Overview
Eukaryote (真核生物, shinnkakuseibutsu) refers to all organisms in which genes are not distributed in the cytoplasm as in prokaryotes (bacteria and archaea), but are integrated in the nuclear membrane. It includes mainly unicellular protists and all multicellular organisms.
This type of organism appeared on the Earth 2.7 billion years ago. At that time, the Earth's atmospheric oxygen concentration increased rapidly due to the emergence of photosynthetic cyanobacteria. This brought a crisis of extinction to prokaryotic organisms (including cyanobacteria) that were vulnerable to oxygen.
At this time, a type of archaea, (probably) the ancestor of eukaryotes, had acquired the ability to breathe oxygen by capturing part of the genome of a parasitic aerobic bacterium (rickettsia) and turning it into mitochondria by taking over its life-giving. Bacteriophages parasitize them.
Once parasitized inside the cell, the phage builds a spherical structure inside the cell, replicates its own genome inside the structure, and assembles virions (viral particles) near the surface that serve as containers for the genome. Normally, the completed phage eats its way through the host cell membrane and exits the cell, but some archaea send their own genome into the phage structure, hijack its function, and eventually use it for their own proliferation. This is the root of the cell nucleus, which protects DNA from harmful oxygen and also allows for efficient replication of large numbers of genomes. They eventually evolved into eukaryotes by developing an endoskeleton. Later, a type of eukaryote evolved into a plant, which acquired the ability to photosynthesize by also incorporating cyanobacteria and using them as chloroplasts.
Oxygen concentrations on Earth continued to increase. This has consumed all the greenhouse gases carbon dioxide and methane, resulting in the first global freeze (Snowball Earth), the Huronian Ice Age.
