One can see a sort of Darwinian view of the purpose of life in a get whatever you can philosophy that leads to a consumptive life best summed up by the bumper sticker, "He who dies with the most toys wins".
Altruism (helping others) can provide greater satisfaction, but without an object, altruism can turn into serving self. Political systems such as Communism or Fascism often attempt to get people to serve the state as they would in a religious system.
Religious systems often draw strength from cultural tradition.
If one uses religious or political systems as a starting point rather than a final answer, the might prove more useful.
To find a purpose one has to first decide if the universe was created or if it created itself, the next question would be what should we do in an unrestricted environment. The various utopian experiments throughout history all ended in failure. This points to something intrinsically wrong in us that damages anything we collectively attempt. Since we are all born helpless, ignorant, and selfish, one might conclude that we have yet to evolve past selfishness.
As has already been cited, the creator view is often rejected because it is assumed that the existence of suffering. The assumption is that a good God would not make his creation suffer. However, upon closer examination the only way a creator could prevent suffering would be to make his creation one of robots that only do what they are told. If we are created with free will, it would seem inevitable that we would be selfish and begin to harm ourselves and others.
It would seem at first that both the created and uncreated views lead to a world of suffering from which the nihilist would conclude that all is pointless. However, there is a path out of this quandary. If one were to hypothesize a creator, one would have to consider why he would create beings that should be expected to run amok without some plan to recover at least some.
If one investigates various recovery theories such as Islam's achievement system with it's five pillars. Christianity with it's redemption system, or Hinduism and Buddhism with it's karma and reincarnation, one has to consider that the information you get is usually presented with accumulated contributions by humans often over centuries. As a result, one has to cut through all sorts of distortions.
If one looks for clues to the purpose of life in religious systems, one would have to consider what it was in humans that would constitute a salvageable characteristic. would it be achievement, improvement, an elevation of truth over self, or some other characteristic..
It would seem that a purpose derives from how one sees creation. A good starting point might be a ruthless search for truth.