How many people have been born since the first humans lived on Earth?

The number of humans ever born is probably around 80 billion, though many died in infancy. Out of this total, almost half have been born in the last two millennia (since the year 1 AD). One human in five (15 billion) was born during the last two centuries and almost one in ten (8 billion) is still alive. This recent pattern of rapid population increase is specific to human beings. 

For more information:

Jean Bourgeois-Pichat, Du XXe au XXIe siècle : l’Europe et sa population après l’an 2000, Population, n°1, janvier-Février 1988, [FR] for the method of calculating the total number of humans who have existed, read pages 32 and following [FR]
Jean-Noël Biraben, The rising numbers of humankind., Population and Societies, n°394, October 2003