RNA and protein system. It sits a long way downstream of the actual origin, so everything above happened before it.

The timing. Earth formed about 4.5 billion years ago. The oldest widely accepted traces of life, layered microbial structures called stromatolites, are around 3.5 billion years old, and contested chemical hints reach further back, toward 3.8 to 4.1 billion years. Either way, life appeared within a few hundred million years of the planet becoming habitable, which is fast, and is sometimes read as a sign that the steps involved are not wildly improbable.

What is still missing. We have many of the pieces in isolation. Building blocks form readily. RNA can store and catalyze. Membranes self-assemble. Vents and ponds offer energy and a way to concentrate ingredients. What no one has done is join them into a continuous, demonstrated path from plain chemistry to a compartment that copies itself and evolves. There is no agreed line where chemistry ends and life begins, no fossil of the transition, and no synthetic cell grown from scratch. The soup, in other words, is a real and useful idea. It is the opening line of the story, not the conclusion.