Remember that, human and all livings are a form of energy. Based on physics, Energy can not be destroyed or created but it changes the form. from this we have to realize that, the energy from million yrs ago still around but have changed form  in way that adapt to situation. 

So, Religiously people do believe in souls and these souls are around us . let say we assume that  souls as energy as scientific term. What if these souls that we have in us are the same from the million yrs ago. So, that's why we might experience deja vu coz its already happened otherwise how we certain that its we had seen or feel  before. I have discuss these concept to my physic lecturer and did some research also otherwise i wont crap here   ...However its just a opinion only..

hope u can see the point in what i trying to explain here...

yeah i get it.this is a good question how it relates to ancient.
U have to Note that Deja vu is something thats already happen that is why we experience the same feeling as we had before.

First:

okay, let say we go a place somehow we feel that we have been here but we can't recall it. but act it was our first time to be at the place.
hold on there...let us go backward million yrs ago..the same place was there either land or water.wat differ was  how it had changed the place in aspect we seeing it now in term of building road and etc.But the atmosphere and energy around there was not changed at all..somehow we have been here.
So, wen it comes nowadays when we go a place which we feel deja vu and our mind state will form a memory based on the object or building around the place as if we have been here. This is for places or more on concrete object.

How bout feeling and thought that happen everyday?
have u experience that u have said this exact word or fact to someone before. these words been said  before. there was another theory in this matter as well which is still in research . " when human are sleeping(deep sleep) where u dun aware what is happen around ur energy(soul) partial out of the host and it going all places with different energy form(energy that experience many event b4). Thats y we dream and things and word were said happen exactly as we have experienced before.

Second:

This is my own theory...How bout in trillion yrs ago there was a same identical world in same position of our earth where ppl like us had lived and this world had been destroyed due to universe cycle (something like BingBang Theory). somehow the energy is still there. thats y we might experienced deja vu.

Act, i have summarized in simple word for better understanding.