Your cells contain ancient tenants that shaped your entire existence

Health · Published 20 October 2026 · Updated 20 October 2026 Swipe more like this →

In short: You are not a single organism; you are a biological collective. Deep within your cells live ancient bacterial residents that were absorbed by ancestors billions of years ago. These endosymbionts have transitioned from independent life forms into essential components of your own biology, providing the energy and nutrients required for survival.

The theory of symbiogenesis suggests that complex eukaryotic life evolved through these intimate mergers. Around 2.3 billion years prior, an alphaproteobacterium was taken into an archaeon (probably from the Asgard superphylum) by means of phagocytosis. This event gave rise to mitochondria, the organelles responsible for providing energy to almost all living eukaryotic cells today. Approximately 1 billion years ago, a similar process occurred when cells absorbed cyanobacteria, which eventually became chloroplasts—the machinery that allows plants and algae to produce energy from sunlight.

These relationships are often obligate, meaning the symbiont cannot survive without its host. While they reside within host cells, they replicate via binary fission rather than mitosis. Over millions of years, these endosymbionts typically undergo genome shrinkage, discarding genes that the host now provides. For example, in some insects like the pea aphid, the endosymbiont Buchnera sp. synthesizes essential amino acids the aphid cannot get from its diet of plant sap, while the symbiont relies on the host for other life essentials.

Nature also utilizes these partnerships for defense and niche specialization. In Drosophila flies, the spiral bacteria Spiroplasma poulsonii acts as a defensive symbiont, producing ribosome-inactivating proteins to attack invading parasites. Furthermore, some marine worms lack any digestive or excretory systems entirely; they are nutritionally dependent on chemoautotrophic bacteria that fill their entire bodies. These interactions create a 'holobiont'—a collective entity where the host and its internal residents function as a single biological unit.

The transmission of these partners is diverse. Vertical transmission occurs when symbionts move from parent to offspring (such as via mother's milk in tsetse flies), often leading to highly reduced genomes. Horizontal transmission involves acquiring facultative symbionts from the environment, like nitrogen-fixing rhizobia in legume roots. These complex dynamics show that life is less a solo journey and more a sophisticated history of cooperation.

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