What is primary endosymbiosis?

Primary endosymbiosis is the hypothesis that the first eukaryotic cells began as a result of the coming together of different types of microbes. Animal cells all contain mitochondria, and these organelles show many signs of having once been free-living prokaryotes that were taken up by early nucleated cells.

This hypothesis explains many features of mitochondria and chloroplasts. These structures have their own DNA, for example, that is organized in a ring shape common to bacteria but unknown among eukaryotes. A likely explanation for the partnership between cell types is that a nucleated cell absorbed a bacterial cell, without digesting it, and came to rely on the nutrients its "prey" secreted.