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Peter A. Jolliffe
Agroecology Program
Faculty of Land and Food Systems
University of British Columbia
Vancouver, BC V6T 1Z4
Canada
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Summary: The long relationship between humans and plants has mutually affected both partners. Our primate predecessors mainly existed as vegetarian tree-dwellers.
Conditioned by this, primates evolved several key traits, including the ability to grasp and manipulate objects and excellent vision. Through much of its several million years of existence the genus Homo survived as a hunter-gatherer, with plants being a normal part of the diet. The post-glacial agricultural revolution, involving the domestication of plant crops, supported the development of the first Neolithic cities. This eventually led to other technologies, more complex societies and civilization. Humans presently exploit many plant species in diverse ways: for fibre, building materials, pharmaceuticals and aesthetics. We should not, however, underestimate key the role of plants as food. They presently contribute more than 90% of the dry matter in food consumed by humans. And, through their primary production, they support the functioning of global ecosystems upon which we ultimately depend. Two strategies have largely been responsible for increases in agricultural production during the past two centuries: (1) expanding the area of cultivated land, and (2) increasing food output per land area by intensification of inputs. Looking to the future, human population increase will drive increased demand for food, and these strategies will be less effective than they have been up to now. It seems essential, therefore, that humans re-think their connections with plants, and work to establish a new and more sustainable partnership.
Introduction
Almost exactly 40 years ago I was fortunate to attend a paper given by noted cosmologist Carl
Sagan, delivered at meetings of the American Association for the Advancement of Science in
San Francisco in December 1965. At the time, Sagan was working to prepare for the first space missions to Mars, and he was concerned with the problem of how to detect the existence of life on other planets. The title of his paper, however, was “
Is There Life on Earth
?” Creatively,
Sagan had inverted the problem and wondered whether satellite-based imagery, using technologies available at that time, would definitively prove that Earth supports intelligent life.
This was only 8 years after Sputnik, and remote sensing was primitive by today’s standards.
Indeed, after analyzing the available satellite images of Earth, Sagan concluded that those data alone were insufficient to prove the existence of intelligent life on Earth. The best evidence he could find was from images of clear-cut areas in the Canadian boreal forest: patches of snowfall on logged areas stood out as regular white tiles against the darker background of the surrounding forest. As always, Sagan was a compelling speaker and I found it striking that, looking from space, the clearest indication of human presence on Earth at that time was offered by our influence on vegetation.

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Here, I will give an overview of some of the main relationships that have existed between humans and plants, from our primate ancestors to the present day. Within the space available, a comprehensive treatment of the many links between plant resources and human development is not possible. Instead, I will sketch some broad perspectives on these relationships, reflecting on humans, food and agriculture. In doing this I will emphasize how our relationships with plant resources have changed, and how those changes have operated to transform both ourselves and the plant species that we depend upon.
Ancient Ancestors
Anthropologists have broadly delineated the evolutionary sequence leading from early primates to Homo . We descend from arboreal quadrupeds which mainly lived as vegetarians. This is evident in our digestive systems and in the structure of our teeth and facial muscles, well adapted to masticate and process plant food. Our hands, being adapted for tree-climbing, are wellstructured for grasping and manipulation, and our binocular colour vision is highly effective in distinguishing objects in dense vegetation. Clearly, through our primate lineage, we have been partly sculpted by our ancient relationships with plants.
Insights into our early relationships with plants are offered by observations of our closest relatives today, the bonobo ( Pan paniscus ) and chimpanzee ( P. troglodytes ). Both species have highly diversified diets, being primarily frugivorous but also consuming leaves, flowers, and pith
(Rafert & Vineberg 1997; Newton-Fisher 1999). Both species have colour vision, and by their behaviour they are known share with us a high ability to distinguish aromas and flavours associated with palatable and nutritional dietary sources. Field observations on bonobo in the
Congo basin found that it exploits over 110 species of plants as food sources, although more than
80% of their diets came from less than 10 plant species, especially fruit of Dalium (Rafert &
Vineberg 1997). Chimpanzees spend about 7 hours a day feeding, much of that time in trees. At
Budongo Forest Reserve in Uganda, for example, ground-level herbaceous vegetation was little used for food, and chimpanzee diet was dominated by arboreal fruit of four plant species and leaves of two species. Figs were considered to be the staple diet, being consumed through most of the year (Newton-Fisher 1999).
There is a small non-plant component in the diets of both bonobo and chimpanzee. For bonobo this is mainly invertebrates with possibly some freshwater shrimp or fish. Chimpanzees also prey upon young baboons and monkeys (Rafert & Vineberg 1997). Pioneering observations by Jane Goodall at Gombe National Park in Tanzania revealed that chimpanzees exploit plant tools in feeding on termites. More recent studies (Sanz et al . 2004) have found that reusable wooden tools are regularly employed by chimpanzees to extract termites from aboveground and subterranean nests. Their tool forms are specialized for different tasks, and tool assemblages differed between different communities of chimpanzees. It is not known for certain whether our common ancestors shared all of these attributes, but collectively the evidence suggests ancient and fundamental connections between plants, human physique, tools and the origins of culture
(Balick & Cox 1996).
The Post-glacial Agricultural Revolution
For much of their several million years of history, members of the genus Homo existed as scavengers and as hunter-gatherers. Their stone and bone tools indicate that meat was an important part of their diet, and like more modern Inuit peoples, big game hunters such as Cro
Magnon may have used plants as a secondary source of food. However, patterns of dental wear

3 confirm that even for the big game hunters plants remained as a staple part of the human diet, and the important use of plant materials for fire during glacial periods is not to be discounted.
During the several thousand years following the last glacial retreat, humans in several parts of the world independently underwent a revolutionary transition from hunter-gatherers to agricultural societies. Changes big game abundance associated with post-glacial climate change may have pressured people to undertake cultivation (Pringle 1998). The transitions to agriculture were gradual, and the timelines and details are not fully understood (Pringle 1998). They are best documented for the Near East where by 9000 BP Jericho was established as a small settlement, and where Catal Huyuk emerged in Anatolia as the first known city by 8000 BP. Numerous religious sites have been found within Catal Huyuk, and there has been some speculation about role of psychedelic plants in the development of religion there. Syrian rue containing the compounds harmine and harmaline grows abundantly at the Catal Huyuk site, and Amanita mushroom, ergotized rye and cannabis are common in the area.
The domestication of animals may have shortly preceded plant domestication and served as a bridge to settlements and to plant cropping systems. Although the dog had been domesticated before the end of the last ice age, sheep were the first food animal domesticated.
This had occurred at least by 10,000 BP in the Near East (present day Iran, Iraq, Turkey), followed in less than a thousand years by goats. The maintenance of sheep or goats leads people to identify with grazing territory; it reduces the need to be nomadic and is conducive to settlement. With settlements, humans can tend crops. There is some evidence for cultivated rye production as early as 13,000 BP in the Near East (Pringle 1998), and evidence for domesticated barley dates back to 10,200 BP in present-day Israel (Evans, 1993). In Mesoamerica, there is evidence for domesticated Cucurbita pepo from as early as 10,700 BP, although extensive adoption of agricultural cultivation did not occur rapidly in the western hemisphere. Other early centres of plant domestication were in present day Thailand (Piper, Ricinus) and China (broomcorn millet, rice), all before 8000 BP (Evans 1993; Hancock 2004). There was also yam cultivation by that time in sub-Saharan Africa, and most of our major crops had been domesticated by 5000 BP (Hancock 2004).
The centres of plant domestication were invariably centres of settlement, and while we don’t understand the full process there is little disagreement with the idea that these changes in the way humans related to plants were fundamental steps toward the development of advanced human societies and cultures. It is also clear that the process of domestication resulted in rapid genetic change in the plant species involved (Hancock 2004). Therefore, the post-glacial agricultural revolution not only changed people, it also altered the plants they were exploiting.
Also, as agriculture developed humans did not discontinue using wild plant species, which has continued to the present (Balick & Cox 1996). For example, an ethnobotanical survey of plant use by extant Native American peoples documented the use of 4029 species and 44,691 usages (Moerman 1998). More than half of those uses were medicinal, and in addition to food plants were also used for fibres, dyes, ornaments, shelter, cleaning agents, fertilizers, incense, fragrance, insecticides, lubricants, musical instruments, preservatives, soap, smoking, snuff and weapons. Clearly, the connections between humans and plants are extensive and profound.
The Present
Over the past several centuries there have been immense changes in human societies and populations. In part, this has been enabled by the radical expansion and intensification of

4 agriculture. Once again, humans have been revolutionizing their relationships with plants, with important consequences to human development.
An important contributor to this has been human exploration and travel. Disparate and distant cultures have been brought into contact, enabling crops to flow from their early centres of distribution to other areas throughout the world. The globalization of our major crops has led to many interesting results. For example the tomato, native to the Pacific coast of South America, is now a characteristic ingredient of Italian cuisine. The United States has become the world’s major rice exporter. The diversification of food crops available throughout the world has helped with human dietary balance and is important for our food security.
Another major contributor to food increase has been through scientific advancement. It would take considerable time to document this in any detail, but science has affected agriculture comprehensively in key areas of plant genetics and breeding, nutrient management, irrigation and stress tolerance, pest and disease control, and post-harvest handling and storage. By the late
1960s, the determinants of crop performance were sufficiently well understood that physiologists were able to provide plant breeders with recipes for high-yielding attributes in cereals, which were the basis for the ‘green revolution’ in rice (Matsushima 1970; Yoshida 1972). As before, human activities have affected the plants around us. An example of this is the development of weed resistance to the herbicides atrazine and simazine following extensive use of these chemicals on crops for weed control. A broader example is the observation that stomatal densities have decreased in a number of plant species as atmospheric carbon dioxide levels have risen over the past several centuries (Woodward 1987).
Agriculture is inherently a land area-based operation, and the expansion of land area devoted to agriculture is thought to be one of the two most important causes of increasing global food production during recent centuries (Evans 1993). Exploration and migration by peoples has led to the expansion of agriculture into regions where cultivation was not extensively practiced before, such as North American prairie. The diversification of crops available to farmers, together with the development of hardy types and techniques for growing crops in formerly unsuitable regions, have also increased the potential land area useable for food production. At present, however, the extent of arable land suitable for agriculture is not increasing significantly.
Indeed, it may even be declining due to a set of largely human-related activities that either divert arable land for other purposes, or erode the agricultural capabilities of that land: e.g
. salinization, desertification, laterization, urbanization, and industrialization.
The second main driver of recent food increase has been intensification of agriculture, resulting in higher food outputs per unit land area. Maximum crop yields increased impressively with the advent of chemical fertilizer use, such as phosphate fertilizers through the work of
Lawes and Gilbert in Britain in the 1840s, and through the intensive use of nitrogen- and potassium-containing fertilizers since the early 20 th
century. Mechanization has been part of the intensification of agriculture, not only for cultivation, chemical applications and harvesting, but also for irrigation, transport and storage.
Intensive agriculture is hallmarked not only by high levels of production but also by a narrowing of the genetic base. Low biodiversity in intensive agriculture is helpful for mechanization and has several causes: elimination of species through weed and pest control, and by the use of monocultures of a limited number of genotypes. Mechanization has led to a decline in human labour per unit land area, and the reduction in opportunities to work in agriculture is partly responsible for the migration of people from rural to urban areas.

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These changes have led to a disconnect between many humans and the plants they depend upon. It is paradoxical that although we know more about plants today than ever before, they are often less present our everyday lives than was the case in earlier societies. Overall, humans now use thousands of plant species, but I know from talking with my students that many people do not appreciate how much we rely on only a few dozen plant species for most of our food needs. In terms of food, cereal crops are of paramount importance. Just three cereal crops
(wheat, rice, maize) supply about half of the world’s food, and collectively cereals constitute about 2/3 of the total (Evans 1993). On a dry matter basis, animals and fish supply less than 10% of the total, although they are the source of about 18% of dietary protein (Evans 1993).
The Near Future
So, like our primate ancestors, plants remain our predominant source of food. In the rest of this article I will focus on the sustainability of human food supply, although human impacts on forest systems are another major concern (Williams 2003).
There is cause to believe that the main strategies we have been using to increase global food production, ‘farm more land’ and ‘intensify by providing higher resource inputs’, may be losing their potential. As mentioned earlier, arable land is being lost due to a number of processes, so prospects for continuing this strategy for agricultural expansion are therefore diminishing. Some suggest that humans should overcome the terrestrial limitations to food production by farming the oceans. I worry about relying on this option for at least two reasons: effective technologies for extensive open-ocean farming either don’t exist, or their widespread use may lead to adverse ecological consequences in systems where human over-exploitation of fisheries has already been detrimental (Jenkins 2003).
Perhaps there is more latitude for expanding food production through further agricultural intensification. After all, much existing arable land is not being used to full capacity. However, this strategy is eventually limited by the pattern of resource-dependence of plant production systems. This has the same form as the ‘diminishing returns’ pattern in economics where stepping up inputs successively results in less and less improvement as the total level of inputs rises. So, if we double fertilizer inputs once again we will not get nearly the same boost in yield as we achieved through the previous doubling. Average crop yields have improved in many crops during the past generation, but not in all cases. For example, long-term surveys of horticultural crop yields in southern Canada show increases until the mid-1980s, followed by yield declines and/or greater variability (McKeown et al . 2005). Maximum achievable yields in many major crops do not seem to have changed very much in the past 30 years, despite a considerable advancement in biological science during that time.
It is not my purpose to discredit intensive agriculture and the benefits that can be through its use. However, it should be recognized that there are risks attendant upon increasing agricultural intensification and expansion. These includes the requirement for higher energy inputs into agriculture, involving additional fossil fuel use, higher energy costs, depletion of nonrenewable resources and added greenhouse gas emissions. Ecological principles suggest that, due to low biodiversity, intensive crop monocultures are inherently less stable than more diverse systems (Gliessman 1998). The high risk associated with extensive planting of a few genotypes was exemplified by the Southern corn leaf blight disaster in 1970-71 (Ullstrup 1972). A new race of Helminthosporium maydis (syn. Cochliobolis heterostrophus ) attacked corn inbreds and hybrids that possessed Texas male-sterile cytoplasm, which was more than 80% of the dent corn grown in the U.S.A. at that time. Massive crop losses occurred, amounting to 250 million bushels

6 of corn in the state of Illinois alone. Intensive cropping systems are not closed nutrient cycles. In consequence, high inputs of fertilizers are followed by high outputs to surrounding ecosystems, disturbing their functioning and composition. If humanity simply operates to appropriate an increasing fraction of planetary environmental and biological resources, it inevitably acts to disturb natural ecological balances and ecosystem services that are fundamental to our survival.
Beyond the biological risks, the shift from low- to high-input agriculture can have negative socio-economic consequences, being disruptive to local economies and traditional farming systems.
At the start of this discussion I mentioned Carl Sagan’s talk in 1965. When he made that presentation the world had about 3.5 billion human inhabitants, and atmospheric carbon dioxide concentrations were less than 320 parts per million. Now there are about 6.5 billion people, and carbon dioxide levels are near 380 parts per million and continuing to increase steadily because of human activities. Current estimates are that about 860 million people on Earth suffer from significant dietary malnutrition. Forecasts project a slowing in the rate of human population increase over the next generation, with global population reaching about 9 billion people in the year 2050 (Cohen 2003). In the five weeks since I was asked to write this article, the total number of people on Earth has increased by about one-quarter of the present population of
Canada.
It seems to me that, while these trends are well known, we have nevertheless become complacent about our abilities to cope with them. We see regular evidence that convinces us of the power of science and the rapid advancement of technology. It is easy to conclude that surely, if future difficulties in food sustainability come to pass, we ought to be able deal with such problems through the power of science and the invention of new technologies.
I would agree that prospects are good for many helpful advances in science and technology. However, for reasons suggested above I suggest that we cannot simply assume our past approaches to agricultural development can simply be extended into the future in order to meet increasing human food needs. In my opinion, humanity is about to take a gamble that in the next generation, as human populations increase by about 50%, new approaches will be invented that will generate major increases in food supply without causing ecological havoc. And, it is gambling that the new approaches will arrive in a timely way, avoiding widespread human suffering.
Some agricultural scientists I know are not overly concerned about our prospects for increasing food production in the near future. Since the year 2000, however, I have attended a number of (>10) seminars and discussion groups where reputable agricultural scientists have publicly expressed doubt that today’s agriculture is sustainable. These were all people with extensive international experience - about half of them being department heads or senior administrators at academic institutions. They included scientists from Denmark, Britain, the
Netherlands, the United States, Chile, Australia and Canada, and their skepticism about agricultural sustainability was for many different reasons. They expressed concerns about: impacts of climate change, rising costs of energy and other agricultural inputs, degradation and alienation of agricultural land, disruption due to due to political and social change, vulnerability to disease and pests, the loss of traditional knowledge, demographic change, and the poor ability of agriculture to recruit young farmers. This is a long list, but it is interesting to reflect that few of these concerns are addressable through narrow scientific approaches.
This may all seem rather gloomy, but on balance I am optimistic about what can be done to meet these challenges if humans work collectively toward a common goal. At the heart of this

7 issue is the development of an ethical framework that has long-term sustainability and the wellbeing of humans and nature as its goal. Matters of future food supply might first appear to be the
‘problem’. But, in my view it is more accurate to recognize that we are being confronted by problems of human behaviour. We need to find ways to address modes of human behaviour that act contrary to a sustainable future.
Plants are, as they always have been, a fundamental part of this. As human development proceeds we should not lose track of the knowledge base associated with the farming practices of indigenous peoples (Gliessman 1998), which have often been sustained for hundreds or even thousands of years. Although even in those cases there is no assurance that scaling-up such practices for the expansion of food production would not lead to some of the same difficulties we have encountered with intensive farming. I am optimistic about the future because in the past humans have demonstrated a considerable ability to adapt. In working toward sustainability, it seems that we will again have to reinvent how we interact with plants, how we exploit them, and how we sustain them.
Acknowledgement
I thank Dr. Quazi Abdul Fattah, Professor at Dhaka University and my friend for almost 40 years, for inviting me to prepare this article. I thank my colleague Dr. A. R. Reisman for helpful comments on a draft of this paper.
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