I'd really like to know where they came up with those numbers. For instance, the graph shows the USA near zero until a few centuries ago, but still non-zero, p

I'd really like to know where they came up with those numbers. For instance, the graph shows the USA near zero until a few centuries ago, but still non-zero, particularly around 1000CE where it goes up a little before going back down until it begins its steady climb just after 1700. There was no "USA" in 1000CE. And seriously, how they they possible come close to quantifying relative GDPs in ancient times, or even as early as the 1700s or 1800s?