In addition to boasting about witnessing part or much of an event,1 historians were especially quick to boast if they essentially saw it all from the beginning.
• Flavius Josephus (37-100 AD): “I, on the other hand, have written a truthful account of the whole war [My insert: i.e. from the beginning] and its individual details, having been present myself at all the events [My insert: i.e. having seen it all]… was not unacquainted with any thing whatsoever that was either said or done in it [Against Apion (1.9) 1.47-50]
• Thucydides (460-400 BC): “For I well remember how, from the beginning to the end of the war… I lived through the whole of it, being of mature years and judgment, and I took great pains to make out the exact truth. …associating with both sides, with the Peloponnesians quite as much as with the Athenians… I will now proceed to narrate… the events of the war which followed.” [History of the Peloponnesian War 5.26]
The phrase (e.g. “eyewitnesses from the beginning”) functions as a technical term frequently employed to donate historiographic intentions.
• Examples of historians using this lingo abound.
1st-2nd century historiographers did not strive to ground their material in witness approval directly nor nearly directly. See here.