How life's building blocks formed

Today, every new cell comes from a cell that already exists. That raises an obvious puzzle: where did the very first cell come from? Biologists can’t watch it happen, but they can test which steps are chemically possible.

The cell theory

The cell theory sums up what biologists know about cells in three statements:

1. All living things are made of one or more cells.
2. The cell is the smallest unit that is alive.
3. New cells arise only from cells that already exist, by cell division.

Part 3 holds on modern Earth, but the first cell had no parent cell to come from.

Four overlapping stages

Modern cells depend on three kinds of large molecules. DNA stores genetic information, RNA helps turn that information into proteins (and does other jobs), and proteins do most of the cell’s work. DNA and RNA are chains of small units called nucleotides, and proteins are chains of small units called amino acids. So researchers picture life starting in four stages that overlapped in time:

Stage 1: molecules before life

Molecules built around carbon, like those in living things, are called organic molecules. Making them without the help of living cells is prebiotic synthesis (“before life”), also called abiotic synthesis (“without life”).

On today’s Earth, such molecules wouldn’t last long. Oxygen in the air would react with them and break them down, and living things would eat them. Early Earth had almost no free oxygen gas and no organisms, so organic molecules could slowly build up in the oceans. The result was a watery mix called the prebiotic soup.

Scientists agree that life arose from nonliving chemicals, but they debate where the first organic molecules came from. Three hypotheses get the most attention, and more than one could be true.

The reducing atmosphere hypothesis

Building organic molecules from simple ones requires redox reactions, in which electrons pass from one molecule to another. Methane and ammonia hand off electrons easily. An atmosphere rich in them, and lacking oxygen, is called a reducing atmosphere, because it pushes electrons onto other molecules.

The Miller–Urey experiment tested this idea by sealing those gases in glass with water vapor and zapping the mix with sparks to mimic lightning. Within a week, it produced simple starting materials (hydrogen cyanide and formaldehyde), which combined into amino acids such as glycine. Later runs made sugars, lipids, and adenine, one of the bases in DNA and RNA.

Critics note that early Earth’s strong ultraviolet light destroys methane and ammonia, and that volcanic gases are mostly CO₂ and N₂. Still, reducing pockets may have existed, and organic molecules can also form from neutral gas mixtures.

The extraterrestrial hypothesis

Some meteorites, called carbonaceous chondrites, carry amino acids and nucleic acid bases, so meteorites could have seeded early Earth with organic molecules. Some of that cargo does reach the ground; how much the heat of the trip destroys is still argued over.

The deep-sea vent hypothesis

Deep-sea vents are cracks in the ocean floor that release gases hotter than 300 °C, along with water rich in metal ions and hydrogen sulfide (H₂S). Where hot vent water meets cold seawater, the temperature drops sharply over a short distance, and organic molecules could have formed along this gradient. At alkaline vents, the gradient is instead between alkaline vent water and more acidic seawater. That gradient can drive the formation of organic compounds such as pyruvate and of amphipathic molecules (a water-loving end and a water-avoiding end) that could have formed early membranes.

Support comes from today’s vents, which host animals and bacteria that run on vent chemicals instead of sunlight, and from fossils of ancient microbes found near old vents.

Stage 2: linking units into chains

A polymer is a long chain made by linking many small units, called monomers. To carry enough information for a working genetic system, a polymer probably needed at least 30–60 monomers.

Water makes this hard. Linking two monomers releases a water molecule, and the reverse reaction, hydrolysis, uses water to break the link. In an open soup, hydrolysis competes with chain-building. So many scientists think chains formed on solid surfaces, such as clay, or in evaporating tidal pools.

Experiments have produced both RNA chains and short protein chains on clay. Chains can also form in water, though: a simple volcanic gas, carbonyl sulfide, links amino acids under mild conditions, so the soup itself may have worked.

Check yourselfWhat made deep-sea vents a good place for making organic molecules before life existed?

Vents release very hot gases and mineral-rich water into cold seawater. The steep gradient between hot and cold water (or, at alkaline vents, between alkaline and acidic water) supplies energy that can drive the formation of organic molecules.