What is the main idea presented in this passage?
[In about 1200, a] major population shift saw [the Ancestral
Puebloans] begin to move back into the cliff alcoves [or
recesses] that sheltered their ancestors centuries before.
Why did they make this move? We don't know. Perhaps it
was for defense; perhaps it was for religious or
psychological reasons; perhaps alcoves offered better
protection from the elements. Whatever the reason, or
reasons, it gave rise to the cliff dwellings for which Mesa
Verde is most famous.¹