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Energy has a special role on this web site in the form of the plant energy that has fuelled human bodies as food, notably the energy concentrated in the grains of the storable cereals that powered the Neolithic Agricultural Revolution across the world, and the fossil fuel plant energy that drove the Industrial Revolution. This key role of plant energy in the coevolution of plants and people is described in more detail in the article Plant-People Big History.



To live is to consume energy & resources


Energy is the capacity for ‘doing work’, for ‘making things happen’ and ‘getting things done’. Energy is what drives change and growth; and all activity is an expression of energy flow.

Though this might sound abstract and scientific, it is obviously energy and, more specifically, plant energy, that has determined the long-term course of human history – a fact that hardly creeps into our history books which are more concerned with the proximate (short-term) aspects of our past, that with ultimate (long-term) reasons.

Physics and biology are connected to long-term human history through the Sun’s energy that is concentrated in plant chemicals during photosynthesis. Photosynthesis is the miraculous biological reaction on which all life depends. Plant chemical energy, created and stored during photosynthesis, is then converted into biological energy when plants are consumed (either directly, or indirectly as meat) as food. Food is the metabolic engine that keeps bodies going.

But if we want to keep cities going, then we need to supplement the work that can be done by bodies with additional and different kinds of energy.

Historically, simple social goals were achieved by burning the biological energy in human muscle – hunting animals, gathering plants, building shelters, talking, and so on. More complex social goals were later achieved by harnessing additional kinds of energy – initially animal muscle energy for transport, pulling cars, and ploughing fields. Later, we tapped into the concentrated plant energy of fossil fuels. These additional energies were made more efficient by using technology – both the technology of physical tools and machines, and the mental tools that helped to organize, administer, and simplified daily tasks.

Over the full course of human history the daily quantity of biological energy needed to sustain one person has remained about the same (1500-2000 kcal/person), while the social energy required to sustain that same person has increased about 100 times (to around 200,000 kcal/person). This social energy has become increasingly woven into the fabric of human society in ever more complex ways.

This article describes in more detail the role of energy in long-term human history as outlined in the article history in 10,000 words – not only the kinds of energy and their modes of expression, but also their availability, capture, and use. It discusses the way that, although from prehistoric times humans have also harnessed the energy of water, wind, and fire, by far the most significant and limiting factor on human existence has been the availability, capture, and use of plant energy.

Today, though plant energy remains our source of biological energy, for the first time in human history, it is non-plant renewable energy sources that are being increasingly used to drive social systems.

Kinds of energy

We know that the moon exerts a pull on the Earth that creates our tides – but it is the energy from the Sun that drives the Earth’s climate by influencing winds, the patterns of rainfall and evaporation, and the heating of the oceans that generates climate-affecting water currents.

Although in physics we learn about kinds of energy like chemical, kinetic, and potential energy, it is clear that energy use can take on many different kinds – just one of these being the kinds of energy that relate directly to human use.

We think of humans using non-renewable fuels like coal, gas, oil, and nuclear power, and renewable fuels like solar, wind, bioenergy, and hydro. With the exception of nuclear energy, all these kinds of energy can be traced back to the Sun, which is the power generator for life on Earth. As we have seen, it is the Sun’s energy that drives living bodies via the food energy that is provided by plants.

For our own survival, we need to understand, as best we can, how this life-support process works.

The Sun

The Sun

Ultimate source of the plant energy that powers the entire community of life

The Sun is 4.5 billion years old.
The temperature in the core (a hot, dense plasma of ions and electrons) is 15 million degrees celsius, generated by nuclear fusion.
It takes just over 8 minutes for the Sun’s light to reach Earth
Sun-worship makes a lot of sense
Courtesy Wikimedia Commons – Kelvinsong – Accessed 17 September 2018

Plant energy

Plants are consumers of the Sun’s energy, but primary producers in the food chain of energy that supports its animal consumers –  which is all animals including humans. Within nature there is a sustainable turnover of this energy, along with nutrients, organic matter, and water; and this remains stable over comparatively long periods of time. The maintenance of this planetary cycle of production and consumption is called the natural economy and the benefits we humans derive from the natural economy are now called ecosystem services.

The human demand on global ecosystem services has increased over time as our dependence on plant primary production has changed from simple plant foraging and gathering into industrial agriculture, and the manufacturing, transport, and communication systems that are dependent on plant-based fossil-fuels.

Historically simple needs and activities have changed into complex processes. The need for shelter has become the building and construction industry; the advantages of mobility first served by walking and running, changed first into horse and animal transport, then into modern high-energy transport systems; the need for water has changed into vast dams, pipelines, irrigation systems, and treatment plants; and our use of simple materials found in local environments has turned into a global manufacturing sector sharing the world’s planetary resources as part of a growing global economy.

It is ecosystem services that must be protected if humans are to remain sustainable – remembering that the human economy, and its production and consumption of goods and services, is a subsystem of the natural economy. However, the human economy is increasingly absorbing nature and the natural economy, mostly because it too depends on primary production – notably the primary production of crops that provide the food (energy) for ourselves, our livestock, and the consumption of other resources.

The focus of sustainability and environmental management has, in the last decade or so, changed from being reactive to proactive – from cleaning up, to prevention. We have shifted towards the more thoughtful environmental management of human consumption, but with human population numbers likely to increase for several decades yet there is much to be done.

The significance of plant energy for human history is at last gaining some recognition (see history in 10,000 words) which explains how human dependence on plant energy has increased exponentially through three major phases of human history (Natura, Agraria, Industria) whose mode of existence and degree of social organization was constrained by the available energy, its method of capture, and use, and how we are now entering a fourth phase, Informatia

Natural Economy

The Cycle of Life Energy
The Natural Economy of Producers and Consumers

All life is powered by energy derived from the Sun. Plants use this energy directly: animals derive it indirectly from plants.

Courtesy CSIRO Publishing. Sustainable Gardens, Cross & Spencer 2009, p. 9

Radiation from the Sun is absorbed by plants during photosynthesis and stored in carbon compounds that are formed from the combination of water taken from the soil, and carbon dioxide extracted from the atmosphere. This fundamental life process captures, in plant cells, energy that was formed by nuclear fusion in the Sun. Leaving the surface of the Sun, this radiant energy travelled through space at the speed of light for about 8.5 minutes before being absorbed by the plants on Earth. It is this energy that passes through the food chain from plants to animals and that powers our own bodies when we eat. Energy does not cycle like water and other elements and compounds, but passes through the biosphere to eventually leave in the form of heat. Our total dependence on plant energy, like our dependence on the oxygen we breathe (also supplied by plants), is not addressed directly in history books because plants are taken for granted: they are a simple necessity of existence, a ‘given’ that requires no explanation in history.

Biological energy

Understanding the role of energy in our daily lives is facilitated by making a distinction between biological energy and social energy.

Biological energy is the metabolic energy that drives our bodies. Though this energy is needed for us to breathe, digest, and excrete, some of it powers our muscles which are the most obvious way in which our bodies adjust to their environments by providing mobility.

In biological systems it is plant energy that drives the body metabolism that builds the muscle that is so important for ‘getting things done’.

Biological energy is the energy that powers all life. It is derived from the plant tissue built up from photosynthesis and consumed as plant food (stored energy) that ultimately sustains the entire community of life.

In humans most of this energy is used to power metabolic activity in the heart, lungs, stomach, brain, and muscles. However, some biological energy is directed outside the body to achieve social objectives. This portion of biological energy can be called social energy.

Human body energy

The human body obtains metabolic energy from the energy contained in plant tissue and eaten as food. This includes the meat protein that is also derived from plants. Plants obtain their energy from the Sun, converting light into chemical energy in a process (photosynthesis) that humans have been unable to replicate as a means of energy capture and storage. Our bodies make use of chemical energy by breaking down biomolecules like glucose, amino acids and fatty acids. To build tissues (anabolism) others must be broken down (catabolism)., a process that goes on simultaneously all the time in the body, though not always at the same rate. The heart, stomach, lungs, muscles, and brain are especially energy hungry.[1]

A short diversion into the food-energy requirements of our own bodies when we are resting and going about our daily lives will help us get a feel for the energy flows that occur in other organisms, within ecosystems, and throughout the human food chain. Like a running engine our bodies are constantly using fuel. At rest they tick over at basal metabolic rate (BMR). The approximate human daily energy requirement is about 12,500 kJ, equivalent to an energy generating capacity (at BMR) of 80 watts, about the same as an incandescent light (about 20 watts of this energy is being used by the brain). A 100 watt light bulb therefore works 1.25 times as hard as our body (100/80 watts). We could create a human energy unit called a human-equivalent (H-e), and say that the 100 watt light bulb is running at 1.25 H-e.


For most of human history people have been condemned to lives of physical toil. While a small elite have enjoyed the release from labour that comes with wealth and privilege, most people have eked out a subsistence living by back-breaking labour in the fields. The magnificent pyramids and monuments of the ancient and classical world were assembled with blood and sweat, without modern machinery.

The Industrial Revolution, it was anticipated, would provide some release from this burden of daily physical toil although the early transition from farm to factory did not seem like an advance. Today, as we struggle with obesity and information overload, we can hardly imagine the work load required to feed and clothe the large families of former times.

The energy we burn in physical activity is derived from the food we eat. Like all other animals, we obtain body energy either directly from plants (as cereals, fruit, nuts and vegetables) or indirectly through the consumption of animal meat (which ultimately derives its energy from plant matter either directly or through an animal food chain).

The Industrial Revolution did eventually remove the need for physical toil, and it did so by combining the use of the concentrated energy in fossil fuels with new and sophisticated technologies. Tractors replaced hoes, scythes, and pitchforks as agriculture was industrialized and transport and communication systems transformed. With the advent of the Industrial Revolution the numbers of agricultural workers rapidly decreased as people moved into the cities, mines, and manufacturing industries.

Europe was developing its global empires as its mills and factories processed the raw materials supplied by labourers in colonial plantations. With this vast influx of almost limitless social energy came an acceleration in growth in populations and economies – and a rapid increase in the complexity of social organization.

The energy needed to support this cultural expansion, the energy coming from fossil fuels, was of little concern because after a while it became almost limitless. But the social energy embodied in today’s goods and services is 100 fold greater than the biological energy that formerly sustained us – and its use has come at an unexpected price . . . climate change.

Social energy

Early kinds of social energy included the wind that swelled ship sails and powered the mills that ground flour and moved water. Fire provided heat for warmth and cooking. Gradually more effective and concentrated kinds of energy became available. This included the energy concentrated in cereal grains that could be stored in preparation for times of less abundance. The energy contained in fossil fuels could be combined with the technological productions of increasingly differentiated, complex, and sophisticated large-scale social organization – like furnaces and large machines that transformed industry and transport and communication systems.

To comprehend the historical explosion in the quantity of our individual use of social energy it helps to understand the idea of embodied energy. The energy expended by our hunter-gatherer ancestors making clothes involved the energy needed to collect the necessary fibres, skins, etc. and to weave of cut them into the desired form. Today materials may be flown in by plane and woven on machines before being transported to shops and sold. Every step of this process involves the expenditure of energy and this expenditure of social energy is embedded or embodied in the final product. The production of today’s clothes requires energy use on a scale that would not be possible without these of new and more concentrated energy sources made more efficient by technological innovation. For us, as global citizens and consumers, it is important to know, and recognize through our behaviour, that energy flows are both ‘direct’ (as when we ‘burn’ the petrol in our cars) but is also ‘indirect’ or ‘embodied’ in the goods and services we use. A lot of energy would have been consumed in the manufacture of our car. When we buy a product we are, in effect, also buying (and, ideally, accepting responsibility for) all the resources and all the environmental and social impacts that were ‘embodied’ in that product . . . except that we rarely have any idea what these are.

The most obvious historical source of concentrated energy was fossil fuels. Some of the ancient energy of the Sun has remained locked up in plants that was fossilized many millions of years ago. This highly concentrated energy, stored in coal, oil and natural gas, drives human industry. It is released, together with carbon dioxide, when fossil fuels are used, returning the long-stored carbon to a very different atmosphere and world from the one in which it was collected.

Fossil fuels are intimately bound up in the history of the biosphere as well as our own history and way of life. The transition to a post-industrial society and a modern standard of living in the West can be attributed to the use of vast quantities of cheap fossil fuels. We now depend on these in almost every aspect of our daily lives, from our alarm clocks in the morning, to our travel to work, the lighting, heating, and cooling of buildings, and the production of the food we eat.

If history is about ‘getting things done’ then we must ask how this is achieved in terms of energy flows. Social energy is energy that is directed to social ends. It may be biological energy, made more efficient when leveraged by mental and physical tools, or it may come from totally different energy sources. Up to the time of the Industrial Revolution, with its machines powered by fossil fuels, the work needed to physically maintain complex societies was provided by human (and to a lesser extent animal) muscles. The awe-inspiring monuments of the ancient world – the Great Pyramids, Stone Henge, the magnificent buildings of Ancient Greece and Rome like the Parthenon and Colosseum, the temples of Angkor Wat – all were achieved using human toil, and not always by choice but as slave labour. Though craftsmen and artisans might willingly provide their labour to such causes the European empires of the 17th and 18th centuries were built on products like sugar, cotton, tobacco, tea, coffee, and rubber that were provided from plantations maintained by slave labour.

Much of human social and political history has been bound up in the subtle linguistic distinctions and working conditions associated with ‘rulers and administrators’, ‘employers’, ‘managers’, ‘employees’, ‘servants’, and ‘slaves’. The owner of a large country estate simply cannot do the cooking, cleaning, and maintenance needed to keep that estate going, for efficiency there must be a division of labour and that, historically, has led to hierarchies and inequities. (For a discussion of hierarchies see Great Chain of Being, and reductionism).

Energy availability, capture, and use

The significance of energy in human history is closely related to, not only its form and mode of application (e.g. cereal grains and fossil fuels used as sources of biological energy and social energy), but also its availability, capture, and use.


The availability of plant energy relates to geography. We see this, for example, in the location of the regions where life-transforming agriculture arose (including cities and civilization) in the valleys of the Nile, Tigris, Euphrates, Indus, Yellow, and Yangtze rivers. Here in the ‘lucky latitudes’ climatic and ecological conditions after the last Ice Age aligned with the availability of domesticable plants and animals (Agraria) that made farming possible. The ready supply of coal in Britain at a time when technology could make clever use of this concentrated form of energy accounts for Britain being at the centre of the Industrial Revolution (Industria).


These factors relate to technology – during Agraria the capacity to harness and process crops and, during Industria, the capacity to process fossil fuels. The modification of transport and communication systems (trains, steamships etc.) made possible by the use of fossil fuels changed the meaning of geography.

While the entire community of life converts plant energy into the biological energy that powers their metabolism, human biological energy is used to power not only our body metabolism as metabolic energy, but the muscles that help us achieve social goals – this portion of our energy use referred to as social energy.  In the course of history, we have achieved social goals by first using our own muscles, then the muscles of animals, and then the concentrated energy of fossil fuels.

Acceleration & Growth

Biological studies on the growth of populations of organisms show that their numbers will increase until either:

a) they come into equilibrium with their environment (determined mainly by resource availability but also predation and other factors) or
b) there is a repeated pattern of flourishing and collapse or c) if numbers greatly exceed sustainability then there may be a sudden and total collapse leading to extinction.

Human populations have tended to grow until reined in by wars, famine, disease, and natural disasters. This was the thesis of the English curate Thomas Malthus in his Essay on the Principles of population (1798). 

Today we live in societies where medicine has greatly reduced the risk of disease and the energy that underpins growth is cheap and abundant. We know that societies thrive when they are growing and that is reflected in our economic theory and practice.

However, growth cannot be limitless and there is always the danger that it will be halted by one of Malthus’s limiting factors if it is not carefully managed. There is now the possibility of artificial birth control, although the use of this as a means of regulating global population in a non-voluntary way is highly controversial. Based on current figures the global human population should plateau by 2050 but by this time will have increased from 8 billion (November 2022) to between 9 and 10 billion – a critical period in global history.

Meanwhile the acceleration and growth in complexity of social organization and technological sophistication continues. Advances in biotechnology are outpacing the demand for bioethical standards while we struggle to find ways of regulating social media and the internet as a source of modern-day crime, not just useful information and friendly communication. While technology and social organization has changed, human nature has not – although, given the choice of an historical period in which to live it is likely that today has more to offer each individual than any other period (see progress).

Energy & CO2

It is the carbon dioxide released when fossil fuels are burned that are the source of the enhanced greenhouse effect that is driving climate change. Finding ways to beat our fossil carbon addiction is one of the greatest challenges for sustainability science. Any management of atmospheric carbon dioxide must start with the knowledge of how carbon cycles between land, plants, the atmosphere, and the oceans. The movement of carbon between the major sinks is known as the carbon cycle and it is closely linked to the flow of energy through living organisms.

Stabilizing the world’s climate will require high income countries to reduce their emissions by 60-90% over 2006 levels by 2050. This should stabilize atmospheric carbon dioxide levels at 450-650 parts per million (ppm) from current levels of about 380 ppm. Above this level and temperatures would probably rise by more than 2o C to produce ‘catastrophic’ climate change.


Australian annual solar radiation (2006)

indicating regions most suited to harvest the Sun’s energy

Plant-People Big History

Energy is an elemental force in the universe that takes many forms. However, it is plant energy[2] that best explains the path of human history.

This article has investigated those kinds of energy (e.g. food, fossil fuels) and their modes of expression (e.g. as biological energy, social energy) that have had the greatest influence on human history. This influence has depended on their availability which, in turn, has depended on geography (e.g. of cereals first used for agriculture in the ‘lucky latitudes’, and the fossil fuel coal that launched the Industrial Revolution in England) but also on technology (as a function of social organization) that was available for its capture and use.

Energy, when harnessed in an organized way, as occurs in nature, encourages growth and diversity. This is demonstrated by both an increase in overall population numbers and an acceleration in organic differentiation. In social systems it facilitates an increase in the complexity of social organization and improved technologies.

Biological energy, the energy that sustains the bodies of living organisms, is derived from the energy of the Sun, captured and stored in plant tissues during photosynthesis – and then taken into the body as plant food which is the ultimate life-sustaining fuel for all living creatures. This energy arises during photosynthesis in an energy conversion from electromagnetic energy to the chemical energy that is expressed as biological energy.

The historical role of plants in the process of human-plant coevolution becomes more transparent when we consider the way humans have devised ever more efficient ways of harnessing energy, first as life-sustaining biological energy (food), supplemented later by the society-sustaining social energy of fossil fuels.

Since both biological and social energy are derived from plants then it is plant energy that has underpinned the growth of individual organisms, human populations, cities, and economies. This has transformed the human environment of biological evolution into one of cultural evolution that has resulted in a global community.

With an increase in the complexity of social organization there emerged the technological possibilities of scale that have facilitated globalization. Over the last 500 years of accelerating growth in social complexity there has been a global redistribution of both humans and plants across the surface of the Earth – a replacement of natural landscapes of wild plants by cultural landscapes of anthropogenic plants.

The coevolution of plants and people reached a cultural climax in the Age of Plants that lasted from around 1550 to 1950, an Age that is now passing as we transition away from fossil fuels.

Human dependence on plant energy has increased through four major phases of human history whose mode of existence and degree of social organization was constrained by kinds of energy, its availability, method of capture, and use.

These four phases are called Natura, Agraria, Industria, and Informatia which are described in detail elsewhere. Human energy was first obtained from wild plants (Natura), then cultivated plants (Agraria), then supplemented by a massive boost of social energy obtained from fossil fuels that fed into an acceleration in the growth of population, social organization, and technology that created Industria and globalization.

Today, in Informatia, our dependence on plants for social energy is waning as the world economy transitions from plant-based fossil fuels to renewable energy sources like wind, solar, biomass, nuclear and hydro as, it seems, we approach a new milestone for humanity . . . peak per capita energy use.

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Daily food needs - 1500-2000


   Natura       -     5000-10,000
   Agraria       -    10,000-30,000
   Industria    -    200-230,000
   Informatia  -   200,000 +



Date of origin

Base state   -       human muscle

Hand tools        -     3.5 M BP
Mental tools     -     3.5 M BP



Fire                     -     1.7-2 M BP
Animal muscle -     12000 BP
Wind & water   -  ...  5000 BP ...
Coal                    -      1600 ...
Gas                     -      1820 ...
Oil                       -      1860 ...
Electricity           -      1880 ...
Nuclear              -      1950 ...


             Natura       -   0.25m
             Agraria      -   0.48
             Industria   -   0.26 - 0.31


The energy derived from the Sun, stored in plant tissue during photosynthesis, then used (as food) to power the bodies of living organisms. Most biological energy drives internal metabolic processes within organisms but some is transformed directly into social energy via muscles and brains.

The food energy needed to sustain an individual human body has remained about the same throughout history (though physically active people require more calories) at about 12,500 kJ. while the human body has an energy generating capacity (at basal metabolic rate) of around 80 watts (about 20 watts of this being used by the brain), about the same as an incandescent light bulb). To derive a physical 'feel' for what this means, a 100 watt light bulb works 1.25 times harder than our body, that is, 1.25 H-e or 1.25 human equivalents.


The energy that powers the social activity that may be directed towards the maintenance or enhancement of social organization.

The energy of human social activity is derived partly from the biological energy that powers human bodies, and partly from external sources like water, wind, animal muscle, fire and more recently, fossil fuels, nuclear fuels etc.

Historically, the proportion of social energy derived from human bodies has decreased over time to become negligeable today. Fossil fuels provided a concentrated, abundant, and cheap source of social energy that facilitated growth in populations and economies. The use of this energy has been leveraged by the increasing efficiency of technology as both material and mental tools.

Media Gallery

Top Countries by CO₂ Emissions per Capita 1950 to 2018

Animated Stats – 2019 – 5:49


First published on the internet – 1 March 2019
. . . 1 October – 2020 revision
. . . 19 July 2022 – revision
. . . 6 October 2022 – minor editing (added Big History para.)
. . . 22 November 2022 – complete revision



World Emissions Flow Chart – Data from year 2000
The flow of energy through complex social organization.
Global greenhouse gas emissions by sector, end use, and gas.
Source: World Resources Institute – see caption

World Emissions Flow Chart
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