The Origin of Life

Four and a half billion years ago the Earth was a ball of molten rock under a sky of steam and carbon dioxide, hammered by leftover debris from the formation of the solar system. Within a few hundred million years, by 3.7 or perhaps 4.1 billion years ago, something on it was alive. How a planet of chemicals produced a cell that could feed, grow and copy itself is one of the great unanswered questions in science. It is not, however, a question without clues.

The ingredients were everywhere

Life is built from a surprisingly short list of parts: amino acids, which link into proteins; nucleotides, which link into DNA and RNA; sugars; and fatty molecules that form membranes. In 1953 a graduate student named Stanley Miller sealed water and a mixture of gases in a flask, passed electric sparks through it to imitate lightning, and within a week found amino acids in the water. Later experiments, with different gases and different energy sources such as ultraviolet light, produced nucleotides and sugars too.

Nature does the same chemistry without a laboratory. Meteorites that fell to Earth contain dozens of amino acids and the bases of DNA. Radio telescopes detect sugars and amino acid precursors in the gas clouds where stars are born. The building blocks of life are not rare or special; they are what carbon chemistry does given energy and time. The early Earth was delivered such molecules by the tonne and made more of its own.

From parts to a machine

Having bricks is not having a house. The hard problem is how the molecules organised into something that could make copies of itself, because copying is where evolution begins. Modern cells use DNA to store information and proteins to do the work, but DNA cannot copy itself without proteins, and proteins cannot be made without DNA: a chicken-and-egg problem.

The leading solution is RNA. RNA is a cousin of DNA that still carries information in cells today, but it can also fold into shapes that act as catalysts, speeding up chemical reactions the way proteins do. Molecules of RNA have been made in the laboratory that can copy stretches of other RNA. In an "RNA world", the first replicators might have been RNA strands that stored their own instructions and did their own chemistry, with proteins and DNA taking over those jobs later because they did them better.

Where it happened

Darwin imagined a "warm little pond". Modern candidates are more dramatic. One is the deep-sea hydrothermal vent, where hot mineral-rich water pours from the seafloor into cold ocean. Vents supply a steady flow of chemical energy, the minerals in their walls catalyse reactions, and their porous rock forms countless tiny chambers in which molecules could concentrate. Some of the core chemistry of modern cells still resembles the chemistry that happens naturally in vent minerals.

Another candidate is hot springs and shallow pools on land, which repeatedly dry out and refill. Drying concentrates molecules and encourages them to join into chains; wetting lets them mix. Volcanic islands on the early Earth offered plenty of both. Fatty molecules in such pools spontaneously form tiny bubbles, membranes that could have enclosed the first replicators.

The common ancestor

Whatever happened, it happened once, or at least only one lineage survived. All life uses the same genetic code, the same twenty amino acids, the same energy currency and the same kind of membrane. Comparing the genomes of bacteria, archaea and complex cells, biologists have reconstructed features of the Last Universal Common Ancestor, LUCA, which lived roughly four billion years ago. It appears to have been a heat-loving microbe that lived on hydrogen and carbon dioxide, consistent with a home in hot, mineral-rich water. It was not the first living thing, but it is the one from which everything descends.

Reading the oldest rocks

The oldest convincing fossils are stromatolites, layered mounds built by mats of microbes, from rocks in Australia 3.5 billion years old. Microbes are still building identical mounds in Shark Bay today. Possible chemical traces of life appear in rocks up to 3.7 billion years old in Greenland and perhaps older still in Canada, though these are debated. Earth's crust has been so thoroughly recycled that almost nothing older survives, so the first few hundred million years of life's history may be gone for good.

What we still don't know

We do not know the sequence of steps, the location, or whether the process was inevitable or a fluke. We do not know whether life began more than once and only one line survived. And we have no second example to compare with. If microbes, living or fossil, are found on Mars, or in the oceans of Europa or Enceladus, we will learn more about how life begins than from any experiment on Earth.