It is difficult to believe there weren't if modern humans have been around for 200k years.

Yeah there was even a chapter the Avengers 1000000 BC story that showed that there was mutants during the cave men era

Heck, let's not froget that Eternals, Deviants and Mutants were created by Celestials eons before Apocalypse was even born!

^^I wish you wouldn't call them "cave men".

It's nothing new, we already know mutants like Crule, Selene, Saul and Nicodemus was around during the Hyborian Age and long before that. There was even a island of mutants 15,000 years ago in Uncanny X-Men vol 1 422. The Celestials were on Earth a 1,000,000 years ago, creating the Deviants, Eternals and the X-Gen. Which Firehair, Highwalker and those of the Tribe Without Fear were possibly the result of the early generation of that work?

As for this 2 Billion years ago thing, I don't pay it given the hints of time travel and that they lived among humans but weren't from them. That and they were born from some Ra's al Ghul Lazarus Pit. Plus the "mutants" of Threshold that Theia now told us is not only different from what was revealed earlier but seems to fantastical to be legit.