during your absence from the game, i guess 40 of them died while building the ruins, making room for 40 couples to have 40 children who grew up.
during your absence they can die, get pregnant, be born, build things, do research, plant and harvest crops, become elders, and more. but for some reason, when we are not there to watch, they won't bury their dead.
the only thing i can think of is that they must have a weird fetish and like it when we watch them bury the dead. creepy *shiver*