According to the well-known paradigm, eukaryotes are defined by the presence of a nuclear membrane surrounding their DNA, which is wound up into several pairs of chromosomes, as well as the presence of endocytosis and an endomembrane system, thereby compartmentalizing different cellular processes and separating them from one another inside the cell (de Duve 2007). This molecular characteristic separates them from prokaryotes whose genome is much smaller, and condensed into a single, circular DNA moleculep. The transition from prokaryotes to eukaryotes (PET) counts as a major obstacle for evolutionary theory between these two types of cells (Maynard Smith and Szathmáry 1995).
Until now there have been two basic theories on the evolution of cellular complexity. According to one theory, cellular complexity came about via endocytosis between bacteria and archaea. The problem with this theory is that, in practice, newly made constituents are always inserted into pre-existing membranes (omnis membrane e membrana), which subsequently divide (de Duve 2007). Membrane fusion between archaea and bacteria has never been observed. According to another theory, membrane invagination occurred after the digestion of engulfed material by the cell (Lonhienne et al. 2010). The invaginated membrane grew more and more convoluted, and then more specialized into different compartments, such as the endoplasmic reticulum (ER), Golgi apparatus and lysosomes (de Duve 2007).