The TL;DR of it is "Hell if we know; probably, but it's really tough to prove."

The longer answer is this:

Berserkir exist in literary sources all across Scandinavia, but typically not in contemporary sagas. The earliest appearance is in Hrafnsmál, a fragment of skaldic verse that mentions berserkir twice. After that, they show up in 13th century and later sources as wandering duellists or shocktroopers in royal retinues, but never as the mindless warriors of popular culture.

Egils saga, for example, has Egill fight a berserkr duellist names Ljotr, who doesn't go into a berserk fury at any point in the duel. That said, though, Egill, his father Skalla-Grímr, and his grandfather Kveldulfr do go into a state of fury that's something like berserkergang but it's never called that in the saga, and it's due to their troll-heritage rather than being a berserkr.

Archaeologically, it's tough to say just what berserkir were. There was clearly a concept of them in, at the very least, the 9th century (when Hrafnsmál was written) and we have engravings and carvings of (possibly) warriors (maybe) wearing animal skins while (potentially) performing (what could be interpreted as) either a ritual dance (because if you don't know for sure what it is, call it ritual) or a ritual sacrifice. Unfortunately, we have no written explanation of what these figures represent, nor do we have a written account of a ritual which sounds similar to what the engravings depict, so we can't tell for sure.

You might also be interested in several earlier questions in this subreddit on berserks (and whether they existed).