4.1 The Origin of Life

Life as we know it is built from organic molecules (compounds containing carbon, such as amino acids and nucleotides). These are the building blocks for proteins and nucleic acids (DNA and RNA).

In 1953, chemists Stanley Miller and Harold Urey conducted a landmark experiment to test whether these molecules could form under early Earth conditions. They created a closed system filled with gases thought to be present in Earth’s early atmosphere—methane, ammonia, hydrogen, and water vapor—and introduced electrical sparks to simulate lightning. After just a few days, they found that amino acids had formed spontaneously.

This experiment showed that organic molecules can arise naturally from simple chemicals when given the right conditions. Since then, similar experiments have produced sugars, lipids, and even nucleotides. On early Earth, these organic compounds would have accumulated in the ocean where they could mix and react over time. This is often referred to as primordial soup because it contained all of the ingredients for life.

Scientists have proposed other hypotheses for how organic molecules may have formed. These include deep-sea hydrothermal vents, where mineral-rich water and heat could drive chemical reactions, and extraterrestrial delivery, where organic compounds may have arrived on Earth via meteorites and comets.

Organic molecules alone aren’t enough. Life requires macromolecules (large, complex molecules like proteins and nucleic acids) to perform essential functions such as catalyzing reactions and storing genetic information.

But how did small molecules link together into these larger structures? Scientists believe that natural catalysts may have played a key role. For example, clay minerals can attract and concentrate organic molecules on their surfaces. In lab experiments, researchers have shown that nucleotides can spontaneously link into short RNA chains when dripped onto certain types of clay.

Another important factor may have been the rhythmic action of waves on early Earth. As waves splashed onto hot rocks or mineral surfaces the organic molecules ftom the primordial soup would have the opportunity to interact. As the water evaporated in the heat, molecules became more concentrated and were more likely to bond.

Experiments simulating these conditions have successfully produced all of the major macromolecules in the lab, supporting the idea that simple environmental processes could have driven the assembly of life’s first functional molecules.

The cell membrane serves as a boundary that separates a cell’s internal environment from the outside world. A key step in the origin of life likely involved enclosing organic molecules within a simple membrane, forming structures known as protocells. These weren’t true cells, but they exhibited some life-like properties.Protocells likely formed spontaneously from fatty acids which can naturally assemble into spherical vesicles in water. These vesicles could trap molecules like RNA and other macromolecules inside of them and create a protected space where chemical reactions could occur more efficiently.

Protocells made in lab have been shown to mimic early cell-like behavior, including growth and even division.

A defining feature of life is the ability to reproduce (to make copies of itself). In modern cells, this process depends on DNA to store genetic information and proteins to carry out the work of copying and expressing that information.

But this raises a fundamental question: how did the first living systems reproduce before DNA and proteins existed? This is often referred to as a “chicken-and-egg” problem. DNA needs proteins to be copied, but the instructions to make those proteins are stored in DNA. So which came first?

One possible solution lies in RNA. What makes RNA special is that some types can do two jobs: they can store genetic information and help chemical reactions happen. Some RNA molecules can even help copy other RNA molecules.

This means that early life may have relied on RNA molecules that could both store information and help copy themselves. Early RNA wasn’t perfect and likely made lots of mistakes when copying. Over time, the RNA strands that were better at making accurate copies would have increased in number.