If a woman marries at about 16, survives until about 40 and has regular sexual intercourse, she will, on average, have at least a dozen livebirths. Even if half of the children die soon after birth, each married couple will leave six descendants. You can see that if this happened for many generations population would grow very fast indeed.
Human beings, like many animals, have the reproductive capacity to increase their populations in a way which leads to what is often called exponential or non-linear growth. That is to say, it follows a sequence: 1,2,4,8,16,32 and so on. The curve of growth becomes steeper and steeper. We can see this happening today. While it took over 100,000 years for human populations to reach 6 billion, it will only take one further generation to reach nine billion.
What are the laws of population?
Populations in history were periodically decimated by three types of disaster: war, famine and disease. People kill each other and destroy their means of livelihood. Famines and food shortages lead to deaths or decline in births, diseases kill.
There has been an alternating pattern in history. Some large accident or invention occurs; a new source of food is discovered (rice, potatoes, maize, herrings) or new ways to grow and harvest plants are developed. There will be a boost to the food supply. Health will improve, people can afford to marry younger, their infants will survive. There will be a spurt in the population. For a time there will be a golden age.
Then, at a certain point, population will build up to such a level that it will usually trigger what we would now call negative forces. People will jostle up against each other and there will probably be violence and war. They will over-exploit the seas and lands and a slight shift in weather patterns will lead to famine.
As populations build up they will also face another basic problem. There is a biological law that micro-organisms multiply and vary much faster than long-living species such as humans. Their natural ability to do this means that as human societies succeed in building up their populations, the micro-organisms that predate on them build up and vary even faster.
Their human hosts crowd together and create a rich material culture of other animals, clothing, food, drink, and masses of excrement. In these conditions, the normal tendency is for the vectors which carry the bacteria and other organisms (flies, lice, fleas) to multiply. The viruses, which only survive with high population densities, also thrive.
Hence there has been a strong tendency through human history for most diseases to increase in variety and incidence. Humans are the prey and as they increase in numbers, so do the organisms that live in and off them. This is a law which is likely to halt the development of all human civilizations. It did so in the past with bubonic plague, malaria and dysentery; it may be doing so today with AIDS or something worse.
Often these catastrophes feed off and exacerbate each other. Ravaging armies cause famine and spread disease. The population will then collapse with huge suffering. The cycle will then repeat itself. This explains why despite the natural potential for growth, the number of people has grown quite slowly over long periods. It is a dreadful cycle, from misery to misery, with brief periods of happiness between. Given the growth in absolute numbers, each time there is a disaster, suffering is greater, a million people dying in a war or famine, rather than a thousand.
Why does more work often lead to less results?
We are constrained not only by our biology, but also by a fundamental economic law which is fundamental to the analysis of why the growth of population does not lead to ever-increasing economic growth. The law states that in most situations there is an ideal level of work that can be put into a task. After a certain point, however hard one works, the returns begin to diminish. Five people working on a stretch of land may produce five tons of grain. Fifty people working on the same area will not produce fifty tons, but a great deal less. An essay which is quite good and takes three days to write will probably not be twice as good if you work on it for another three days.
This law of diminishing returns is one of the keys to the negative effects which result from continued population growth. Through good fortune or invention humans open up a new resource. The first gains are immense. The fish are plentiful, the burnt down forest produces excellent crops. Soon, however, the returns on further input of labour begin to diminish. The cleared forest gives less and less, the marginal lands on the hill-sides are less fruitful than the valley bottoms. This means that after a certain point a growing population fails to provide extra benefits. Each new mouth, after a certain number, is a net loss. We live in a finite world with diminishing resources.
Are there limits to energy for humans?
The amount of energy available to humans on earth up to about 1750 was very limited. For almost all of human history the major energy source was the sun, transformed through plants and animals by photo-synthesis into something humans could use for food or heating. Yet plants and animals can only appropriate a tiny part of the energy which flows onto the earth as sunlight. The development of agricultural systems increased the efficiency of this absorption but still set very firm limits on the quality and quantity of human life.
Almost all resources on earth, even air and water, are finite and tend to degrade with use. At first, with few people, there was plenty. Millions of years of sunlight had been banked in the soils, forests and oceans on the planet. But many of these are non-renewable so that with use they decline in quantity and quality.
Deserts expand, the mountains lose their tree cover, the sees become barren, the forests are felled. Humans as they become more successful exploit nature and often wreak havoc on their environment. Within a few generations the resources are used up. The movement from island to island in the Pacific, exhausting each in turn, is but one example. Another is the huge ecological stress in China, North Africa, the Middle East and the Amazon. Again and again the returns from nature have started to decline through over-exploitation. Now even air and water are under threat.
One partial response to these ecological and economic laws is to improve the technology and thereby tap a new source, or increase the efficiency of energy transformation. Yet this is also a trap. New crops or improvements in old ones (potatoes, rice, wheat, vines) often lead to an increase in yields. Yet the new resource often leads to a dead end.
Thus potatoes trapped the Irish into a situation where they became so dependent that when the blight arrived and aid was refused there was mass starvation. Rice is such a good food that it encourages the growth of dense populations. The dispensing with non-human labour and a movement away from any chance of industrial development is a likely consequence.
Too much can often result in too little. Resources run out. The problems of initial over-abundance, of too rich resources, are as much a trap as under-abundance. They often lure populations down a route which, when it reaches its limit, is impossible to escape from and leaves people in a particularly exposed situation, as we may find with our current dependence on oil.
What hope is there?
You can see in what a fragile and contradictory world we live. We compete with other species and often we have been too successful and eliminated them entirely. Yet there are others, especially microbes, which are more effective than us. We are largely composed of bacteria and surrounded by quickly evolving viruses. Although we now have a growing understanding through the developments in genetics, this is not a final triumph. One day micro-organisms will eliminate, or greatly diminish, the human species, just as we have eliminated or diminished others.
Another constraint is energy. The first law of thermo-dynamics is re-assuring since it reminds us that energy is limitless and irreducible. Yet the second law describes the actual world in which we live where energy dissipates, becomes less concentrated and hence less useful. We temporarily overcame some of the constraints on the amount of energy available when we started to use stored carbon energy as a result of the industrial revolution. The development of high-temperature super-conductivity, or nano-technologies, of more effective use of solar energy, may again delay the final outcome. Yet it cannot postpone it for ever.
Or perhaps we will have too much energy and so we will pollute our world beyond recovery. All human activities, that is the manipulation of atoms and the use of energy, cause ‘externalities’. Everything we do tends to leave a deposit, even if it is only temporary (like sound pollution) and relatively harmless.
We can see this in garbage, in water and land pollution, through the use of insecticides, fertilizers, genetic engineering, the production of greenhouse gases. We all know that the more energy we use and the more of us there are who are living at a high standard of material consumption, the more these problems will grow. One way or another, either a shortage of the energy we can tap, or the damage which the release of that energy causes, is very likely to trap us.
So we shall find ourselves caught in the law of diminishing returns. We will try, being a creative and energetic species, to overcome these problems. We will dig away furiously in order to try to escape from the traps we find ourselves in. It is impossible to predict whether these attempts will lead us to new frontiers in the stars or into rapid extinction.