SAM said:
What was the source of this estimate?
Mulitple books and journal articles over the years.
Most recently (hence fresh in mind and ready to hand) and a good source of references as well as discussion, a book called "1491" by Charles Mann.
http://en.wikipedia.org/wiki/1491:_New_Revelations_of_the_Americas_Before_Columbus
The discussion is layman level, pop history and anthropology. But it's solid.
Edit in: Simple table of the estimated populations of the continents during the colonial years in Africa, and links. Note behavior of population of Asia, before modern medicine and already crowded, poor sanitation, etc. If you like, try separating India under the British and the various Indonesian colonies out from the dataset
http://www.statistics.gov.uk/StatBase/xsdataset.asp?More=Y
A simple bar graph comparing Africa with the rest of the world, during the colonial years in Africa.
http://downloads.cambridge.edu.au/e...ageProofs/Contents/Section 3/Section3_3.3.pdf
As far as population booms (not stable dense populations, which mey be adapted to local circumstances) leading to trouble,
http://www9.ocn.ne.jp/~aslan/pfe/pop.htm
look at the green numbers followed by projections of large jumps - those mark the fastest booms in the larger countries on earth, in progress as of the late 90s. See any countries you recognize from the bad news headlines as of 2007 ? Much over a doubling in 50 years seems to be dangerous, barring unusual circumstances - it's hard to accomodate that.