2. Biological background
The history of life on Earth has been marked by a series of crucial watersheds, each with enormous consequences for the future of the biosphere.
The first watershed, of course, was the appearance of the earliest forms of living organisms over 4000 million years ago. They were single-celled bacteria and were the most complex form of life on Earth for over 1000 million years.
Then, around 2800 million years ago, micro-organisms capable of photosynthesis emerged. Apart from providing energy for life on Earth as we know it today, this process releases free oxygen into the atmosphere. Some of this oxygen becomes converted into ozone which floats up to the stratosphere where it acts as a filter, protecting the Earth’s surface from ultraviolet radiation from the sun.
Other watersheds include the appearance of multicellular organisms between 600 and 700 million years ago, the colonisation of land by plants and animals around 400 million years ago, and the first flowering plants 160 million years ago.
The history of life has also been marked by a number of mass extinctions, the most severe of which occurred around 250 million years ago when around 95per cent of all marine species and 70per cent of land species were wiped out. Another mass extinction took place about 65 million years ago when many forms of life disappeared, including all the dinosaurs and flying reptiles. Some groups of reptiles survived, including the snakes, lizards, crocodiles and turtles.
Some birds and mammals also survived, and during the next 50 million years an extraordinary diversification occurred in these two groups. By 6 million years ago there were upright-walking primates in Africa.
The earliest forms of living organisms, single-celled bacteria, were in existence
Micro-organisms capable of photosynthesis were in existence.
Multicellular organisms came into being
The colonisation of land by plants and animals began
The most severe of the mass extinctions in the history of life (more than 90 per cent of all species wiped out)
The emergence of the first flowering plants
A mass extinction bringing an end to many forms of life, including all the dinosaurs and flying reptiles
Great diversification among birds, mammals and flowering plants
Homo erectus in existence
Homo sapiens in existence
Interdependencies in the living world
Our existence, and that of all other animals, is entirely dependent on the growth of green plants — because plants, through photosynthesis, capture light energy from the sun to manufacture the complex energy-containing organic molecules on which life depends. Nearly all this energy ultimately leaves the biosphere in the form of heat.
An essential characteristic of life on Earth is the cycling of the nutrients that are taken up from the environment, built into the tissues of living organisms, and then eventually released again to become available for incorporation into new life. These nutrient cycles are essential for the sustainability of life.
The fertility of soil is also dependent on its organic content, which plays a vital role in the nutrient cycles. It consists of decomposing plant and animal matter as well as a profusion of different kinds of living organisms.
There are thought to be some 7 to 15 million different species of living organisms on Earth today. About half of them may be extinct by 2075 as a consequence of human activities.
Humans in nature
At the time that the dinosaurs disappeared, about 65 million years ago, there existed a small group of shrew-like, tree dwelling primates. Among them were the ancestors of humankind.
By 180 000 years ago people with the physical characteristics of modern humans, and classified as Homo sapiens, were living in Africa; and by 60 000 years ago some members of this species had reached Australia. About 45 000 years ago Homo sapiens displaced another human species, Homo neanderthalensis, in Europe.
Humans today are biologically the same animal today as their ancestors who lived long before the advent of farming – that is, an animal genetically adapted through natural selection to the life of hunter-gatherers. This fact has important implications for our understanding of ourselves and the challenges that face us in the modern world.
Over the millennia, and as our species spread across the globe, some divergence occurred in the genetic characteristics of human populations, resulting in observable physical differences between people living in different parts of the world.
An extremely important feature of the evolution of Homo sapiens was the development of the human capacity for culture, and consequently the gradual emergence of human culture itself. The most essential aspect of this capacity for culture is the ability to invent symbolic language and to use it for exchanging information and ideas, and so to create, accumulate and share knowledge, beliefs and assumptions.
The ability to invent new technologies and to pass on this technical knowledge from one individual to another and from generation to generation can also be regarded as an aspect of culture.
The evolutionary emergence of the capacity for culture represents another of the great watersheds in the history of life on Earth. Human culture emerged as a new kind of force in nature, and it has had far-reaching consequences not only for humankind but also for the rest of the living world.
One of the outcomes of the capacity for culture was the emergence of religion, involving belief in supernatural spirits and gods. Religion became a universal feature of human societies, although there have always been enormous differences in the beliefs of different religious groups. However, in our current society there are many people who do not accept any religion.
Differences in religious beliefs have been, and still are, a major cause of violent conflict between human groups.
Four ecological phases
The history of Homo sapiens falls into four distinct ecological phases:
This was by far the longest of the four ecological phases, lasting at least 180 000 years (or 7000-8000 generations). 
Ecologically the most important culturally-inspired activities in this phase were the deliberate use of fire and the manufacture and use of tools and weapons.
As in the case of all other animal species living in their natural habitats, for most of the time most members of hunter-gatherer bands are likely to have been in a state of good health. Indeed they had to be in order to survive and successfully reproduce under the demanding conditions of their lifestyle and environment.
Because of the relatively low population density, people would not have suffered from such respiratory and enteric virus infections as colds, influenza, gastric flu, measles, smallpox and German measles. Nor are they likely to have experienced bacterial infections like cholera, plague or tuberculosis. However, bacterial infections following injury would have been a constant hazard.
Phase 2 - The early farming phase
Homo sapiens had probably been in existence for at least 170 000 years before farming began in several parts of the world around 10 000 to 12 000 years (or 400-480 generations) ago. This development marked a turning point in cultural evolution. It was a precondition for all the spectacular developments in human history since that time.
Phase 3 - The early urban phase
This phase began around 9000 years (360 generations) ago when fairly large clusters of people, sometimes consisting of several thousand individuals, began to aggregate together in townships. Many of these people played no part in the gathering or production of food. Occupational specialisation became the hallmark of urban societies.
Although the new conditions offered protection from many of the hazards of the hunter-gatherer lifestyle, malnutrition and infectious disease became much more important as causes of ill health and death.
This phase was ushered in by the industrial revolution, which began a little over 200 years (or 8 generations) ago. It has been associated with profound changes in the ecological relationships between human populations and the rest of the biosphere. The following developments have been especially significant:
- the introduction of machines that use extrasomatic energy, mainly from fossil fuels, for performing various kinds of work 
- the discoveries and applications of electricity and radioactivity
- the spectacular growth of the chemical industry
- the invention, manufacture and use of weapons of mass destruction.
Phase 4 has seen a marked increase in human life expectancy, especially in the developed world – due mainly to improved hygiene and nutrition, but also to immunisation and the use of antibiotics. The global population increased from about 1 billion in 1800 to 2 billion in the 1930s, and it is now approaching 7 billion.
This massive growth in the human population, together with the explosive increase in the intensity of techno-industrial activities, is resulting in progressive ecological disturbances across the whole planet.
We are now living in the final part of this fourth, high consumption, phase of human existence. It is simply not sustainable ecologically and therefore, in the long term, not sustainable in any other way. Continuing business as usual will inevitably mean the breakdown of civilisation.
What lies ahead? This will depend on decisions made, or avoided, by human societies across the globe in the very near future. Can we hope to move rapidly into a very different ecologically sustainable and healthy Phase 5 of human existence? Let us hope so. The alternatives do not bear thinking about.
Human culture as a force in nature
The capacity for culture was of major biological advantage in the evolutionary environment of our species, and in more recent times it has resulted in an amazing increase in the number of humans on Earth.
Apart from its practical advantages, culture adds richness to human experience. It did so in the lives of hunter-gatherers – as in storytelling, musical traditions, dancing and other forms of artistic expression. It does so today in so many ways. Culture makes a huge contribution to the sheer enjoyment of life.
But there is another side to the picture. The consequences of our capacity for culture are not all good.
In fact, as cultures evolve they have often come to embrace not only factual information of practical value, but also ideas and assumptions that are sheer nonsense, leading to behaviours which are equally nonsensical.
Sometimes these cultural delusions have resulted in activities that have caused a great deal of unnecessary human distress, or damage to ecosystems, or both. We refer to culturally inspired activities with these characteristics as cultural maladaptations.
There are numerous examples of cultural maladaptation in human history. 
Cultural maladaptations today
The worldview and assumptions of our dominant culture today are resulting in cultural maladaptations on a scale and intensity never seen before in the history of humankind − maladaptations that are totally incompatible with the survival of civilisation.
There are two sets of changes underlying the major ecological difficulties facing humankind today:
- The huge increase in the human population. There are now about 1000 times as many people on Earth as there were when our ancestors first started farming around 450 generations ago – 70 per cent of this increase has occurred in the past 80 years.
- The massive intensification, especially in the developed countries, of energy and resource use and technological waste production associated with industrialisation, consumerism and economic growth. 
The human species is now using about 12 000 times as much energy and emitting about 12 000 times as much CO2 as was the case when our ancestors started farming some 10 000 years ago. Ninety per cent of this increase has occurred in the past 80 years.
In addition to these ecological issues, weapons of mass destruction constitute another horrendous threat to our future. According to recent estimates, there are around 25 000 nuclear warheads in existence. It would not take many of these to bring an end to civilisation.
Among the other undesirable outcomes of our aptitude for culture is the fact that, largely as a consequence of urbanisation, culture has separated us psychologically from the rest of the biosphere. Instead of feeling part of nature, humans tend to see themselves separate from, and even in some way superior to, the rest of the living world. This mindset is one of the major factors blocking social reform aimed at achieving ecological sustainability.
Ultimate biological limits - an important biohistorical principle 
Through their capacity for culture, humans have long been consciously manipulating the processes of life to their advantage – or to their perceived advantage.
Up to a point, nature tolerates these manipulations. Indeed it offers considerable scope for people to enrich their lives through culture.
However, there are limits, and these are determined by the simple rule that the essential underlying processes on which life depends must not be violated.
For example, the biochemical and physiological processes inside our bodies on which our health depends must be kept intact. We can make changes in our diet up to a point to our advantage – as in the practice of the culinary art. But if we omit essential nutrients like ascorbic acid, or if we add certain unnatural chemical agents, the consequences can be fatal.
We can also somewhat modify our natural sleeping patterns. But there are limits. If we are deprived of sleep for too long we cease to function biologically.
Similarly with the natural environment – the essential ecological processes on which all life depends must be kept intact.
We can practice farming to our advantage. But if we seriously disrupt the natural nutrient cycles, or if we interfere with soil biology so that bacteria and other organisms can no longer play their crucial role in these cycles, then productivity will cease.
Likewise, if we discharge chemical pollutants into the environment that interfere with essential life processes, either directly through poisoning living organisms, or indirectly by changing the climate – then the life support systems on which we depend will ultimately collapse.
When humans overstep the mark by disrupting the essential processes that underpin the whole living system they are in big trouble. There are many instances when this has happened in local regions in the past.
Today our society is transgressing these biological limits on a massive scale and, for the first time ever, at a global level.
There are clearly limits to the amount of damage that humans can do to the ecosystems of the biosphere before they cease to be able to support civilisation. The crucial question is: How far are we from reaching these limits?
Opinions differ on the answer to this question. A middle view is that of the Union of Concerned Scientists (UCS) who stated in 1992 that:
‘No more than one or a few decades remain before the chance to avert the threats that we now confront will be lost and the prospects for humanity immeasurably diminished’.
An important report known as the Millennium Ecosystem Assessment (MA) was published in March 2005. It was called for by United Nations Secretary-General Kofi Annan in the year 2000. It involved the work of 1360 experts around the world. The following excerpts from the statement from the Board governing the MA process summarise some of its conclusions:
‘At the heart of this assessment is a stark warning. Human activity is putting such a strain on the natural functions of Earth that the ability of the planet’s ecosystems to sustain future generations can no longer be taken for granted.
‘Protecting and improving our future wellbeing requires wiser and less destructive use of natural assets.
‘We must learn to recognise the true value of nature – both in an economic sense and in the richness it provides to our lives in ways much more difficult to put numbers on.
‘Above all, protection of these assets can no longer be seen as an optional extra to be considered once more pressing concerns such as wealth creation or national security have been dealt with.
‘This assessment shows that healthy ecosystems are central to the aspirations of humankind.
‘… this is not a counsel of despair. The natural balance sheet we bequeath to future generations depends on choices made at every level and in every corner of the planet.’
With respect particularly to climate change, it now transpires that we have only a decade or so to act in order to avoid catastrophe, by drastically reducing the emission of greenhouse gases and by sequestering carbon through forestation and other means.
1. A generation is here taken to be 25 years. Back to text
2. Extrasomatic energy is energy used by humans outside the human body in various technologies. It is thus distinct from somatic energy which is the energy used in metabolic processes within the human body and provided by food. Back to text
3. A particularly tragic example of cultural maladaptation was the ancient Chinese custom of foot-binding, which prevented the normal growth of the feet of young girls and caused them excruciating pain. This extraordinary practice well illustrates the propensity of culture to influence people’s mindsets in ways that result in activities that are not only nonsensical in the extreme, but also sometimes very cruel and destructive and contrary to nature. This particular cultural maladaptation was mutely accepted by the dominant culture of the Chinese population for some forty or more generations.
Throughout the history of civilisations, different cultures, including our own, have come up with a fascinating range of delusions about how social wellbeing, or prosperity, can best be achieved, and some of these delusions have led to blatant examples of cultural maladaptation.
For example, according to the dominant culture of the Mayan civilisation, prosperity could best be achieved by pleasing the gods, and the best way to please the gods was to torture, mutilate and then sacrifice human beings.
Behaviour based on this delusion can be seen as a cultural maladaptation because it clearly did not do the Mayans any good: their civilisation collapsed suddenly, probably for ecological reasons, around 900 AD. And it certainly caused a great deal of unnecessary human suffering.
Again, the point to be emphasised is the fact that while there may well have been a handful of sceptics among the Mayans, the great majority of them really believed that the torture and sacrifice of humans was an entirely appropriate behaviour. Cultural gullibility is indeed a fundamental characteristic of our species. Back to text
4. Figures for energy use provide a fair indication of the overall impact of humans on the biosphere. People in some of the developed countries today are using around fifty times as much energy per capita as was the case when farming began. Most of this increase has occurred very recently. Back to text
5. Biohistory is the study of human situations, past and present, against the background of the story of life on Earth. Back to text