Here's a fun fact for you...
Today, 30 million people in China live in cave (http://www.nydailynews.com/news/world/30-million-people-china-live-underground-caves-article-1.1046872).
At the end of the Upper Palaeolithic, the entire world population of humans was between 1 million and 10 million.
Therefore, there are far more people living in caves today than there were in "cavemen times."
talk about affordable, low carbon footprint housing!
A small number of people in the UK lived in caves within living memory: [en.wikipedia.org]