Self is not a verb, so it does not have a past tense.
Happened is the past tense of happen.
The past tense of happen is happened.
The past tense of happened is happen.
History repeat it self is a pharasal word, used when somthing is happening or soemthing is about to happen that happend also in the past.
The word self doesn't have a past tense as it's not a verb.
The past future tense of "happen" is "would happen." For example, "He said the event would happen next week."
Not currently - but probably in the past RNA did the job. And before that it is possible that self-replicating proteins did it too.
past
What is going to happen when you repeat this experiment on self D.C generator?
Past
Happened.