We won’t go into a study of demography here, suffice it to say that variations in the number of humans are due to two factors:
Above all, these two factors depend on the capacity of the population to produce enough food.
The number of surviving children per woman is by far the leading factor. If you factor in single people who leave no descendants and those who die before reaching an age to produce offspring, you need 2.1 surviving children per woman to keep a population stable. Here, exponential growth enters the picture: with more than 2.1 children per woman, the population will eventually double. Inversely, with fewer than 2.1 children per woman, the population will eventually decrease by half.
The second factor, lifespan variability, has an impact on the number of humans as long as it is changing. Since we are not eternal, the lifespan of a population eventually stabilizes, and so does the number of humans.
Obviously, there are other factors entering into it, as well. I do not claim to be able to resume the science of demography in just a few lines. Let’s just keep in mind that variations in population numbers are due mainly to variations in the number of surviving children per woman.
There is always an equilibrium between the capacity of a population to provide for its own needs and its numbers and composition. When there is enough to eat, humans proliferate, just like bacteria do, until their comfort becomes an issue – and then, they start regulating to ensure better comfort for fewer numbers.
Here are some statistics on the most populous countries in the world (taken from the the "Population Reference Bureau" website).
The value that counts is the one for "natural growth". The term "increase" is inappropriate because, just as for economic growth, it implies a positive value – whereas the value can be negative, e.g. for Europe, Russia, or Japan. It becomes negative when fertility, minus infant mortality, decreases and the life span stabilizes. There is a lag of about one generation between the moment that fertility varies and the moment that the natural population growth starts to vary.
The natural increase in the most populous country, China, remains positive, even though the fertility rate is below 2. This is due to the increasing lifespan. Before long, the lifespan will stabilize and they will start feeling the effects of the only child. Natural population growth will become negative, meaning the population will decline. Fifty years or so from now, it will be only one-half of what it is now.
The country with the world’s second largest population is India, where the natural population growth rate is higher than the global average. The fertility rate is dropping there, but evidently not enough yet to turn the population curve around. India will become the most populous country in the world.
The populations of Europe and Japan are stable, but should decline over the next few decades as the baby-boomers start to die off.
Both Nigeria and Pakistan are countries where infant mortality is high, but where the natural growth also remains high, due to high fertility.
The population growth rate is dropping. We need to keep in mind that for a 2% per year growth rate, exponential growth winds up doubling the population after 35 years, and for a 1% per year rate, after 70 years.
We are approaching the 1% rate, and will eventually reach a negative rate, for which the same principle applies as for positive growth. A 1% decline per year will decrease the population by half over a period of 70 years.
Infant mortality is still falling, thanks to improving conditions in development countries, but will never reach zero.
Fertility, however, is also declining. The rate will soon drop below the 2.1 replacement level, announcing a population decrease over a single generation.
Of course, I have not been able to find a population forecast curve that takes the possibility of a decline in natural resources into account. All that can be said is that if you add together:
the world population will continue to grow for some years and then start to decline.
I will therefore refer to the Bourgeois-Pichat curve used in the Jean Laherrere article using only the part that includes 1950 to 2050 in order to minimize potential errors.
There is always an equilibrium between the number of humans and their capacity to provide for their basic food needs. At a certain level of technological skill, the number of humans fluctuates as a function of variations in the weather. The quantity of nutritional resources varies, then the number of humans varies accordingly.
In the future, as the technology will decline, the quantity of accessible nutritional resources will decline then the population will start to decrease and once again to fluctuate more or less as a function of weather variations.
The evolution of humans here is taken only in the quantitative meaning, I have no intention of entering into a philosophical debate on human evolution. The word evolve doesn’t imply an improvement. A disease, for example, evolves for better or worse. It is the comparison with our 17th and 18th century technological evolution that leads us to believe that humans evolve for the better.
Unfortunately, no one is capable of providing subjective criteria to quantify this evolution – there are as many criteria as there are individuals.
On the other hand, you can say that humans are still always warring among themselves, that they are still just as greedy and vain. So if there is a qualitative evolution, what is it?
As for happiness, I refer the reader to autobiographies written by ordinary people who have lived in rudimentary conditions, for example La vie de ceux d'avant from Albert Cotte editions "Les Alpes de lumière".
So we can consider that:
So we will stick to a study of population change only.