A fascinating way to look at any culture, either ancient or nascent, is to look at what they deem to be right and wrong. Some behaviors will be strongly rewarded, while other will be strongly dissuaded. The latter, the taboos, are very interesting indeed.
A taboo, then, is something that is forbidden to do. Many cultures have many examples, but three areas of importance will overlap considerably into all cultures in some form or another. These three areas are edible foods, sexual partners, and the handling dead bodies. In the case of the latter, it may involve the prior two.
The first to discuss are forbidden foods (i.e., food taboos). A primary example is the fact that many cultures refuse to eat pork and pork products. Others eat pork at will, with no ill effect, so why would one culture denounce it? Clearly, tainted pork can kill you. Trichinosis, which occurs in undercooked meat, is more prevalent in swine. So after a few illnesses or deaths, pork becomes forbidden.
Sexual behaviors are culturally defined most everywhere, and some types of sex are taboo almost all the time. Primary among these is the act of incest. But why is this act so inherently reprehensible to most all of humanity? Is it because when we see the results of it in nature we are repulsed by the resulting deformities and deaths? Why would some ancient cultures support the act, like the ancient and royal Egyptians are rumored to have, while others will kill any who do so? What defines this idea?
Another quite common taboo is that of eating other humans. Some cultural groups have practiced this tradition, only occasionally, without doing too much internal damage. Others have been decimated by disease that only occur within cannibalistic societies. Could the macabre practice be forbidden for baser reasons than the grossness factor? While it is understandable that some, if starving, would resort to such activities, most of us would never even consider the idea. Why would this be so?
Taking these three examples, from the many thousands available, begins to show why forbidden acts become so vehemently forbidden in the first place. It is to ensure survival above all else. Biological imperatives become cultural mores for the same reason, and that is to keep the tribe alive.
So perhaps it may be that morality is not a question of what we think is good or bad on a conscious level, perhaps it is a way to rationalize what we do on a subconscious level. We are thinking animals, so we tend to create meaning where there may be none. Taboos have evolved into extremely important codes of conduct over time. Maybe they began as mere instincts for survival.