The Origin & History of Life

Earth is about 4.5 billion years old, and evidence shows life has existed for around 3.5 billion years, diversifying ever since. The oldest widely-accepted fossils are of cyanobacteria (as stromatolites) from Western Australia.

The first cells

Membranes may have formed spontaneously, and the first cells may have used RNA as their genetic material — with ribozymes (RNA molecules that act as enzymes) doing early catalysis. This is the 'RNA world' idea.

Prokaryotes first, then endosymbiosis

Fossil evidence shows prokaryotes existed before eukaryotes. The endosymbiotic theory explains how eukaryotes gained organelles: a larger cell engulfed free-living prokaryotes that became mitochondria and chloroplasts — which is why both still have their own DNA and double membranes.