There has been a study run by an organisation called ‘Population Reference Bureau’ which demonstrated that the number of people that have ever lived on Earth is 108 billion, while today the population is 7.1 billion and rising.
The author of the study, Carl Haub, counted 50,000 B.C. as zero year as that is when the modern homo sapiens is widely believed to have made its appearance.
Around 8,000 B.C. the population was around 5 million and by 1 A.D. which was Roman Empire experts estimate that there were at least 45 million people. In 1650 the number reached half a billion!