Jeon’s colonies of amoebae seem perfectly happy living with their permanent guests, the x-bacteria, inside of them. This kind of relationship — two or more different species living in close association — is called symbiosis.

Each amoeba and its x-bacteria work together for mutual benefit — but they are still separate organisms. Each bacterium or amoeba divides on its own, gets its own energy, uses its own genes, and makes its own proteins (mostly!). However, with their close relationship, it seems possible that after many years of evolving together, these cells could become not just a team, but a single integrated organism with a common set of genes and proteins. A future scientist discovering the descendents of Jeon’s amoebae might not guess that this one “amoebacterium” was once two distinct organisms.

Evidence like this points to the likelihood that the “merging” of two simple organisms has also happened under natural conditions. Long ago in evolutionary history, two cells formed a symbiotic team that, over millions of years, evolved into a single organism. The result of this union was the first eukaryotic cell — the type of cell that makes up the human body. We humans owe our existence to two bacteria that teamed up in a symbiotic relationship over a billion years ago!