Re: How can more people be alive today than ever lived?

Greetings, Patrick:

It is indeed a mathematical estimate, but I am not sure that the stated
results are true.  Quite a few years ago I encountered an estimate to
the effect that the total number of humans who ever lived was about 70
billion -- in which case the assertion you are Asking about would be
false.

Yet I do not recall how far back the estimate I encountered considered
humans to have existed.  Should we pick the earliest upright hominids,
or should we pick anatomically modern humanity, or something in-between?
For the purposes of the estimate being made here, I will choose the
anatomically modern types, which, according to the fossil record,
started appearing about 100,000 years ago.

Since I will be considering the anatomically modern human type, I can
make the associated assumption that one average generation is 15 years
(because in those days the perpetuation of a tribe was so important that
women became baby-factories just about as soon as they reached puberty, at 
about age 14).  I can also use an average lifespan of 25 years, since
we all know that life in those days was "nasty, brutish, and short".

A quick-and-dirty estimate of the total number of humans can be made by
multiplying the number of years (100,000) by the global population, and
then dividing by the lifespan (25).  (We assume that each 25 years the
entire population is replaced by all new people.)  The first major
modification to the estimate takes into account the major growth in
population that accompanied the develoment of agriculture about 10,000
years ago.  So let's begin with a 90,000-year span of pre-agriculture
humanity.

Next, it has been established that a pure hunter-gatherer society needs
between 7 and 500 square miles of territory to perpetually support each
person, depending on the local environment.  Let's assume 250/person, so
we can include the whole 57,000,000 square miles of land area of the
Earth:  We obtain a global population of only 1/4 million.  (And people
wonder why it is tough to find fossils of ancient humans!)  Immediately
I must mention other estimates, that the total global population was
much more:  from 5 to 10 million.  That CANNOT be right; if we use 7.5
million and divide the global landscape the other way, then we get 7.6
square miles per person -- PROVIDED people were using Antartica,
Australia, and both Americas 100,000 years ago, at near-maximum hunter/
gatherer capacity!  In deference to the other estimates, since I have no
idea how they generated those numbers, I shall double my figure to 1/2
million, but no more.

To make this estimate rather easier, I will note the fact that hominids
have just about always occupied every food-containing environment they
could reach.  (My 1/2 million includes/partly-excludes the previously-
mentioned continents by simply using the large number of 125 square
miles per person, but you are welcome to use any more accurate factor
you can derive.)  Finally, as the anatomically modern humans replaced
other hominids around the world, the total number of ALL hominid
hunter-gatherers remained relatively constant.  I'll ignore the rate at
which numbers of modern humans grew from nearly zero to that global 1/2
million, and include the replaced hominids in this estimate (they were
not necessarily so different that they couldn't have been replaced
partly through interbreeding).

So if we take 90,000 and multiply by 1/2 million and divide by 25, we
get 1.8 billion humans.  Immediately I begin to suspect that the
estimate of 70 billion that I had previously encountered was probably
based on EITHER: millions of years of hominid types, and not just
100,000, OR: a total global population of 5 to 10 million -- or some
mixture of both.  I also suspect that we will still fail to CURRENTLY
have more people alive today than have ever lived...(perhaps I should
return to my original global population estimate of 1/4 million?).

When agriculture was invented, so much more food became available that
one major cause of infant mortality was greatly reduced -- and average
lifespan increased to perhaps 33.  The closely related invention of
cities, and the associated larger population of defenders, had several
consequences.  For one, while more food was available, it was not
necessarily equal-quality food; agriculturalists in those days didn't
worry about balanced diets, while hunter-gatherers automatically
obtained balance by simply eating everything they could find.  The
resulting moderate dietary deficiency of city folk retarded the physical
development process somewhat.  In other words, the age of onset of
puberty gradually rose to roughly 17 years -- and the length of a
generation rose along with it.  Simultaneously a cultural shift
occurred, regarding the "proper" age for a woman to be allowed to
marry/breed.  Nowadays we do worry about diet, and so girls are once
more reaching puberty at 14 or so -- their BODIES are theoretically
ready, even if the culture doesn't agree.  Yet note one fact that even
modern culture usually fails to mention:  The more fully-grown a woman
is, the more easily she can bear children.  Even for early city folk,
girls didn't necessarily stop growing, as they reached age 17 while
awaiting menarche.  So the rate of death in childbirth probably went
down a bit, which in turn enhanced population growth (the more women who
survived childbirth, the more ADDITIONAL children they could have).

Other factors that helped the total human population to continue to
expand were inventions like irrigation and iron axes, that allowed more
environments (savannahs and forests) to be agriculturalized.  On the
other hand, diseases and wars caused greater numbers of deaths to people
living in cities than to people in rural areas, so we may with some
reason consider the overall growth in humanity's global population to be
rather steady -- until the Industrial Revolution.

The generally-accepted start of the Industrial Revolution is 1750, 9750
years after the Agricultural Revolution.  It has been estimated that the
total human population in 1750 was at most about 800 million.  An annual
growth rate of about 0.075% per year would cause 1/2 million to become
750 million after 9750 years -- which I shall consider close enough for
the purposes of this Answer.  If we now assume every 33 years the entire
population was more-than-replaced (thanks to that steady annual growth),
then we could take 'snapshots' of the total population, and add them all
up.  I wrote a simple computer program to do this; the number that came
out at the end was more than 30 billion!

So now it immediately seems very likely that no special effort will be
needed to reach that old estimate of 70 billion....  In the last 250
250 years the population rose from the preceding 750 million to a current
value of 6 billion; the average annual rate of global population increase
was MORE THAN TEN TIMES the previous rate: 0.83% per year.  Lifespan has
also increased to a global average of perhaps 60 years.  I modified my
simple computer program to take snapshots for 190 of the last 250 years;
the total number of people seems to have been at least 8 billion.

So while the total number of people alive today is about 6 billion, the
total number who ever previously was alive (as of 60 years ago) seems to
be roughly 1.8 + 30 + 8 billion, or 39.8 (call it 40) billion.  Depending
on how stupidly we continue to litter this planet with people, while at
the same time causing lifespan to grow longer, we could indeed someday
obtain a total existing population that is greater than the total number
who ever previously lived.  Personally, I don't think it is a worthy
goal; we are already straining the biosphere to support only 6 billion.

I fully recognize that this estimate has been rather crude, but I hope it
is an acceptable Answer to your Question.