Mitochondria are indeed thought to have arisen from incorporation of free-living organisms, but interestingly they mostly retain their own replicating DNA that

Mitochondria are indeed thought to have arisen from incorporation of free-living organisms, but interestingly they mostly retain their own replicating DNA that is not incorporated into our genome [1]. 1- https://en.wikipedia.org/wiki/Mitochondrial_DNA https://en.wikipedia.org/wiki/Mitochondrial_DNA