Primitive bacteria. Artwork illustrating how early life forms from the Proterozoic era (around 2.5 billion to 543 million years ago) may have looked. At centre left is a cut-away through a unicellular prokaryotic bacteria cell. Prokaryotes are organisms without a cell nucleus. Their appearance preceded more complex organisms, the eukaryotes (algae, plants, fungi, animals and protists). The column at right is a colony of cyanobacteria. Cyanobacteria are able to perform photosynthesis and are thought to be responsible for the dramatic increase in oxygen levels in the Earth's atmosphere which enabled new forms of life to evolve.