Catholic Church Law, Structure, and Governance

Last night somebody asked the following question:

“How do we know the origin of ordination of our priests and that it links back to the apostles?”

The answer I gave was the following (edited for brevity):

“Priests are ordained by bishops. For each bishop we have a complete lineage that dates back to the original apostles.”

This answer, I double checked, is correct. In fact, I learned from one of my classmate-priests that the archives in the Vatican have complete apostolic succession documentation for all bishops in the world. Nevertheless, I could not answer the follow-on question which was:

“When did the Church begin documenting these lines of succession?”

I once again followed up with one of my classmates, who knows much more about these matters than I do, and the answer I got was: “Very early. Already by the time of St. Iraneus (ca. 120 AD) the practice was well established. The importance of succession and of the laying of hands was clear to the earliest fathers as early as we can trace".

I hope this is helpful.