Was thinking about the kids in the mausoleum and it led me to a theory. I think we have all made the assumption that that screen shot was from later in the game after we have lost some villagers. But suppose it is from the start of the game and those are the villagers that inhabited that portion of the island before our new tribes arrival and our new tribe has buried them.

Our new friends arrive in some fashion that has stranded them and they find the whole island dead or dying at the start of the game, including those villagers they had to bury. They then disscover the "Tree Of Life" is not giving off the energy it once did to substain life and their only chance for survival is to find out and correct whatever is killing the "Tree of Life"