I'd like to have an estimate for the total number of human beings that
have ever lived, including those living today (~6B). Part of what
makes this tricky is the issue of when to start counting. A good
answer will lay out an argument for when the earliest human beings
lived, and then analyze population trends from there.

Dear curiousone99,

I have located a staid estimate that is also explained in detail. The
question how many people have lived on Earth so far has been answered
- as far as "answering" is possible in such a complicated case - by
demographer and population expert Dr. Carl Haub, of the Population
Reference Bureau.

His estimate is based upon the premise that only the population of
Homo Sapiens, the modern man, should be counted. He writes:

"Fixing a time when the human race actually came into existence is not
a straightforward matter. Various ancestors of Homo sapiens seem to
have appeared at least as early as 700,000 B.C. Hominids walked the
Earth as early as several million years ago. According to the United
Nations' Determinants and Consequences of Population Trends, modern
Homo sapiens may have appeared about 50,000 B.C. This long period of
50,000 years holds the key to the question of how many people have
ever been born."

So he develops his estimates starting from the assumption that modern
men appeared 50,000 years BC. Beginning at that initial point, and
using several ways to estimate and calculate past global population
development and trends (which you will find explained when following
the link below), he comes to these results:

- The total number of people that have lived on Earth from the time
Homo Sapiens appeared to 2002 is about 106.5 billion.

- The 6.215 billion people living on Earth in 2002 equal a percentage
of 5.8% of those ever born.


Please follow this link to read a detailed explanation of Dr. Haub's
estimate:

Population Reference Bureau: How Many People Have Ever Lived on Earth?
By Carl Haub
http://www.prb.org/Content/ContentGroups/PTarticle/0ct-Dec02/How_Many_People_Have_Ever_Lived_on_Earth_.htm

I would like to add that I did not find any other approach to this
often-asked question that looked as reliable and well-founded as the
one offered by the Population Reference Bureau.
Just in case you are interested in establishing contact to them in
order to request additional information directly, here is how to
contact them:

Population Reference Bureau
1875 Connecticut Avenue, N.W.
Suite 520
Washington, DC 20009-5728
USA

Phone: (800) 877-9881; (202) 483-1100
Fax: (202) 328-3937
E-mail: popref@prb.org
Website: http://www.prb.org/


Sources:

Deutsche Stiftung Weltbevölkerung: Frequently Asked Questions - Wie
viele Menschen haben jemals auf der Erde gelebt?
http://www.dsw-online.de/magic_kopf1.shtml?faq/nav_faq.html?faq/b_faq_11.html

Deutsche Stiftung Weltbevölkerung: DSW Newsletter Jahrgang 2003 - Wie
viele Menschen haben je gelebt?
http://www.weltbevoelkerung.de/magic_kopf1.shtml?newsletter03/nav_newsletter03.html?newsletter03/b_nl031_5.html

Search terms used:
"jemals gelebt"
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"How many People have ever lived on Earth"
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"How many People have lived on Earth"
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"How many humans have ever lived"
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Hope this is what you were looking for!
Best regards,
Scriptor

Yes, this is exactly what I was looking for. Thank you.