How do you estimate that?
You need three things: how long humans have existed, roughly how many there were in each era, and how many children were born per 1,000 people. You don't need the number of deaths. By the way, around 8000 BCE only about 5 million people were alive.
Those of us alive
Today's 8 billion people are nearly 7 % of everyone ever born. Many of the others didn't grow old: according to an estimate by Carl Haub from 1995, about 40 % never reached their first birthday.
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