Does anyone know where I can find the best scientific estimates of how many humans there were that ever lived?

Try here;

http://www.prb.org/Template.cfm?Section=PRB&template=/ContentManagement/ContentDisplay.cfm&ContentID=7421

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Since the house is on fire let us warm ourselves. ~Italian Proverb

Here's a paragraph (from World Overpopulation Awareness) that seems to summarize some of the information on that webpage:

quote:

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World Population: How Many Have Ever Lived. An unknown writer claimed that "three-quarters of all the people who have ever been born are alive today". That erroneous statistic became accepted as fact. However, there is enough information to make a good guess as to how many people have ever lived on Earth. According to calculations a total of 106.4 billion people since man appeared about 50,000 B.C. That means that 5.8% of all the people who have ever been born are alive today. Every year, global population increases by about 78 million people. It is estimated that humanity is consuming the earth's resources 20% faster than they can be sustained. Until the modern era, world population grew slowly. During the next eight milleniums, population grew at .05% per year, reaching 300 million in 1 A.D. During the following 16 centuries, the annual growth rate fluctuated, partly because of the Black Death, which ravaged 14th century Europe. Today, there are six times as many people alive as at the start of the industrial revolution, 13 times more than when Columbus set sail and 20 times more than during the Roman Empire. There's an assumption that in pre- history women had as many babies as they could, so the birth rate would have been fairly high. Average life expectancy in Iron Age France have been pegged at only 10 or 12 years. There is considerable debate about when the human race actually came into existence.
http://www.overpopulation.org/faq.html
http://www.population-awareness.net/

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Someone named Vernon Nemitz arrived at a "crude estimate" of 40 billion.
http://www.madsci.org/posts/archives/may2000/957452021.Ev.r.html

Here's a quotation that is interesting:

quote:

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If growth continued [at the current rate] for about 900 years, there would be some [sixty million billion] people on the face of the earth . . . This is about 100 persons for each square yard of the Earth's surface, land and sea. A British physicist, J. H. Fremlin, guessed that such a multitude might be housed in a continuous 2,000-story building covering our entire planet . . . Fremlin has made some interesting calculations on how much time we could buy by occupying the [other] planets of the solar system. For instance, [at the current rate] it would take only about 50 years to populate Venus, Mercury, Mars, the moon, and the moons of Jupiter and Saturn to same population density as Earth . . . It would take only about 200 years to fill [the remaining planets] "Earth-full." . . . What then? . . . Using extremely optimistic assumptions, [Professor Garrett Hardin of the University of California at Santa Barbara] has calculated that Americans, by cutting their standard of living down to 18% of its present level, could in one year set aside enough capital to finance the exportation to the stars of one day's increase in the population of the world.
~ Dr. Paul Erlich, The Population Bomb

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That sounds more realistic.