Short version of our aging paper
Our ancestors who delayed reproduction when environmental cues predicted famine were more likely to survive to reproduce after the big die-off. Delaying reproduction therefore increased relative representation in the smaller post-famine gene pool.
Biological responses inherited from those ancestors are still triggered by cues that predicted past famines, such as eating less or eating "famine foods." These responses can therefore extend lifespan, with a decrease in potential fertility as a side-effect. But most of us don't want to achieve our maximum possible family size anyway.