The Participatory Anthropic Principle, or PAP, is the idea that the universe requires observers, because without observers the universe could not actually exist. This controversial claim is based on the traditional Copenhagen interpretation of quantum physics, which requires an act of observation to resolve the superposition of states in a quantum wavefunction. It is one particularly intriguing variant of the anthropic principle.
It appears that PAP was first proposed by the renowned physicist and outside-the-box thinker John Archibald Wheeler, though it's unclear to what extent Wheeler really intended this suggestion to be taken seriously. One of the earliest references to the concept was when Wheeler discussed the idea of living in a participatory universe related to his classic "It from Bit" concept, which rests at the heart of quantum information theory. Here is a quote from Wheeler in 1990:
It from bit. Otherwise put, every 'it'—every particle, every field of force, even the space-time continuum itself—derives its function, its meaning, its very existence entirely—even if in some contexts indirectly—from the apparatus-elicited answers to yes-or-no questions, binary choices, bits. 'It from bit' symbolizes the idea that every item of the physical world has at bottom—a very deep bottom, in most instances—an immaterial source and explanation; that which we call reality arises in the last analysis from the posing of yes–no questions and the registering of equipment-evoked responses; in short, that all things physical are information-theoretic in origin and that this is a participatory universe.
-- Wheeler, John A. (1990), W. Zurek, ed., "Information, physics, quantum: The search for links", Complexity, Entropy, and the Physics of Information (Redwood City, California: Addison-Wesley)
At first glance, there is a fundamental problem with this approach in that it took several billion years after the Big Bang for the universe to reach any condition where it's reasonable to expect life to have formed (and certainly for it to have evolved through the known processes of Darwinian evolution). However, one possible way out of this is to accept that the universe existed in a superposition of states for billions of years until there was finally some sort of observer, at which point the wavefunction would have collapsed into the state that allowed that observation to take place. (This approach was applied by molecular microbiologist Johnjoe McFadden in his 2000 book Quantum Evolution.) This scenario seems like it would be fully consistent with the Copenhagen interpretation of quantum physics, and certainly in line with the claims made by Wheeler in a 2006 radio interview:
We are participators in bringing into being not only the near and here but the far away and long ago. We are in this sense, participators in bringing about something of the universe in the distant past and if we have one explanation for what's happening in the distant past why should we need more?
The Participatory Anthropic Principle is a controversial theory and is not accepted by a majority of physicists as being a principle that actually applies to the universe. Though the Copenhagen interpretation is often applied to resolve the fundamental problems of quantum physics, such as those in the quantum double slit experiment, most physicists don't really believe that an observer is required to resolve all interactions. (The fascinating role of a conscious observer in quantum physics is the subject of the book Quantum Enigma, if you'd like to learn more on the subject.)