All Google is giving me is "how many humans have ever existed" and the answers all vary wildly. I figured since year 0 we may have a more accurate number.
You're right: I think much of the difference relates to early assumed birth rates. Carl Haub, the source of the widely-cited 108 billion, suggests 60.44 bn born from AD 1 to 2011. I think it's still too high because of the large initial population and the high birth rate to 1650, but a total around 50bn seems broadly right. For "ever" I find something nearer 70bn, near the bottom of the various estimates offered.
Just as a small quibble - there is no year zero in history. The year before 1AD was 1BC (same with the CE / BCE convention). However astronomy does have a year zero, so astronomical and historical dates are out of sync by a year in the BC range.