Most of the life we see, including plants and animals are classified in the domain Eukaryota. Eukaryotic cells are more complex than prokaryotes, they contain organelles, and the DNA is linear and found within the nucleus. The complex eukaryotic cell enabled a whole new era of life on Earth, because these cells eventually evolved into multicellular organisms. But how did the eukaryotic cell itself evolve from a simple prokaryotic cell? Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union - endosymbiosis. What evidence supports the theory of endosymbiosis?

The endosymbiotic theory is a theory regarding the origin of eukaryotic cells from prokaryotes, which states that the organelles that are specific to eukaryotes evolved from prokaryotic organisms with a single cell.

The evidence of the theory is the formation of eukaryotic organelles. The theory specifies is that the mitochondria and the chloroplast (as well as other plastids) are prokaryotes that used to be free-living, but have entered a eukaryotic cell about 1.5 billion years ago, and somehow managed to survive in the conditions inside it. What happened after the prokaryotic cell survived inside the eukaryotic cell is that it started relying more and more on its host, so most of the genes were transferred to the genome of the eukaryotic cell. We can see this in today's mitochondria which has its own genome, with only genes that are completely necessary for its survival, while most other genes it requires are a part of the nuclear DNA.