Question

The endosymbiotic theory suggests that mitochondria orginated from where?

Answer 1

Itself

Explanation:

It is believed that mitochondria was its own entity before it moved into cells.

It was among the first organisms.

Atoms and molecules connected in different ways until a perfect combination occured: One molecule could take other molecules and replicate itself.
Eventually they became mitochondria, amongst other things. At some point though, mitochondria moved into cells instead.