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The population numbers in Country-Year data came from the HYDE project, with data interpolated for the missing years. The GDP numbers came by interpolating the GDP/capita data from the Maddison database and multiplying it out. All numbers were normalized against the world totals.

Numbers from before the first entry for each country were extrapolated backwards assuming a 2% growth rate back until the subsistence level is reached. Note that I am using $400 (1990)/year subsistence level from the first version of the database rather than $691 level from the second. The $691 level had too many countries at below subsistence. The 2% growth rate seems to give reasonable results.

Note that pre-industrial economies were never much above subsistence and the GDP proportions are effectively population proportions sufficiently back in the past.