The Darwinian "theory of everything" has always stood above its presumptive competitors because it came packaged with several "big problems," which could just as easily have been the theory's undoing as its vindication: altruism, sociality, organs of extreme perfection, and animal-built structures. With respect to the last one, the basic problem is that when we build, we act as purposeful, intentional and designing agents. Yet it is Darwinism's core assumption that such agency has no place in guiding evolution.
When animals build things, sometimes appearing to anticipate, match or exceed our own capabilities as architects, what are we to think? Do we conclude that other creatures can also act as intentional agents? In that case, the Darwinian vision of a world without such agency is undermined. Or do we conclude that our own intentionality is a quality apart, with no precedent in the living world from which we sprang? Drawing such a conclusion would be tantamount to succoring Darwin's bête noir, Platonic essentialism. This problem is not trivial: Indeed, it drove a wedge between Darwin and his "co-Darwinist," Alfred Russel Wallace. Yet Darwin himself, confronted with the magnificent structures built by bowerbirds, resorted to attributing them to the birds' pursuit of "pleasure"—a purposeful agency if ever there was one.
