Prokaryotic organisms were probably the earliest known organisms, appearing about 3.5 billion years ago. They are most closely related to cyanobacteria. Methanogens are characteristic archaebacteria. Photosynthetic prokaryotes are limited to the eubacteria. The first eukaryotes known are fossils 1.3 to 1.5 billion years old.
Eukaryotes differ from earlier prokaryotes in that they have a nucleus.
Eukaryotes are thought to have evolved through the phenomenon of endosymbiosis. It is likely that there is some form of life elsewhere because by the sheer mathematical probability that with so many stars and so many planets, life should have evolved somewhere out there. For carbon-based life forms to evolve, a planet must have temperature and atmospheric characteristics similar to those on earth.