As we'll never know the truth until they start shoveling dirt on us, and I've yet to meet someone that's come back with any viable info, why strain the brain?
That said, the very fact that, at some time in most every human beings life, the desire for answers arises, leads me to believe that it's in our make up, instinctual for lack of better term.
If one supports evolution (I do not, another time), then at what point did the meaning of life become a concern?
After the primordial ooze but before the Wheel?
Simplistically, for me it's a gut feeling. Every time I've ever failed to follow I've regretted in the long run. That's my proof.
Mankind has twisted to suit/ enslave/ profit from fellow man, granted.