Based on a few assumptions (modern h. sapiens sapiens start date 200,000 BCE, the Toba bottleneck reducing the population to 5000 individuals, average historical (before 1900) life expectancy at birth being 28 years), I worked up a few numbers.
By my calculations, 48.8 billion people have walked or are walking the Earth. 7 billion of them are around right now. But how many people were alive to experience various and sundry events?
The following chart will show you.
(click for a bigger version)