There's a tendency to think of manners as mere useless ritual -- table etiquette in particular is often dismissed as a matter of "knowing which fork to use," of interest only to grandmothers and Judith Martin. Paying attention to manners means you're anal-retentive, sexually frustrated, and don't know how to enjoy life.
Ha! Turns out the development of table manners may have been a major step in the formation of human civilization. Dr. Mary Stiner of the University of Arizona has been researching Paleolithic remains at Qesem Cave in Israel. The earlier Lower Paleolithic hunters were evidently just as capable of bringing down big game as the later Upper Paleolithic humans were. But their descendants were noticeably more polite about serving it out.
Earlier bones show a variety of cutting methods used on the same bones, suggesting a kind of self-service scramble which wasted some of the food. Later animal bones show a more orderly approach indicating that one person carved the meat, and didn't waste as much. Table manners ("meat-sharing rituals," in anthropologist jargon) were a survival trait allowing tribes to make better use of the animals they killed. One suspects they also cut down on dinnertime fights over who got the bigger piece. Over time, one can assume that well-mannered tribes out-competed their boorish neighbors.
So: mind your manners or your descendants won't thrive.