[slavik81]

The impact of the raw number of people we have now should not be overlooked, either. Today, there are more people in the United States than there were in the entire world during Roman times.

We have efficiencies of scale that they could never realize just due to our population size. Or course, we also have new problems due to that scale. We'd have a lot less to worry about in terms of wild habitat loss and CO2 emissions in a world with in a world with 20x fewer people.

[acqq]

It is believed that the number of people at previous times was also limited by the energy available. See how the population graph looks like -- the real exponential growth starts at earliest around 300 years ago:

https://en.wikipedia.org/wiki/File:Population_curve.svg