Any such exercise can be only a highly speculative enterprise, to be undertaken with far less seriousness than most demographic inquiries. Nonetheless, it is a somewhat intriguing idea that can be approached on at least a semi-scientific basis.
To start the deduction we need to know the world population, this is elucidated in the image below:
If we look at the numbers, It took us 10,000 ( New Stone Age, home sapiens exclusively) years to reach a billion people, but in the next 127 to add the next billion, 33 years to add another billion. 14 to add the fourth billion. Now for every 13 years we are adding a billion people more or less the pace is stagnant.
If we look at through the ages, population has been very stagnant, until the medical advances starting 19th century, industrial revolution in end 19th and 20th century. Round about after the world war, we seems to have hit a critical mass and just took off!
So How do we calculate as to how many people actually lived on earth?
Any estimate of the total number of people who have ever been born will depend basically on two factors: the length of time humans are thought to have been on Earth and the average size of the human population at different periods. Now since there is an issue with both of them the answer below will just have to be at best a semi scientific guesstimate, scientific though. Some theorists believe that homo sapiens walked millions of years ago, some believe that they walked during 50,000 B.C, with no proper consensus, we will start looking to estimate after 8000. B.C- Agrarian Era.
At the dawn of agriculture, about 8000 B.C., the population of the world was somewhere on the order of 5 million (Guesstimates from the Graph above). The slow growth of population over the 8,000-year period, from an estimated 5 million to 300 million in 1 A.D., results in a very low growth rate—only 0.0512 percent per year. Now to estimate the cumulative population, we have to estimate the life expectancy, the first estimates of life expectancy comes from Iron Age France which was around 26 Years. (Life expectancy), but one has to note that this takes into account only the infant mortality rate, not the pre-natal mortality. So we assume for the entire period plagued with vagaries the life expectancy including prenatal mortality should have been around 15 years (Assumption for the sake of calculation). For the species to survive then, easy math one should have a high birth rate, say around 80-100 births per thousand to support the population, Else we would have been wiped out.
Now take a hunter gather society, If someone who had played any simulation games, infants are a bane to a hunter gatherer society and we should assume exceptionally high infant deaths, Lack of nourishment, diseases, trial and error to handle pregnancies and just plain infanticide practice.
During the period, there was a 90% chance of an infant to die by the age of 1, that means only 10% of the population ever reached the age of one, if someone survives this, there was only a 25% chance to survive till age of maturity, say 14 as an average. Then after maturity the chances of survival increased to 65%. (Life expectancy). If we take this into consideration, it is estimated that the infant mortality rates 500 deaths per 1000 births.
Going back to our graph above: By 1 A.D., the world may have held about 300 million people. One estimate of the population of the Roman Empire, from Spain to Asia Minor, in 14 A.D., is 45 million. However, other historians set the figure twice as high, suggesting how imprecise population estimates of early historical periods can be.
By 1650, world population rose to about 500 million, not a large increase over the 1 A.D. estimate. The average annual rate of growth was actually lower from 1 A.D. to 1650 than the rate suggested above for the 8000 B.C. to 1 A.D. period – The Main reason was Bubonic Plague, this has been credited to be rampant since 542A.D not just the 1600’s.
Some Numbers to the Estimate:
For the purposes of this exercise, We will assume that a constant growth rate applied to each period up to modern times. Birth rates were set at 80 per 1,000 per year through 1 A.D. and at 60 per 1,000 from 2 A.D. to 1750. Rates then declined to the low 30s by the modern period. (There are exceptions in a young population countries)
This semi-scientific approach yields an estimate of about at least 100 billion births since the dawn of the human race.
If there were 1000 births in the Bronze age, 500 died, of the remaining 500, 50 lived to year 1 – Which means 950 Died, out of this 12 lived to maturity. Which means cumulatively the population would have increased by 12. which represents about 88% of people who were dead. Now this is 8000 B.C to 1 A.D
In 8,000 B.C to 1. AD the population had increased from 5,00,000 to 300,000,000. That is 295,000,000 living people , the number of people who would have died before reaching Maturity would have been 2 Billion, that would have represented 20 Billion births. If 20 Billion were to be born Alive, about 40 Billion Children born considering the infant mortality rate.
So estimates of how many people stepped foot on Earth in the 19th Century alone: (GeoHive – Global Data index)
1900 – Population was 1.65 Billion with a birth rate of 40 per 1000
1950 – Population was 2.5 Billion with a lower birth rate thanks to the wars say around 35
2000 – Population was 6 Billion with a lowest birth rate 23 Births per thousand..
using the infant mortality rate stats here : Achievements in Public Health, 1900-1999: Healthier Mothers and Babies
We can guesstimate that 10 Billion people were born between 1900 to 2000 and the current population represents on 7% of people who have lived on earth.