Why and how the problem of the evolution of Universal Grammar (UG) is hard

Universal Grammar (UG) is a complicated set of grammatical rules that underlies our grammatical capacity. We all follow the rules of UG, but we were never taught them, and we could not have learned them from trial and error experience either (not enough data, or time). So UG must be inborn. But for similar reasons, it seems implausible that UG was “learned” by trial and error evolution either: What was the variation and competition? And what were UG’s adaptive advantages? So this leaves the hard problem of explaining where our brain’s UG capacity came from. Christiansen & Chater (C&C) suggest an answer: Language is an organism, like us, and our brains were not selected for UG capacity; rather, languages were selected for learnability with minimal trial and error experience by our brains. This explanation is circular: Where did our brains’ selective capacity to learn all and only UG-compliant languages come from? Chomsky suggests it might be a combination of optimality and logical necessity