9. Why was it in the oceans that life first emerged?

Most life forms initially evolved in marine habitats, as the oceans provide nearly 90% of the planet’s habitable surface. The earliest forms on the planet were prokaryotes, simple single-celled organisms without a defined nucleus. These first signs of life appeared about 3.5 billion years ago in primitive seas and oceans. The oldest prokaryotes are cyanobacteria, which carried out photosynthesis and released oxygen into the atmosphere. This represented an immense advance on the path to higher life.