Endosymbiosis explains the origin of mitochondria and chloroplasts, but could it also explain other features of the eukaryotic cell? Maybe. Endosymbiotic origins have been suggested for many structures, including flagella (structures like the tail of a sperm), cilia (hair-like structures that help in locomotion), and even the nucleus — the cell’s command center! However, scientists are still actively debating whether or not these structures evolved through endosymbiosis. The jury is out while more evidence is gathered.

In her theory of endosymbiosis, Lynn Margulis emphasizes that during the history of life, symbiosis has played a role not just once or twice, but over and over again. Instead of the traditional tree of life branching out from a few common ancestors to many descendent species, Margulis proposes that branches have separated, and then come together again many times as individuals of different species set up symbiotic relationships and formed new organisms. This process formed an interconnected tree of life in which organisms have multiple ancestors, even from different domains. As eukaryotes, our ancestors include both the bacteria that became mitochondria, and the archaebacterium that was the host cell.

Why have endosymbiosis and symbiosis been so important to evolution? Why cooperate at all? The answer to these questions points us to one of the basic processes of evolution: natural selection. As Darwin observed, organisms that are fit enough to succeed in the game of survival have a good chance of passing on their genes to the next generation. Any survival or reproductive advantage can help a species out-compete another species or simply avoid becoming extinct itself. It seems likely that the first eukaryotic cells gained a slight edge over their neighbors when the mitochondria, a rich source of energy, moved in with them. Like Kwang Jeon’s x-bacteria and amoebae, the mitochondria and their hosts relied more and more on each other in order to survive. Eventually, neither could succeed alone — but as a team they produced millions of descendants, establishing a whole new domain of life.