Of the many ways that people handled heat before AC [1], most involved thicker walls and living underground. Air Conditioning has allowed the use of modern building materials (drywall & studs, rather than poured concrete or bricks), and therefore greatly accelerated the growth of home ownership.

As with everything, there are pros and cons--cheap electricity has simply led to the use of AC rather than natural cooling.

[1]http://www.mentalfloss.com/blogs/archives/7424