atheism is NOT a "worldview" - Beliefnet
All moral law is ultimately the mores of the tribe. That which allows the face group of the tribe to function. The tribe may adopt a moral law giver in the form of a shaman or imaginary super shaman, but either will of necessity codify the mores of the tribe: Be nice to members of the tribe, protect the children of the tribe, respect the leaders of the tribe and protect the traditions and lore of the tribe.
In the modern world tribes are larger than a face group, and dispersed among other tribes in the society, but have common tribal values. Some are built around a religious tradition, others are built around business traditions, and another is based around the traditions of the university community. The university communities are typically split into the scholars and the warriors, and loyalties to each group carry beyond the campus with the warriors transferring loyalties to professional warrior teams, either sport or military.
The above is grossly stereotyped of necessity, there are major differences within each 'tribe.' Religious groups in particular are split into smaller and smaller groups some as small as a parish, each with its own mores and most with it's own higher moral law giver providing an absolute higher moral law for the tribe. Of course it is too much to expect that these absolute higher moral law agree on much of anything except that they are right and all the other absolute higher moral law givers are wrong.
It should be noted that there are atheists in most of the tribes, and the atheists generally adopt the world view of the tribe with the exception that the absolute higher moral law giver, if there is one, is an imaginary myth.