Kenorland prehistoric landscape. Artwork showing a landscape at the time of the Kenorland supercontinent (2.7 to 2.1 billion years ago) during the Archean Era and Proterozoic Era. Life had emerged on Earth in the oceans, but consisted only of simple cells (prokaryotes) and biofilms of micro-organisms such as cyanobacteria, forming structures called stromatolites (bottom). These produced oxygen (bubbles) by photosynthesis, leading to the oxygenation of the Earth's atmosphere around 2.4 billion years ago. The land was barren with no life, and irradiated by ultraviolet light as the ozone layer had not yet formed.