[link|http://members.bellatlantic.net/~bjmcg/story2_peple_lived.html|Here]. It seems reasonable in broad outline.
50000 BC population 2. ;-)
8000 BC population 5 M. 1.1 B births to that point.
1850 population 1.3 B.
1998 population 5.9 B, 110 B total births to that point.
Ignoring the starting point as being 2 people, the numbers seem to make sense based on changes in births per 1000 people over time.
Cheers,
Scott.