7.13 Origin of Life on Earth

From simple to living

Scientists hypothesize life arose through stages: simple organic monomers formed from inorganic molecules on early Earth, joined into polymers, became enclosed in membrane-bound droplets (protobionts / protocells), and eventually gained self-replication. The Miller-Urey experiment showed organic molecules can form from simple gases under early-Earth conditions.

The RNA world

The RNA world hypothesis proposes that RNA — not DNA or protein — was the first genetic material, because RNA can both store information and act as an enzyme. RNA enzymes are called ribozymes. The earliest cells were likely simple prokaryotes, appearing around 3.5 billion years ago.

Key terms

Origin of life (order)

Monomers → polymers → membrane-bound protocells → self-replication.

Miller-Urey experiment

Showed organic molecules can form from simple gases under early-Earth conditions.

RNA world hypothesis

RNA — able to store information and catalyze reactions — was likely the first genetic material.

Ribozyme

An RNA molecule that can act as an enzyme.

Protobiont / protocell

An early membrane-bound droplet of molecules, a possible step toward cells.

Try an AP-style question

Which hypothesis did this support?

1. Life on Earth began from living cells carried in by meteorites
2. Free oxygen was abundant in Earth's early atmosphere
3. Amino acids can be made only by enzymes inside living cells
4. Organic molecules could form from inorganic molecules under early-Earth conditions