The origin of life on Earth is a question of great interest. The origin of eukaryotic cells is yet another question that has boggled evolutionary biologists, naturalists, cell biologists, and molecular biologists alike for quite a while. Although the question is far from settled, evidence suggests that some of the organelles in the present-day eukaryotic cells have arisen from symbiosis events. This article examines morphological and some molecular evidence for the endosymbiotic origin of mitochondria and plastids in eukaryotes. Many of these arguments are based on a classic paper by naturalist and microbiologist Lynn Margulis and follow up work from other scientists. She described eukaryotes as multigenome systems, where all biochemical reactions are encoded in the DNA of either the nucleus or the subcellular organelles of symbiotic origin.