The single key factor governing all life on earth is energy. Energy is used to power the metabolic processes of bodies and, in so doing, it allows bodies to direct some of this bodily energy into an interaction with the organisms’s environment. A distinction may be made between energy that is directed inwards and expended in powering internal bodily metabolic processes (biological energy), and energy that is directed outwards to meet environmental or social needs social energy. There is a sense in which all bodily energy is used to support the organism’s existence within a broader environment but the distinction here draws attention to, say, the energy used to power muscles that, say, take an organisms from one place to another rather than the energy needed to make the heart pump.
This energy distinction is especially useful in the case of humans who have managed energy by adopting two very different modes of existence.
Firstly, by minimizing the expenditure of bodily energy by adopting the life of a hunter-gatherer and, secondly, by harnessing additional energy sources from the environment to build social organization.
Plants provide the energy that powers the bodies of almost all the world’s living organisms. Plant energy is energy that is ultimately derived from the Sun and stored as ‘biological potential energy’ in plant chemicals during photosynthesis. It is these chemicals that, when consumed by organisms as food, are then ‘burned’ to drive the metabolic processes that sustain life.
Humans use some of this biological energy to do social work outside their bodies. They also use additional forms of energy – like the wind that propels sailing ships, the energy of flowing rivers that turns mills, and fossil fuels that drive machinery – to increase social activity and build social organisation. The energy used for social activity we call ‘social energy’. Plant biological energy is critical for survival and reproduction and biological growth, but social energy has played a further role in promoting a different kind of growth, the growth of economies, populations, and social organisation – especially over the last 500 years.
In these two ways plants have underpinned all the major human socio-economic transformations because they have, until now, provided both the food energy that powers human bodies and the additional social energy (mainly as fossil-fuels) needed to build and maintain complex human societies.
Plants as drivers of human biological and social change – Historically, social organisation has increased in complexity as existing energy sources were used more efficiently and new, accessible, and more concentrated forms of energy were discovered. In pre-industrial societies it was mostly biological energy powering the muscles of slaves, workers and domesticated animals that provided the energy needed to maintain economic activity. But it has been mostly the social energy of fossil-fuels that has driven modern industrial economies – their systems of resource extraction, production, distribution and consumption. The growth and interconnection of widespread populations, and the diversification of economies, presented the emergent possibilities of social scale, such as the capacity to build ocean-going ships, assemble armies, to mint and exchange coins, to make computers and develop complex technologies.
The food energy needed to sustain an individual human body has remained roughly the same throughout history (though physically active people require more calories), but the energy required for social transformation, the social energy that drives social and economic growth and complexification, has increased dramatically in the modern era, woven ever more deeply and obscurely into the fabric of our energy-intense products and lifestyles.
Our total dependence on plants, 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’. But when basic resources are threatened, the consequences gather historical significance to become part of environmental, as well as economic and cultural history. We see this today in relation to food security, climate change and our human sustainability on planet earth.
Human dependence on plant energy has increased exponentially through three major phases of human history whose mode of existence and degree of social organisation was constrained by the available energy, its method of capture and use. We are now entering a fourth phase. For convenience we have referred to these four phases as Natura, Agraria, Industria, and Informatia (Fig. 2).
First was the phase (Natura) of nomadic hunter-gatherers in the Upper Palaeolithic which lasted from about 315,000 BP to around 12,000 BP (these dates are still a matter of keen academic debate) when small tribal groups of 10-20(-100) people lived within wild nature at a time when the world human population peaked at around 3 million (Morris 2015). Universally the plant diet was quite varied, consisting mostly of wild greens, fruits, seed, and root vegetables and the meat of hunted animals. Some additional indirect energy was needed to support individual and collective lives – such as that obtained from fires or embedded in the materials used for clothing, and so on. Social activity was achieved mostly using human muscle, so this was a form of existence that benefitted from energy conservation, with minimal possessions, the use of simple domestic and hunting tools, and the highly developed skills of bush craft. The combined total of food energy and social energy was roughly 1.5 to 2 kcal/capita/day (Morris 2015). The migration of prehistoric hunter-gatherers, walking out of Africa to occupy the world, took about 60,000 years sourcing wild animals (ultimately dependent on plant energy) and wild plants as food for muscle-power. Hunter-gatherers placed little emphasis on political and wealth hierarchies but accepted gender hierarchy and violence (Morris 2015).
Second, was the phase of settled farming communities (Agraria) with their domesticated plants and animals which, during the Neolithic Agricultural Revolution, arose independently in 6-12 centres of civilisation (the exact number is debated) across the world, the first appearing about 13,500 years ago in the ancient Near East. Most people in these communities were engaged in farming based on cereals. Grains were a concentrated source of energy that could be stored for year-round use. Some settlements thrived and grew into Bronze Age cities that became trade centres for even larger human groupings of nations and empires.
Effectively governed communities with large populations took advantage of new technologies made possible by their scale of operation in a series of changes that increased the complexity of social organisation. The sedentary existence facilitated population growth in hierarchically governed urban societies displaying many of the characteristics we associate with the cities of today: a division of labour, coinage, monumental architecture, private ownership, sophisticated legal and economic systems, art, written records etc. as proportionally fewer people worked on the land.
In and around the Bronze Age cities there were now specialised (often enclosed) social spaces that contained cultivated plants, spaces that served different social functions and which have persisted to the present day: fields, public parks, avenues, orchards, vegetable and market gardens, vineyards, gardens – both domestic and royal – and the formal plant decoration used around administrative blocs, temples and burial sites. It was during the Bronze Age interaction of trade, diplomacy and military conquest that occurred between Mesopotamia, Egypt and the Aegean during the third to second millennia BCE that ‘. . . gardens emerge as distinctly meaningful spaces’ (Stackelberg 2013).
The increase in social organisation was made possible by the surplus energy from plant and animal domestication and the human population multiplied from a few million people at the dawn of the post-Ice-Age Agricultural Revolution about 10,000 years ago, to 400 to 500 million around 1550 CE at the dawn of phase 3. The combined total of food energy and social energy needed to produce, store and distribute the grain and sustain communal activity had now increased fourfold to around 6-8 kcal/capita/day (Morris 2015). Agricultural societies no longer depended on wild plants but used the energy provided by cultivated plants to feed the muscles of man and domesticated beast over a period that lasted about 10,000 years.
Third was the Modern Era (Industria) which lasted about 500 years from around 1550 to 1950 as a time of rapidly increasing social complexity – the advance of science, technology and medicine that was combined with population growth, industrialisation, democratisation, the development of nation-states, and the global connectivity that flowed from the Age of Discovery and European colonial expansion. This accelerated social change took place when the muscle power of humans and domesticated animals was supplemented by using the concentrated plant energy found in fossil fuels, providing the social energy that powered industry and manufacturing. Industrial agriculture used sophisticated machinery to boost food production in what was, in effect, a second Agricultural Revolution as people moved from farm to factory, from toil on the land to work in the expanding cities. The number of people working in agriculture fell dramatically in technologically developed countries – from over 90% at the beginning of this period to less than 5% today (see Roser 2019).
It was fossil fuels (fossil plants) – first coal (a convenient replacement for the rapidly diminishing supply of timber fuel) and then gas and oil – that were the drivers of increasing global connectivity and social complexity. The outcome of Industria was a world that aspired to energy-hungry Western lifestyles that consume over 200 kcal/capita/day (Morris 2015), a hundredfold increase over that of hunter-gatherers. Plant-based energy use increased from about 38 kcals/person/day in 1800 at the start of the Industrial Revolution to around 92 kcals/person/day in 1900, and 230 kcals/person/day in 2000 (Cook 1971).
Phase four (Informatia) followed in the wake of the devastation of two WWs as economic recovery gathered pace around 1950. Increasingly sophisticated science and technology facilitated globalisation and the more efficient extraction of planetary resources that produced unprecedented economic and population growth – the Great Acceleration – as the world population soared from 2.5 billion in 1950 to 6 billion in the year 2000. It had taken about 200,000 years for the human population to reach 1 billion around the year 1800 and then, fired by fossil fuel social energy, only 200 years more to reach 7 billion.
Cities expanded upwards with the advent of skyscrapers, and outwards as domestic houses and gardens multiplied to form sprawling suburbs. In 1800 about 3% of the world population lived in cities but by 2017 this had increased to 55% (Index Mundi World Demographic Profile 2018) as more land was appropriated to provide the food needed to feed the growing population.
Human appropriation of plant net primary productivity (HANPP) is a metric that tracks the percentage of global net primary production for human food, livestock production and fuel: it includes the loss of potential NPP due to human land use (Fig. 3). It is a benchmark indicator of human impact on the biosphere. From 1910 to 2005 HANPP almost doubled from 13% to 25% while population grew 2.7 times and GDP grew about 17 times (Krausmann et al., 2013; Haberl et al., 2014). The increasing harvest from forests and the additional land occupied by infrastructure has added little to HANPP. It is agriculture that dominates HANPP globally, representing 84–86% of total appropriation of plant growth over the entire period, with 42–46% on cropland and 29–33% on grazing land. We must also consider the impact of plant-energy-dependent domesticated animals. The ‘MacCready explosion’ claims that 10,000 years ago humans, their pets and livestock comprised around 0.1% of the terrestrial vertebrate biomass. Today this total has rocketed to 98% (MacCready 2004). Though a statistic that is difficult to substantiate, this is a stark reminder that beyond human demands for plant food and other resources are the demands on planetary ecosystems resulting from animal domestication.
But there is much more to the social, economic and environmental dimensions of this fourth human transformation. Economically the post-WWII social activity has entered what is sometimes referred to as the post-industrial information society, or knowledge economy, in which the service sector generates more wealth than the manufacturing sector. Politically it is associated with decolonisation, the geopolitical dominance of America, the formation of the United Nations and, latterly, the rise of Asia, particularly China. Environmentally it has been marked by a new epoch called the Anthropocene. Socially perhaps the most obvious transition is from printed to digital communication with the advent of computers and the internet whereby the many forms of knowledge, once only available to the privileged few and therefore a form of social distinction, is now available to all who have access to computers or smartphones. Scientifically Informatia is strongly associated with the penetration of the last major scientific frontier – the human mind, consciousness, mental and other forms of computation, and artificial intelligence.
Throughout history humans have been totally dependent on plants as a source of energy, both the energy that supports biological metabolism and the energy that builds societies. This energy came first from wild plants (Natura), then cultivated plants (Agraria), then the additional energy of fossil fuels (Industria) that facilitated globalisation and increasing social complexity.
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.
Human-plant coevolution – From the account of the four human phases described so far it might be claimed, in general and simplistic terms, that plants have been a major human source of life, health, wealth and happiness. But humans are not the only beneficiaries, the causal connections between humans and plants is two-way, a co-evolution.
Humans lived first as Palaeolithic hunter-gatherers in wild nature with both their biological and cultural evolution strongly determined by factors in the natural environment. Then, as Neolithic farmers, it was cultural factors that increased in significance. Humanity had taken a giant step from a mode of existence dominated by nature to one constrained by culture as, superimposed on slow biological evolution there followed a period of rapid cultural evolution. In domesticating plants and animals, humans had adopted a sedentary way of life that changed their environments of both biological and cultural evolution. The result was modern civilisation, and in this sense, humans were co-incidentally domesticated by plants. Humans whose bodies and minds were fashioned by adapting to natural wild environments, now live in artificial environments, a result of the greatly accelerated increase in social organisation made possible by the energy of cereal grains and fossil fuels. This has created a new set of challenges, a new environment, for both biological evolution and cultural adaptation.
Through the first three phases six significant categories of economically important plants emerged: those used in everyday living (dyes, fibres, resins etc.), medicinal plants and spices, horticultural plantation crops, agricultural crops, forestry timbers, and the ornamental plants of horticulture (first clearly recognised in the agrarian cities of the Bronze Age). There is a further and more recent seventh category, the unintentionally introduced plants that have escaped from cultivation to naturalise in the wild across the world.