Cellular Evolution

Protobionts

Protobionts may have been precoursors to prokaryotic cells. They had abiotically produced molecules surrounded by a membrane or a membrane-like structure. These structures have been proven that they could have spontaneously formed by following the laws of physics and chemistry during the early Earth's development. (Biology 7th Ed. - Campbell)

Protobionts: aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure. Protobionts exhibit some of the properties associated with life, including simple reproduction and metabolism, as well as the maintenance of an internal chemical environment different from that of their surroundings. (Biology 7th Ed. - Campbell)

Evolution of prokaryotes

It is generally accepted that the first living cells were some form of prokaryote. Fossilized prokaryotes 3.5 billion years old have been discovered, and prokaryotes are perhaps the most successful and abundant organism even today. In contrast the eukaryote only appeared approximately 1.5 billion years ago. While Earth is the only known place where prokaryotes exist, some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes; this is open to considerable debate and skepticism.

Prokaryotes diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct types of prokaryotes. For example, in addition to using photosynthesis or organic compounds for energy like eukaryotes do, prokaryotes may obtain energy from inorganic chemicals such as hydrogen sulfide.

This has enabled the bacteria to thrive and reproduce. Today, archeabacteria can be found in the cold of Antarctica and in the hot Yellowstone springs.

Viral Eukaryogenesis

Viral eukaryogenesis is the hypothesis, proposed by Philip Bell in 2001, that the cell nucleus of eucaryotic life forms evolved from a large DNA virus in a form of endosymbiosis within a mycoplasma cell. The theory has gained support as large complex DNA viruses capable of protein biosynthesis (such as Mimivirus) have been discovered.

A number of precepts in the theory are possible. For instance, a helical virus with a bilipid envelope bears a distinct resemblance to a highly simplified cellular nucleus (ie: a DNA chromosome encapsulated within a lipid membrane). To consider the concept logically, a large DNA virus would take control of a bacterial or archaeal cell. Instead of replicating and destroying the host cell, it would remain within the cell. With the virus in control of the host cell's molecular machinery it would effectively become a "nucleus" of sorts. Through the processes of mitosis and cytokinesis, the virus would thus hijack the entire cell—an extremely favourable way to ensure its survival.