It's our level of complexity that has allowed us to ponder this question. But why we think we're here is a vastly different question than why we actually are here. Any other reason but survival through procreation is open to interpretation, belief, opinion, conjecture, etc. Because of this uncertainty, any such theory cannot therefore be correct. And so it is with the meaning of life.
Meaning is defined as that which provides value or significance.
Life is defined as not dead. Quite a mouthful, isn't it. Life's and meaning's vague individual definitions are only the beginning of the problems in determining a meaning of life. I think you'd have a hard time finding 2 people who can agree on what makes a life meaningful, let alone 6 billion and counting. There are more ways of defining the meaning of life than McDonald's has sold hamburgers.