Modern taxonomy classifies life into three domains. The time of the origin of these domains is speculative. The Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but this supposition is controversial. Soon after this, by 2 Ga, the Neomura split into the Archaea and the Eukarya. Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now coming to light. 

Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today’s "Rickettsia" entered a larger prokaryotic cell. Perhaps the large cell attempted to ingest the smaller one but failed (maybe due to the evolution of prey defenses). Or, perhaps the smaller cell tried to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it was able to metabolize the larger cell’s waste products and derive more energy. Some of this surplus energy was returned to the host. The smaller cell replicated inside the larger one. Soon, a stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the host cell acquired some of the genes of the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these in turn could not survive without the raw materials provided by the larger cell. The whole cell is now considered a single organism, and the smaller cells are classified as organelles called mitochondria. 

A similar event occurred with photosynthetic cyanobacteria entering larger heterotrophic cells and becoming chloroplasts. Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes more than 1 billion years ago. There were probably several such inclusion events, as the figure at right suggests. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, it has been suggested that cells gave rise to peroxisomes, spirochetes gave rise to cilia and flagella, and that perhaps a DNA virus gave rise to the cell nucleus, though none of these theories are generally accepted.
Archaeans, bacteria, and eukaryotes continued to diversify and to become more sophisticated and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Around 1.1 Ga, the supercontinent Rodinia was assembling. The plant, animal, and fungi lines had all split, though they still existed as solitary cells. Some of these lived in colonies, and gradually some division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago the first multicellular plants emerged, probably green algae. Possibly by around 900 million years ago true multicellularity had also evolved in animals.

At first it probably somewhat resembled that of today’s sponges, where all cells were totipotent and a disrupted organism could reassemble itself. As the division of labor became more complete in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die. 

Proterozoic development of life
राक्षसी आघात
Rodinia and other supercontinents
पॅलियोझोइक कालखंड