Crunching the numbers: How many people have ever lived?

Adapted from an excerpt by Jonathan Good

What percentage of people who have ever lived are alive today? We know that sometime this year the world population will tick past 7 billion [1]. We know that there are a whole billion more of us than there were in 1999, as if a new China has appeared out of nowhere. The UN forecasts that the world population is expected to rise further, and further, to a whopping 10.1 billion in 2100 [2]. But what about the past? How many humans have walked on this beautiful planet of ours? And what fraction of people who have ever lived are alive today?

The evolution of humanity is a story of humble beginnings. Humans evolved on the plains of Africa an amazing 200,000 years ago [3] but the population for most of human history has been very, very small (for a flavor of the ‘early years’ see Werner Hertzog’s new movie [4]). Estimates suggest that it took 190,000 years for the population to reach 4 million (in a modern context that’s the number of people who ride the NYC subway each morning!). It wasn’t until the dawn of the industrial era in 1800 that our global population hit a seventh of what it is now, inching towards 1 billion. It’s rocketed ever since:

So what happened? Why did the world’s population start growing so quickly, so suddenly?

The world population is driven not only by more people having more children, but also the fact that we’re living longer. In fact, despite the fact that birth rates have been falling in recent decades, the population has continued to rise as those already on the planet lead longer, healthier lives, thanks to modern medicine and improved nutrition introduced in the centuries following 1800. World population growth is essentially birth rate minus death rate - thus, as life expectancy rises, death rates decrease, and the population will continue to rise.

So back to our original question. To estimate the number of people who have ever lived, we examined the average life span throughout human history in conjunction with world population levels. Until very recently life expectancy at birth hovered between 20 and 35 years, but in the past century it has risen to 67 years (it is highest in Monaco, at about 89 years, and lowest in Angola, where people live on average to be just 39 years old) [6].

All this means that in the 200,000 years since Homo Sapiens took her first steps across the African plains, just 57 billion people have ever lived. Astonishingly that means over 12% of all the people ever born are walking the planet at this very moment. Or to put it another way: one in eight people who have ever been born are alive today.

Footnotes

1. http://www.guardian.co.uk/world/2011/jan/14/population-explosion-seven-billion
2. http://www.nytimes.com/2011/05/04/world/04population.html
3. Smithsonian [http://humanorigins.si.edu/evidence/human-evolution-timeline-interactive]
4. For an amazing 3D experience of this check out the new Werner Hertzog movie "Cave of Forgotten Dreams"
5. We use the UN Department of Social and Economic Affairs numbers where available, otherwise from McEvedy & Jones (1978). For a full set of world population estimates see http://en.wikipedia.org/wiki/World_population_estimates
6. https://www.cia.gov/library/publications/the-world-factbook/rankorder/2102rank.html
7. Data from 1950 on from Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat, 2007. Earlier data is sourced from wikipedia and “Medics, monarchs and mortality, 1600-1800: Origins of the knowledge-driven health transition in Europe” by S. Ryan Johansson. Neolithic assumption is that one quarter of humans lived to 40, one quarter to 20 and one half died as infants.