Endosymbiosis and the three domains of life

Endosymbiosis and the three domains of life

Some of the pathways by which the various endosymbionts might have arisen.

Modern taxonomy classifies life into three domains. The time of the origin of these domains are 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 billion years ago, 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 period 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). Perhaps the smaller cell attempted 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, and soon a stable symbiotic relationship developed. 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. Symbiosis developed between the larger cell and the population of smaller cells inside it to the extent that they are considered to have become a single organism, the smaller cells being classified as organelles called mitochondria. A similar event took place with photosynthetic cyano bacteria 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 some time before one billion years ago. There were probably several such inclusion events, as the figure at left 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. During this period, the supercontinent Columbia is believed to have existed, probably from around 1.8 to 1.5 billion years ago; it is the oldest hypothesized supercontinent.