Derek Thompson of The Atlantic brings to attention that simple little graph you see above: part of a research letter written by Michael Cembalest -- an analyst with JP Morgan -- it shows the world's economies from 1 A.D. all the way to 2008. It's been downhill since 1950.
I'm guessing that your first question, if you started scanning from the left, is: Wait, India was by far the biggest economy at the dawn of AD? Yup, India.
In Year 1, India and China were home to one-third and one-quarter of the world's population, respectively. It's hardly surprising, then, that they also commanded one-third and one-quarter of the world's economy, respectively.