The main question regarding the evolution of complex life on Earth is by far the one related to the origin of eukaryotic cell. The theory providing the most credible answers regarding the origin of eukaryotic cell is endosymbiotic theory, which claims that organelles such as mitochondria and plastids evolved from prokaryotic endosymbionts, claim that is now regarded as common knowledge. In the case of mitochondria, evidence firmly point toward an -proteobacterial ancestor, whilst its host is believed to be of archeal origin, owing to the fact that eukaryotes and archea possess similar ribosomes in their cytosol. The uptake of mitochondria is a crucial endosymbiotic event set at the very beginning of eukaryotic lineage, because it allowed eukaryotes to attain their novel properties which came at high energetic price. While the nature of the host that acquired plastids was undoubtedly that of a eukaryote, the plastidial predecessor that came to reside inside its host is believed to belong to a group of nitrogen-fixing cyanobacteria. The nitrogen fixation itself is thought to be the key metabolic aspect which mediated the symbiosis of cyanobacteria. Essential process that allowed integration of the symbionts was endosymbiotic gene transfer, the same process that transformed the host into heterotrophic from the perspective of energy metabolism. Protein import and protein targeting then come forth as necessary, allowing the organelles to maintain their biochemical identity. Despite of gene transfer and protein import, under selective pressure eukaryotic organelles still retained parts of their genomes, in order to regulate the de novo synthesis of proteins involved in electron transport chain. Although a vast number of questions regarding eukaryotic cell still remains unanswered, it is evident that endosymbiotic theory plays a significant role in the pursuit of the eukaryotic origin. Combined with the tools of molecular biology, it provides distinct explanations to numerous problems and phenomenons associated with the origin of organelles and eukaryotic properties.