Board Thread:Warhammer 40k General Discussion/@comment-25149066-20140730050740/@comment-24944355-20140730074409

Actually in the Ciaphas Cain novel The Greater Good it was discovered that Tyranids were active in the galaxy something like 17,000 years earlier. A group of Tyranids were frozen & later revived and were from a totally different nest than current Tyranid incursions & the two groups were violently opposed to each other.

As to the Tyranids attacking during the latter crusading era, they would have been creamed. A united imperium under the Emperor with 18 primarchs & no true rivals would have turned the Tyranids into mulch. I'm not saying it would have been easy, Tyranids are tough buggers, but without having to worry about Chaos, Tau, Necrons or Eldar all of which were much weaker threats at that time the imperiums would have clobbered any Tyranid incursion.