Effects of the genetic bottleneck event on life expectancy

In biology / paleobiology, it is argued that there must have been a genetic bottleneck event at some point in human history. That is, there must have been very few humans on earth at one point of time. I read this once, but sorry, no source at hand.

Creationists argue that the deluge was that genetic bottleneck event.

According to the bible account, human life expectancy gradually decreased after the deluge. This cannot come just from climate change (now unfiltered cosmic radiation etc.), because then the effect would be immediate.

My thesis (perhaps a new one) is now this: the life expectancy decreased because of the genetic bottleneck event. The effect unfolded fully during some generations after the deluge, just as the effect of unintentional incest (e.g. in small communities) unfolds during some generations, not immediately.