Types of Cells

The very first eukaryotic cell is thought to have arisen roughly 3.8 billion years ago, and the leading explanation for how it happened is the endosymbiont theory - the idea that some of today's most important eukaryotic organelles began as separate, free-living prokaryotic cells that were absorbed by a host cell and never left. The proposed sequence runs in five broad steps. First, an ordinary prokaryote grows larger and develops inward folds of its cell membrane, which increases its surface area relative to its volume. Second, some of those inward folds eventually pinch off completely from the outer cell membrane, forming an early internal endomembrane system that wraps itself around the cell's loose nucleoid - creating, for the first time, a proper membrane-bound nucleus, and with it, the very first eukaryotic cell. Third, an aerobic (oxygen-using) proteobacterium enters this new eukaryote, either as prey it failed to fully digest or as a parasite, and instead of being destroyed it survives inside as a permanent endosymbiont - literally, 'a cell living inside another cell'. Fourth, as the wider environment slowly becomes richer in atmospheric oxygen (a change that drove many oxygen-intolerant organisms extinct), this internal partner's built-in ability to use oxygen to generate energy becomes a major survival asset for its eukaryotic host; over evolutionary time it is fully assimilated into the host cell and becomes the mitochondrion. Fifth and finally, some of these eukaryotic lineages go on to acquire a second kind of endosymbiont - a phot …