How many people have lived in the World?

Neither floods nor plagues, famines nor cataclysms, nor even the eternal wars of century upon century, have been able to subdue the persistent advantage of life over death. Gabriel García Márquez The Nobel Prize in Literature 1982, Discourse during the Award Ceremony.

A question that I had even before I decided to become a demographer was how many people had put their footprint before me in the places I was walking on? Sometime ago, a friend of mine asked me the question of how many people have been born on Earth since the beginning of our lives as homo-sapiens. I remembered how fascinated I was with the figure I got the first time. It was like: it is not possible! That occurred when Professor Nathan Keyfitz, from Canada, an incredible great professor, considered one of the best in mathematical demography, was teaching a group of Latin American demography students (I was part of that group) about some of the mathematical equations of population change. Sometime after that, I used Keyfitz figures and added the births occurred after the year of his estimate, and obtained the number of 76 billion.

However, more recently, my colleague Toshiko Kaneda, a senior research associate at Population Reference Bureau (PRB), provided the current estimates on how many people have lived on Earth based on an original version from Carl Haub, former senior demographer at PRB. Using more data than Prof. Keyfitz, Ms. Kaneda estimated that this number is actually much higher. She calculated that 108.5 billion people have existed in Earth since 50,000 years ago, a moment close to when Homo sapiens migrated from Africa and arrived to Eurasia. Obviously, Homo sapiens existed before and they were other advanced hominids as the Neanderthals before them. So, as the PRB recognizes, the figure could be a little higher than that but, given the challenge of defining a precise departing point, the landmark of 50,000 years seems like a good solution. As it is stated below, a large part of the number is related to the population growth of the last centuries.

The truth is that we, human beings living on Earth now, including you and myself, are the result of a complex, long-standing, amazing and terrible fight for survival. Human beings have gone through moments of glory and destruction, evolution and regression, fights against nature and more frequently than we would like, fight against others from the same or similar species; indeed a very long history of life and death. However, paraphrasing García Márquez, the fact that we exist is a demonstration that, at least up to now, life has overcome death. You and me, all of us, we must considered ourselves as an incredible illustration of universal luck.

It took a long time for the Homo sapiens to come to existence after life on Earth appeared about 4 billion years ago. Only about 4 million years ago the first bipedal apes came to existence. Homo sapiens developed about 200,000 years ago in Africa. It is considered that they arrived in Europe about 60,000 years ago. At that time, Neanderthals were already there. They were not so lucky and basically disappeared. However, their inbreeding with Homo sapiens left genetic traces in a portion of the world population, including on me. Other hominids also disappeared as the fantastic Hobbits that inhabited the Island of Flores in Indonesia. They disappeared like many other hominids branches.

What has been the evolution of the world population in the last 50,000 years? PRB uses for the last 10,000 years the estimates produced by some scholars and United Nations and summarized by the US Census Bureau. With the exception of the recent past, these are based on limited data available and many assumptions, so they are less and less reliable as we go further into the past and become completely non-existent before the development of agriculture 12000 years ago. It would be amazing if we had enough data about our ancestors for the past 50,000 years. The possibility of using more sophisticated data, modeling and techniques is not an easily attainable task. An enormous financial effort to support more archaeology and genetic research could help. But even if this happens, we would be getting only some points in time that can shed light on how we have evolved but could not provide a good account of population numbers. We may have to live with the idea that, at least numerically, we can have some estimates for the past 10,000 years, with some degree of confidence for the last 200 years, but not earlier than that.

The estimates mentioned above plus those produced by the United Nations for the period between 1950 and now show the amazing evolution of the total population (see graph below). One can appreciate the explosion of population growth that occurred, particularly in the last century. It took the whole history of human beings to reach the first billion. It was about the year 1800. And about 200 years later, we are 8 billion of people, all of us living on Earth, except the six astronauts that are now in the International Space Station. Actually, it was only after 1800 that the rate of population growth started increasing over 0.5 %, a rate that allows a population to double in 140 years. Before 1800, the average population growth rate from the date we have some data (10,000 BC) and 1800, was less than 0.1 %, a rate that requires a period of about 700 years to allow for a duplication of the population. Let’s say you start with a population of 100 million with an annual increase rate of 0.1 %; it will take you more than 9,000 years to reach the first billion.

When we think about the figure of 108.5 billion of births, it seems like a lot. However, because of the demographic revolution in the last century, one out of ten (10.4%) of all births ever occurred in Earth took place in the last 100 years! Isn’t that amazing? We, survivors of this last 100 years generation, have lived during the period of humanity with the higher population and economic growth and also the period with greater innovation and scientific breakthrough. Unfortunately, as it is well known, it has been also the period of major environmental destruction. About 94% of all CO2 produced in the last five hundred years, around the start of Industrial Revolution, has been produced in the last century and about 50 % in the last 25 years. The continuity of human existence, and our numeric existence, is related to how we evolve as human beings from now on.

Before ending this post, I would like to share with you what I was thinking this morning before finishing this post. I saw that the probability to win the Power Ball jackpot (an US lottery) is 1 in 292,200,000 million. Doesn’t it seem almost impossible to be so lucky and get all that money? Well, the probability of each one of us of being born in Earth is 1 in 108,470,000,000, a really very low probability. If you are reading this is because you are one of us, the surviving legacy of the generation of births of the last 100-120 years. Even better, as Adam Rutherford says in his book A Brief History of Everyone Who Ever Lived, your genetic fingerprint is yours alone.

So, you and me, we are really a very lucky product of the universe. Our most precious resource is not time. It is luck. The luck to have been born, the luck to be among the human beings having ever existed in this little and beautiful blue planet hidden in one extreme of the Milky Way. Be thankful and responsible for this luck but at the same time be humble about our uniqueness. Rutherford says that we are only as unique as any other species. And I would add that probably the main difference we have with other species is that we are the only ones capable of producing systems for storing and using data about ourselves. This is our gift.

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