The cyanobacteria and archaebacteria of the primitive earth are now referred to as prokaryotes (together with the modern bacteria). Approximately 1.5 billion years ago, in an oxygen‐containing atmosphere, the first eukaryotes came into being. Eukaryotes have a nucleus, a nuclear membrane, a number of organelles, a ribosomal structure different from that of prokaryotes, reproduction by mitosis, and other features that distinguish them from prokaryotes.
No one is certain how eukaryotes came into being. The endosymbiotic theory suggests that bacteria were engulfed by larger cells. The bacterial cells remained in the cell, assumed some of the chemical reactions for these cells, and became the mitochondria of these cells. The cells then reproduced and flourished. These cells became animal cells.
An extension of the endosymbiotic theory refers to plants. In this case, pigmented bacteria, such as the cyanobacteria, were engulfed by larger cells. The cyanobacteria remained in the cells and became the chloroplasts of these cells. Photosynthesis occurs in the chloroplasts of modern plant cells.