proto-eukaryote engulfs a bacterium, or the bacterium enters the proto-eukaryote via a flagellum. The bacterium will become what we call a mitochondrion. A phenomenon of endosymbiosis or parasitism, therefore. It is now accepted that mitochondria and chloroplasts originated in eukaryotic cells as a result of a symbiosis (i.e., coexistence between different species that benefits both) between two ancestral “prokaryotic” organisms. It is believed that one of the two “ate” (or entered into) the other but was then unable to digest it (or escape), and therefore this second organism began to live inside the first. Over time, this coexistence would have been maintained and strengthened, until the symbiotic organism fully integrated with its host, giving rise to an organelle called the mitochondrion. There’s a wealth of evidence supporting this theory, linked to a series of similarities between mitochondria and bacteria, which we’ll discuss later. The same may have subsequently been true for chloroplasts.
