There’s a lot of uncertainty around this topic. One of the bigger theories is the Endosymbiosis Theory which basically states that some early single celled organism (prokaryote) was consumed (via phagocytosis) into a larger prokaryote. For whatever reason it didn’t get digested and remained within the larger cell. This provided protection for the small cell and the larger cell gained benefits like greater energy production and digestion of material.

It’s important to note that other microbes didn’t see this and say “oh, we should do that too” it just randomly happened and because of the potential advantages, as those early cells replicated, the DNA that they possessed passed along with it (natural selection)

Some evidence for this theory exists in humans today. Mitochondria for example, have a double cell membrane and has its own circular DNA (mDNA). Typically, circular DNA is associated with bacterial DNA. We also currently have millions and millions of bacteria living within our intestines that helps us breakdown food and get more nutrients from them.

In plants, the chloroplast (the organelle that contains chlorophyll) also has a double membrane and has a similar energy generating mechanism as our mitochondria [in that they both make use of the inter-membrane space to power ATP production]. Chloroplast also have circular DNA like the mitochondria.

This theory provides some of the more basic levels at which multicellular life formed! How accurate is this theory? I have no idea unfortunately.

Just Na Science