Book Summary: “People, Plants and Genes: the Story of Crops and Humanity” by Denis Murphy


“People, Plants and Genes: the Story of Crops and Humanity” by Denis Murphy

Title: People, Plants and Genes: the Story of Crops and Humanity
Author: Denis Murphy
Scope: 4 stars
Readability: 3 stars
My personal rating: 5 stars
See more on my book rating system.

Topic of Book

Murphy explores the role that genetics played in the ability of humans to domesticate certain plants and the impact that had on human history.

Key Take-aways

  • Relatively few plants have played an outsized role in human history, primarily wheat, rice, corn and potatoes.
  • Each of these plants had certain unique genetic characteristics that make them suitable for domestication.
  • Remarkably these characteristics are controlled by one or two closely related genes. This small number of “master genes” determines whether a plant can be domesticated or not. This means that when humans choose the best plants based upon one characteristic, they were accidentally selecting for all the other necessary characteristics.
  • Humans are unusual among large carnivores in that we can eat a wide variety of foods. This flexibility of our diets played an important role in success.

Important Quotes from Book

One of the conclusions that may surprise some readers is that crop domestication in the Neolithic period almost certainly owed its success more to the structure of plant genomes than to the botanical skills of early protofarmers. Indeed, it is now widely accepted by geneticists that most or all of the ancient crop domestications were unconscious processes of plant–human coevolution, rather than deliberate strategies based on knowledge and foresight by the people involved.

Despite often leading to a reduction in individual human fitness, farming was generally a highly adaptive strategy at the population level. In particular, farming enhanced the competitiveness of the growing agrarian societies compared to the smaller groups of hunter–gathers. We will also see how people have become modified genetically in response to farming, and how most of us carry relatively recent mutations that are directly attributable to our intimate associations with plant and animal domesticants.

In the comparatively few primary centres of crop cultivation, a relatively narrow range of locally available edible plants was domesticated as the major food staples. Wherever suitable species were available, it was the large-grained cereals that were the most favoured candidates for cultivation as staple crops. The most obvious examples are rice, wheat, and maize; these three plants were among the earliest domesticates and are still by far the most important crops grown across the world, supplying well over two-thirds of human calorific needs. The second most popular class of staple domesticants were the starchy tubers such as yams and potatoes, but these crops were not as versatile as cereals, especially as regards long-term storage, and this limited their more general use. The major class of supplementary crop is the pulses, or edible seeded legumes, which provide useful proteins and nutrients lacking in cereals and tubers, as well as replenishing soil fertility with nitrogen compounds.

Domestication of these different crop species did not occur at the same time or in the same place. Several overlapping, and sometimes lengthy, primary domestication processes were in progress around the world over a period of at least eight millennia from about 13,000 BP until 5000 BP.

Moreover, it now appears that the systematic cultivation of crops was preceded in most places by an extremely lengthy preagricultural phase of plant husbandry. During this period, many geographically unconnected groups of humans started to collect, process, and even manage certain favoured plants for food use, while still relying on a nomadic hunter–gathering lifestyle to sustain the bulk of their livelihoods.

One remarkable aspect of early preagricultural human societies is that, right across the world, out of over 7000 plant species that were regularly used for food, only a tiny number of mainly grassy species were eventually selected and domesticated to serve as the primary dietary staples…  Even today, cereals still supply 80% of our global food needs.

It is especially noteworthy that, despite all the impressive developments in agriculture and breeding over the last twelve millennia, the dozen-or- so plant species that were originally chosen by early Neolithic farmers remain our most important dietary items to this day.

Because they are descended from relatively small groups of migrants, most non-Africans are genetically- speaking a rather uniform population. In contrast, sub-Saharan Africans, being a much an older population, tend to be more genetically diverse.

From 75,000 to 12,000 BP, there was an extended period of particularly unstable climatic conditions covering the period when modern humans became dispersed across much of the world.

See page 37 for Climatic fluctuations over the past 5 million years.

Research over the past decade, as summarized in Figures 1.1, 1.2, and 1.3, has led to a new paradigm of abrupt climatic changes, often over a timescale of a few decades or centuries, rather than over many millennia, as was the traditional view.

Evidence from Greenland and Antarctic ice core data, and other sources, suggests that many of these drastic warming and cooling events happened very quickly indeed, sometimes within a single year.

See page 39 for Climatic fluctuations over the past 100,000 years.

Most of the warming at the end of the Younger Dryas occurred over as little as 20 years, between c. 11,640 and 11,620 BP, with an equally rapid rewarming at the end of the Oldest Dryas after 14,680 BP.

The practical consequence of these very recent genetic findings is that we no longer need to think in terms of humans moving out to populate the world in a small series of epic mass migrations. The emerging paradigm is rather of many slow journeys by small bands of a few score people.

See page 40 for diagram with Technosocial and climatic contexts of human evolution.

The vast majority of such groups probably came to grief in various ways, leaving the few successful ‘migrants’ to become the genetic founders of populations that would eventually be numbered in the tens of millions.

During the Upper Paleolithic Era (c. 50,000– 11,500 BP) human populations in the Mediterranean Basin and Near East gradually changed their hunting patterns. In particular, archaeological evidence has revealed that people began to hunt much smaller animals, switching from the likes of deer and gazelle to rabbits and birds.

This was the first of several steps down the food chain that were made by these Paleolithic people. As population pressures grew, and even the smaller prey animals became ever scarcer, the next step was to use plants of all kinds as an increasingly prominent dietary component. These dietary shifts would have occurred in localized areas.

By the early Upper Paleolithic, many human populations exploited large protein-rich prey as a major component of their diet. In this respect, these people occupied the ecological niche of climax carnivores, such as wolves and the larger cats. But there was a crucial difference between people and true carnivores. The more successful climax carnivores, especially the large cats, have specialized to such a degree that they now find it very difficult to move away from this particular ecological niche, that is they are obligate carnivores.

In contrast, humans are facultative carnivores who have retained a more generalist form of physiology and dentition.

A significant feature of this relatively rapid movement across trophic levels,81 which is a highly unusual ecological phenomenon, is that lower trophic levels can support larger populations. Hence, there are more plants (in terms of biomass) than herbivores, and more herbivores than carnivores, while the climax carnivores at the top of the food chain have the smallest populations of all. By moving down several trophic levels, humans were able to increase their populations, albeit at the expense of higher energy expenditure in terms of food collection and processing. Their dietary flexibility gave humans a powerful tool, enabling them to adapt repeatedly to climatic changes and associated demographic

changes in prey populations. It has also enabled them to migrate into a huge diversity of new ecological zones that lie well beyond their African homeland.

These and similar developments elsewhere in the world during the late Paleolithic set the scene for the much more extensive use of cereals, from 23,000 to 13,000 BP, and led to the first experiments in plant cultivation.

The dietary resilience of many late-Paleolithic populations was called upon when the world entered what is called the ‘Last Glacial Maximum’, from 25,000 to 15,000 BP. As its name implies, the Last Glacial Maximum was a full-blown ice age.

As in previous ice ages, many temperate and subtropical forests died out and were replaced by grasses, including members of the cereal family. Across vast regions of the world, only a few relict woodlands survived as isolated refugia, surrounded by huge expanses of treeless, dry grassland. In some areas, these prairie-like ecosystems supported large populations of grassy plants that had somewhat larger-than-average starchy seeds. These plants were to change the course of human development: they were, of course, what we now refer to as

the cereals.

The breakthrough that made it not just feasible but positively beneficial for people to start exploiting cereals on a larger scale was the discovery that cereal grains could be ground and processed to render them more edible.

Grinding breaks down the hard, fibrous cereal grain to release the easily digestible starch granules contained within. This serves two purposes. Firstly, it enabled people to save enormously on the wear and tear of their teeth, compared to eating raw, unprocessed grains.

The second, and more immediate, reason for grinding cereal grains is that it enables us to produce a much more attractive, sweeter tasting, more nutritious, and calorie-rich foodstuff.

On page 49 Geographical distribution of six of earliest cereal and legume crops.

Riverine (riverside), lacustrine (lakeside), and littoral (coastal) communities, who are able to subsist largely on aquatic resources that are available for much of the year, often adopt sedentary or semisedentary lifestyles.

After a 10,000-year ice age, the global climate changed yet again at about 15,500 BP, with a rapid warming and deglaciation that was especially marked in the northern hemisphere.105 This may have lessened any reliance on cereals for people in the Near East because, as forests became re-established, other more attractive edible plants and animals were available for exploitation. By 14,000 BP, the climate in almost all parts of the word was at least as warm and moist as today, and in some areas it was even warmer.

The second factor favouring a resurgence of plant growth was the huge increase in

atmospheric CO2 concentration the Ice Age.

By far the most important climatic prerequisite for successful agriculture is a reliable supply of water. Availability of water is a much more serious limitation on plant growth than temperature.

All the major climatic episodes of the Holocene Era affecting agricultural and societal development, from the Younger Dryas to the post-medieval ‘Little Ice Age’, have involved a cooler rather than a warmer climate. Moreover, in each case, the cold was not the real problem; rather it was a reduction in rainfall, sometimes by as little as 10–20%, that often resulted in widespread collapse of agricultural systems and the complex societies that they underpinned.

A more reliable limit for dependable farming over a longer period (of centuries) is the 300-mm isohyet.

In the case of the Near East (see Figures 10.1 and 10.2), the millennia-long evolution of agriculture had its origins in the valleys of the Levant and the upper reaches of the Tigris/Euphrates at the foothills of the Taurus and 300-mm isohyets and in adjacent wetter areas, with plentiful supplies of water, grazing, game, fruit, nuts, and wild cereals. Gradually, the early rain-fed form of agriculture spread to slightly dryer regions of Northern Mesopotamia that lie between the 200- and 300-mm isohyets.

The second farming strategy is to bring water to the crop via irrigation systems, such as canals or ditches. Irrigation was probably first used in the Near East by the Samarrans as they migrated along the Tigris/Euphrates Basin into more arid regions below the 200-mm isohyet and eventually founded the enduring civilization of the Sumerians.

Archaeological evidence suggests that the Natufians were one of the first human cultures to adopt a predominantly sedentary lifestyle based in semipermanent villages, and that this occurred well before the development of agriculture.

How some people became farmers:
Agriculture probably could have (and maybe did) evolve before the Holocene, but the climatic conditions were far too variable to support its continued existence for more than a millennium or two before the next cold, hot, or arid episode made it impossible to continue.

To put this into a contemporary context, the magnitude and rapidity of the post Younger Dryas climatic change far exceeds even the direst predictions of the various models of putative anthropogenic climate change, which have received so much attention over the past decade.

The loss of animal species during this period was greater than at any other extinction event over the preceding 20 million years.

It is therefore of interest that the period during and immediately after the Younger Dryas Interval is marked by the first good evidence for the use of systematic cultivation and selection of crops by any human group. It is likely that this series of large climatic shocks was a major factor in the emergence of agriculture, but other factors were also important … over the past 110,000 years, there have been no fewer than 23 climatic events of comparable magnitude (albeit not as sudden) to the Younger Dryas.

A parallel development that probably occurred after the development of preagricultural cereal husbandry, but before that of agriculture, was the increasing tendency towards sedentism that is found in many populations in the Near East at this time. Such sedentism took the form of groups of people who tended increasingly to stay in one relatively restricted area, sometimes in permanent dwellings, for an extended period.

As with wheat and barley, we now know that many of the most important domestication-related traits in the rye genome are determined by only a few genes. This would have made it much easier for agronomically suited varieties to arise via unconscious selection in as little as a few decades,

The key preconditions for the relatively rapid domestication of this cereal can be summarized as follows:

(1) a genetic predisposition that enabled certain wild plants to respond to cultivation by the speedy development of domestication-friendly traits;

(2) an environmental shift (normally climatic) that denied the local human population access to existing alternative edible staples, both plant and animal;

(3) sufficient environmental stability to enable continued cultivation of the domesticated crops in the longer term;

(4) the ability and willingness of the human population to exploit the new resource by engaging in possible taboo-breaking activities such as tilling and harrowing the soil.

Both the acquisition and subsequent rejection of agriculture are becoming increasingly recognized as adaptive strategies to local conditions that may have occurred repeatedly over the past ten millennia.

As agriculture developed and spread, human populations increased and spread out; towns grew up; animals were domesticated; crops were improved to produce higher yields; and new crops were introduced from other regions. This set up a kind of positive-feedback loop that made it gradually more and more difficult to reverse the process of agriculturalization on anything but an extremely localized level.

Crops and Genetics:

It is estimated that there are at least 400,000 plant species, many tens of thousands of which are edible or useful in some other way. In principle, each of these tens of thousands of plants should potentially be suitable to cultivate as crops. Despite this seeming plenitude of botanical wealth and many millennia of experience of domestication and breeding, we still only cultivate about 150 species of food crops. Even more remarkably, the vast majority of the world food supply comes from fewer than 20 major crops.

The Centres of Origin concept is significant in two ways. First, it shows that crop domestication happened independently in different areas of the world. Second, it demonstrates that such domestications were relatively rare events—hence the small number of primary centres of diversity.

See page 58 for map of center of origin of major crops.

This implies that it is not necessarily the activity of the humans that is the primary determinant of crop domestication, but rather the availability of the ‘right sort’ of plant.

Therefore, if edible plants with the genetic potential for domestication happened to be present in an area, then the chances for the development of agriculture would have been increased. On the other hand, if no edible plants in a region possessed such genetic attributes, no agriculture based on indigenous crops could have developed, no matter  how clever or resourceful the local human population.

An analogous argument has been used to explain the remarkably small repertoire of domesticated herbivorous mammals. For example the majority of our domesticated animals originated in Eurasia (cattle, sheep, goats, horses, and pigs), plus a few in

the Andes (llamas, alpacas), but very few examples can be found elsewhere in the world. In particular, over the whole of sub-Saharan Africa, with its massive diversity of animals of all kinds, not a single species of native herbivorous mammal was ever domesticated.

See page 59 for Table of key domestication-related traits in crop plants.

The second point to come out of the Centres of Origin concept is that some very different types of plant have been domesticated in each of the centres. In addition to the ‘big three’ cereals (rice, wheat, and maize) there were potatoes, squash, and the various pulses, such as lentils, peas, and beans. These plants are members of widely diverse families, as different in genetic terms as are reptiles, birds, and mammals. Yet these very different types of plant have each responded in broadly similar ways during their journey to domestication. In contrast, there are close relatives of readily domesticated cereals, pulses, and root crops that have never been domesticated.

One genetic feature shared by the most successful crop species is that the control of this suite of genetic traits, often collectively called the ‘domestication syndrome’, resides in a very small number of genes.

Once again, these findings lead to the conclusion that the potential of a plant to be cultivated (or not) as a crop may reside more in its genetic endowment, than its nutritional or other qualities.

Crop plants are especially likely to be polyploids.

The grains of our cereal crops are mostly made up of a starchy endosperm, and it is this triploid tissue that is by far the major source of calories for human societies around the world. Hence, whenever you eat a slice of bread, a forkful of pasta, or a pinch of rice, you are eating the product of a triploid genome.

Looking specifically at crop species, we see that polyploidy has been one of the

main driving forces of genetic variation, and that it has played an especially important role in the process of crop domestication.

In contrast to autopolyploids, allotetraploids are much more likely to give rise to radically different species from their parents over a short timescale, possibly as brief as a few generations or even immediately after they are formed. This is because allotetraploids are hybrids with a complete set of genomes from two dissimilar parents of different species. This dramatic reshuffling of its genetic endowment means that an allotetraploid organism will automatically constitute a new species, carrying a mixture of characteristics from each parental species.

In addition to its role in speciation, polyploidy is of considerable adaptive significance for plants.249 For a start, polyploid individuals tend to have larger average cell sizes and often produce larger adult forms. This may or may not be useful in an open ecosystem but, in the context of incipient domestication, a larger plant might mean larger fruits or seeds, which would be of great interest to a hunter–gatherer or aspiring farmer.

In addition to increased size, polyploids can have other advantages, including improved resistance to insect pests.

The key to successful domestication of a crop lies, of course, in its genetic endowment. To put it simply, if a plant happens to have the right genetics, it will be much easier to domesticate into a crop than most other plants. To a great extent, our most successful crops have been selected, not just because they are good sources of food or other products, but also because their genetic organization has lent itself to the selection of a limited number of traits that makes them easier to manage and cultivate than their wild ancestors. These traits are collectively known as the ‘domestication syndrome’, a concept developed by Jack Harlan and others.

Three interrelated genetic factors have greatly facilitated the manipulation of domestication-related traits in the major crop plants: (1) some of the most important traits are regulated by just one or two genes; (2) many domestication-related genes are located in small clusters in the crop genome (see Table 5.1); and (3) even when a trait is regulated by many unlinked genes, it is commonly found that a very small number of ‘master genes’ can have a huge influence on expression of the trait.

See page 75 for genomic clustering.

One of the most telling features of the domestication process in the major ancient crops is its variability in terms of frequency, duration, and localization. For example some crops, such as potatoes,331 barley,332 emmer wheat,333 einkorn wheat,334 cassava/manioc,335 maize,336 and bottle gourd,337 seem to have been domesticated only once. This contrasts with squash,338 cotton,339 millet,340 and common beans,341 where data suggest multiple domestication events.

Domestication of cereal crops:

One of the most striking features of domesticated plants and animals is the relative genetic similarity of the present-day members of each of these species. In other words, domesticated plants and animals tend to be less genetically variable than most (but by no means all) wild species. In many cases, this genetic uniformity is due to so-called ‘domestication bottlenecks’ whereby all members of a domesticated species are often descended from a very few (and sometimes just one) selected individuals.

See page 80 for graphic on Evolution of Major Cereal Crops.

The first grasses date from almost 100 million years ago and the most ancient group of cereals are the rices, which appeared about 40 million years ago. Several groups of rice then developed in south and east Asia. In western Asia, the major group of cereals were wheat, barley, and rye, which diverged from a common ancestor about ten million years ago. The ancestors of maize and sorghum split about 15 million years ago and wild sorghum species became important edible plants in Africa, while teosinte served the same purpose in Mesoamerica until its mutation into the more cultivation-friendly form known as maize. The evolution of domesticated forms of these various cereals did not occur until the Paleolithic/Neolithic transition, from about 13,000 to 5000 BP, and was determined by a new suite of human-created environmental conditions such as cultivation and harvesting.

See page 81 for Recent Evolution of Domesticated Wheat

Wheat:

Emmer wheat and barley went on to become the twin crop staples of the early agrourban civilizations of Mesopotamia, Egypt, and the Indus Valley. Emmer was eventually superseded by modern breadwheat about 2000 years ago, mainly because the latter proved to be much easier to harvest and thresh.

Today, breadwheat is cultivated in temperate climates throughout the world and has acquired considerable cultural significance among European and Near Eastern societies.

As with rice (see below), the seed-shattering trait in wheat is regulated by a single gene (Br) and can therefore be readily selected against by farmers.

Moreover, since the Br gene is closely linked to eight other DNA regions that regulate additional domestication-related traits, selection for Br mutants would be more likely to enable farmers to ‘accidentally’ select for favourable variants of these other traits as well. So, perhaps all the early farmers had to do was select non-shattering seeded varieties (which they would do automatically as most seeds from shattering varieties would be lost before harvest) and they would also automatically have selected eight other useful traits ‘for free’. As we will see in subsequent sections, it is precisely this kind of genetic linkage between domestication-related traits that appears to have been one of the key factors that favoured the cultivation and successful domestication of most of our ancient crops.

Barley:

Unlike wheat, barley was probably domesticated only once, in the Jordan Valley of the Near East, and all subsequent forms of cultivated barley may be descended from this one event.376 Because barley is mostly self-pollinating, it is relatively easy to fix new genetic variants into discrete breeding lines and there are hundreds of modern varieties and thousands of land races of the crop known today.377 Domesticated forms of barley tend to have shorter stems, larger grains, and more robust structures to hold the grains on the ear of the plant.

Barley was the principal cereal crop throughout the Near East in prehistoric times and was a major dietary staple of the early Mesopotamian and Egyptian civilizations.

For many millennia, wild barley was harvested from mixed cereal stands with wild wheats and other grasses. As domesticated varieties of barley were adopted in the millennia after 11,000 BP, the crop was still commonly grown alongside the domesticated wheats, einkorn and emmer. In some regions, however, barley gradually decreased in importance as a staple crop, as the new forms of wheat started to provide better yields and superior

grain, especially for breadmaking.

Rye:

While rye may have been one of the earliest domesticants, it never became established as an important human dietary staple.

The crop staged something of a resurgence, as agriculture spread to the cooler climates of northern and eastern Europe. Here, the cold hardiness and drought tolerance of rye, which outperforms many other cereals in this regard, made it a useful and resilient crop. During the Hallstatt period of 3200–2500 BP, rye became established in such regions, where it was better adapted to the relatively poor, light soils and the harsher winters. Rye bread soon became a popular staple, surviving today in the numerous dark breads of Central Europe.

Oats:

Oats were domesticated much later than the other temperate cereal crops of Near Eastern origin.

Oats grew well along the Atlantic littoral and, following their introduction into Britain by the Romans, they soon became a staple cereal in the damp and misty climates along the Celtic Fringe of Europe, where they are still consumed with enthusiasm today, for example as a porridge.406 Oats were spread across the temperate regions of the world by European colonists after the sixteenth century CE and had reached Australia and the Americas.

Domestication of Non-Cereal Crops:

From the earliest days of plant cultivation in the Near East, the Indus and Nile Valleys, China, and Mesoamerica, the important cereal staples were normally supplemented by various types of pulse crops, which are invaluable dietary sources of essential amino acids that are deficient in most cereal crops. Other cultures grew root crops, such as potatoes and yams, for many centuries before adopting cereals as additional staples… although the non-cereal crops constitute an extremely heterogeneous group of plants, they share many of the same genetic attributes that facilitated cultivation of the major cereal domesticants.

Lentils:

Wild lentils are found in the earliest preagricultural grain assemblages in the Near East, and can probably be considered as one of the ‘founder crops’, along with barley, emmer, and einkorn wheats.

 The domestication of lentils involved two stages, loss of seed dormancy and development of non-shattering seed pods, each governed by a single mutation. Loss of dormancy, probably occurred between 11,000 and 9000 BP in the core habitat of the wild progenitor,

L. orientalis, namely the region now occupied by southeastern Turkey and northern Syria. These non-dormant varieties rapidly spread south to the Jordan valley and it was here that the second stage of domestication, non-shattering pods, had already occurred by 8800 BP

By 8000 BP, lentils were present throughout the Near East, from Anatolia to the Levant and from Mesopotamia to Central Iran.

Lentils then appear to have travelled as part of a cereal-dominated suite of crops that spread to southeast Europe and predynastic Egypt by 6000 to 5000 BP, and eastwards to Afghanistan and the Indian subcontinent by 4000 to 3000 BP.464 The grains of cultivated lentils are especially rich in protein, which at 25% of the seed weight makes it the most protein-rich crop after soybean.

Potatoes

Potatoes are quite unlike the other major crops that we have surveyed so far, in that they are grown for their starch-rich roots and only rarely propagated from seed. The edible part of the potato, selected by early Andean farmers, is a modified starchy root, called a tuber. This means that aspiring potato farmers would have been interested in very different genetic traits compared to grain farmers…

Because the tubers of wild potatoes normally contain bitter-tasting and potentially toxic alkaloids, the primary trait of interest to farmers would have been low alkaloid content.

In fact, potatoes are exceptionally high yielding in most European soils, achieving as much as 50 tonnes/ha, and are also one of the most nutrient-rich vegetable crops. Quite apart from their very high amounts of complex starchy carbohydrates, potatoes are rich in vitamins B6 and C, as well as folate, niacin, protein, iodine, and many other minerals.491 Gradually, the Andean potato became more accepted by farmers and by the nineteenth century it was an important crop in northern Europe.

People and the Emergence of Crops:

The eventual fates of these civilizations depended on complex interactions between social, environmental, and biological factors—one of the latter being the nature of the major crop(s) being cultivated by each society.

The important insight that like (normally) gives rise to like applies as much to plants as it does to animals and people, would have led early farmers to preferentially propagate seeds from better performing plots. The custodians of the best plots might even have bartered their superior grain for goods, services, or future favours, hence becoming the first seed merchants.

The very practice of clearing, sowing, weeding, harvesting, and storing grain would have provided the conditions that favoured the evolution of what we now know as ‘domesticated’ varieties of each type of cereal and even the evolution of new species.

It is likely that the major mechanism for the spread of the temperate cereal crops across Eurasia was via the transfer of seeds and of farming expertise from one group to another, possibly in the context of reciprocal trade. It has also been proposed that agriculture may have been spread by the physical replacement of non-farming cultures by farming cultures. While there may have been several instances of such forcible spreading (possibly including millet farming in northern China—see Chapter 11), this is not now regarded as the principal mode of agricultural dissemination across Eurasia.

It is also evident that agricultural diffusion did not necessarily occur separately for each type of crop. For example, in the case of Eurasia, a package of crops including emmer and einkorn wheat, barley, peas, lentils, and flax, was disseminated as a group.536 Again, this is reminiscent of the global spread of other ‘bundles’ of related technologies, such as ancient metallurgy, or more recent examples such as electronics and information technology.

Although Asian rice became the major staple throughout much of southern and eastern Asia, its dissemination took many millennia. In the meantime, other crops such as barley and beans were also being domesticated in the some of the same parts of Asia. Gradually, rice began to out-perform the other grain crops and eventually it partially or completely replaced most of them as the preferred dietary staple across much of the region.

Recent evidence from a US/European collaboration has come to the surprising conclusion that most of the initial changes needed to convert a wild teosinte to domestic-type maize can be achieved by the modification of only three major genes.

This genetic alteration could have been effected quite readily if early maize cultivators could recognize the physical differences in the mutant plants. By selecting seed from these mutated plants for subsequent sowing, the mutation would become fixed in the crop population over subsequent generations. Mutations in any of these three key genes would have led to easily observable and obviously desirable traits in the crop, including reduced branching, softer kernels, and tighter adhesion to the cob. This means that the Mesoamerican preagriculturalists would have been able to select agronomically superior triple-mutants of maize in a relatively short time.

Following the initial domestication of maize in this small area of southern Mexico, the crop was spread throughout Central America over the next few millennia. Intensive cultivation of maize was the staple form of agriculture and provided the subsistence base of the later Olmec, Mayan, Toltec, Aztec, and related civilizations. To a great extent, it was the advances in maize cultivation by farmers that allowed the development of these and other complex human societies in Mesoamerica, who never forgot their debt to maize.

From its Mexican centre of origin, maize gradually spread through the rest of the Americas along two different routes. The first pathway ran from Mexico to Guatemala, the Caribbean Islands, and thence to the South American lowlands and Andes foothills.578 The second dispersal route was via northern Mexico into the southwestern United States and on to the eastern USA and southern Canada.579 Maize was quickly taken up in those regions of South America where the climate was suitable for its cultivation. But it took a much longer time for it to become a staple crop in North America, possibly due to a combination of climatic

factors and the availability of more attractive food sources. The first archaeological records of maize in the southwestern USA date from between 4000 and 3200 BP. It was not until 1200 BP that maize was more widely adopted by semisedentary Amerindian cultures as far away as southern Canada and New England.

Once the common bean, or one of the other New World legumes, had been domesticated, it is likely that the new legume crop would have been integrated into a combined cropping system with maize and squash, both of which were spreading across the Americas by the time that the legumes were first cultivated. This three-fold cropping system, known as milpa, is still practised today by traditional societies throughout Latin America.

The term ‘milpa’ means ‘maize field’ but refers to something more complex. A milpa is a field, often recently cleared, in which a farmer plants several crops, such as maize, squash, and beans, at once.

Milpa crops are both nutritionally and environmentally complementary. Hence, maize is rich in carbohydrates and oils, but is deficient in the essential amino acids lysine and tryptophan, which are required to make proteins, and the vitamin, niacin. Beans are protein-rich with an abundance of lysine and tryptophan, but lack the essential amino acids cysteine and methionine, which are provided by the maize. As a result, beans and maize make a nutritionally complete meal. Squash is rich in carbohydrates and many vitamins.609 A combination of the three crops in the diet therefore gives a good diversity of essential nutrients, as well as mere calories. The nutritional qualities of a milpa diet may be one of the factors behind the lack of animal farming in most of Mesoamerica.

Soybeans:

Soybeans are, indeed, one of the most versatile of the major crops. As well as being

protein-rich, they contain considerable amounts of starch and oil, thereby supplying the three macronutrients (protein, carbohydrate, and fat) required in our diet… It is estimated that as much as 60% of all processed food products in a typical Western supermarket contain components derived from soybeans.

A further advantage of soybeans, which must have soon become apparent to early farmers, is the ability of the crop to grow in soils too depleted of nitrates to support other types of crop, such as cereals. The reason is that soybeans are legumes and can therefore fix their own nitrogen, rather than relying on nitrogen already present in the soil.

This means that, not only can legumes be grown in nitrogen-depleted soil, their cultivation actually enriches the soil for the next crop. For this reason, legumes are now commonly used as so-called ‘break crops’ that are grown every 3 to 5 years to reenrich the soil after cereal cultivation. This practice is called crop rotation.

Agriculture: A Mixed Blessing:

But, in the longer term, the relatively healthy nomads would have been unable to compete as a group with the scrawnier, but far more numerous, better equipped (with both tools and weapons), better housed, and better organized farming communities. Hence the main selective advantages of agriculture lie at the level of human societies, rather than at the level of the individual.

The two most important advantages of animal domestication are elimination of the time-consuming and potentially dangerous need to hunt game, and ability to utilize more efficiently a resource that is normally unavailable for human nutrition, namely cellulose.

Cellulose, which is the major structural component of plant cell walls, is the most abundant organic molecule on earth and is made up entirely of the simple sugar, glucose. Unfortunately the cellulose molecule is also very resistant to breakdown and only a few bacteria and fungi possess this ability. Grass-eating ruminants, including the most common domesticants such as cattle, sheep, and goats, can digest cellulose thanks to symbiotic bacteria and fungi in their rumens. In essence, these animals act as bioreactors that convert useless (to humans) cellulose into extremely valuable products such as meat and milk.

Evolution of Agrourban Cultures: The Near East:

Agriculture in this region initially developed in relatively marginal habitats close to the 200 mm isohyet marking the practical limit of rainfed farming. For the first few millennia (up to c. 7000 BP), agriculture was mainly localized in the Levant and Northern Mesopotamia/Anatolia, where it relied on rainfall and was relatively extensive. The invention of large-scale irrigation, c. 8000 BP, by the Samarrans and others enabled farming to spread south of the 200 mm isohyet into the latently fertile fluvial plains of the Lower Tigris and Euphrates. This more intensive form of agriculture led to the development of dozens of agrourban city-states, most notably the densely clustered cites of Sumer in the rich alluvium close to the coast.

From its beginnings at about 12,000 BP, it took over 4000 years for agriculture to be disseminated throughout the region that extended from the Nile Valley and Zagros Mountains to the south-eastern shores of the Balkan Peninsula.

8200 BP: This generally warm and moist period, sometimes called the Early Holocene Climatic Optimum, was especially suitable for rainfed farming, supplemented in places by the use of livestock manure, and amenable in localized areas to the practice of intensive tillage.

The post-8,200 BP era marks the beginning of a rich period characterized by a great deal of agricultural and technological innovation and the expansion of complex societies throughout the Near East. It was during this period of societal ‘rebound’ that many significant agrotechnological achievements occurred. Pottery was developed; irrigation and more complex forms of water management allowed both the expansion and intensification of arable farming; new varieties of wheat and six-rowed barley were selected; larger villages and towns sprung up across the region; and by the end of this favourable climatic interlude, metal working had started as the Chalcolithic period saw the first use of copper tools…  This era also witnessed the emergence of hierarchies, state-organized religions, and more rigid forms of social stratification, and, by 5400 BP, written scripts had appeared.

The 4200 BP climatic event. The third and most serious, major climatic episode of the Holocene occurred soon after 4200 BP and had profound societal effects in the Americas, Eurasia, Africa, and China. The subsequent long-term drought is associated with regression or collapse of advanced societies in Mesopotamia, the Nile and Indus Valleys, and Northern China.

Indus Valley

The region is on the margins of a late-summer monsoon belt that has fluctuated constantly over the past 20 millennia, sometimes bringing plentiful, if seasonal, rainfall, and sometimes leading to more arid conditions that did not favour domesticated cereal species. Although the climate of the Indus Valley today is rather dry, with an average annual rainfall of 130 mm, conditions in the early- to mid-Holocene were considerably wetter and rather more conducive to the growth of dense stands of wild grain-bearing grasses.

The greater Indus Valley region extended for about one million square kilometres, making it larger in area than ancient Egypt and Mesopotamia combined.

Instead of the vast state-organized canal networks seen in the Near East, the Harappans and their neighbours seem to have adopted a more bottom-up system of agronomic management based on the long-term building-up and elaboration of small-scale irrigation and field-development schemes, as exemplified later by terrace agriculture.

While the barley/wheat-based Indus Valley civilization was technologically the most advanced ancient society in the Indian Subcontinent, a smaller but important rice-based culture arose in the Vindhyan Hills to the south-west.

The relative isolation of this area and the early development of rice farming imply that it was developed indigenously. Probably because high-productivity intensive farming was not possible in the Vindhyan Hills, these settlements never developed into sophisticated urban centres as seen in the Indus Valley or Mesopotamia. However, they were responsible for the dissemination of rice farming over much of southern Asia.

The Indus Valley cities never recovered from the disaster of 4000 BP, and most of them were lost from view under the shifting sands, not to be rediscovered until well into the twentieth century.

China:

Even relatively intensive millet farming does not produce anything like the yields of other major cereal staples, such as barley, wheat, or rice.

4000 BP: Eventually, a new, seminomadic, pastoralist, subsistence culture emerged that lasted for more than 1600 years.862 The extent of the north Chinese collapse at about 4000 BP was much more far-reaching in its length and severity than the demise of the Akkadian Empire and the fall of Ur in the Near East, and is comparable with the end of the Indus Valley civilization.

But rice did not become a true dietary staple until about 7000 BP, and probably did not reach Korea and Japan until about 3000 BP. As with wheat and barley in the Near East, domesticated rice took several millennia to diffuse from its centre(s) of origin and become a dietary staple across a wider region.880 Rice is a much more productive crop than millet, but wild varieties cannot grow in northern China and even the domesticated crop required too much water for the kind of rainfed farming that could be practiced in most of the region.

By 3000 BP, Chinese rice farmers had discovered the merits of more intensive, paddy-based cultivation. Soon after 2300 BP, the introduction of early-maturing Champa varieties from Vietnam enabled two rice crops to be grown per season while propagation by transplanting seedlings, rather than sowing seed, improved efficiency and yields still further.881 These developments set the stage for the emergence of one of the most enduring and advanced of our civilisations, which today constitutes one-fifth of humanity.

Africa:

We are now becoming aware of a series of innovative and complex agrarian cultures that once inhabited what is now known as the Sahara Desert. Although these cultures disappeared after the rains failed about 5500 BP, they may have helped to spread sorghum and millet farming to many other parts of Africa.

At least some of the Saharan agriculturalists migrated to other parts of Africa, bringing with them their domesticated crops and agronomic know-how.

Following the progressive aridification of the entire region from 7300 BP, there was a large-scale population exodus in two directions. A more southerly group became specialized nomadic cattle-herding pastoralists, establishing was to become a major way of life throughout sub-Saharan Africa that has persisted to the present day.898 Meanwhile, several more northerly groups settled in the Nile Valley, which had been only sparsely populated before this time. This coincides with the development of sedentary cultures along the Lower Nile and, by 7000 BP, wheat and barley introduced from the Near East were being cultivated along the Valley.

Nile Valley

In early Egypt, the major crop staple was barley, which had been introduced from the Levant by 7000 BP. Cultivation of other cereals, legumes, and fruit crops soon followed, although barley kept its status as the principal staple of the Egyptian civilizations for many millennia to come.

Egyptian agriculture was one of the earliest to become highly became intensified, as their barley and wheat crops responded well to artificial irrigation. This led to the coevolution of a highly organized, urbanized society and an equally well-organized system of high-yielding, state-organized cereal farming that had some parallels with the emerging agrourban complexes in southern Mesopotamia.

As in much of post-Ubaid Mesopotamia, Egypt was an intensely bureaucratic state, based on meticulous record keeping by a cadre of privileged officials of whom the Pharaoh was the head… For much of the next two millennia, agriculture continued in more-or-less the same vein in the Nile Valley, until the arrival of the Ptolemies.

Rest of Africa:

For many millennia to come, most of these other African farmers continued to use hoes and digging sticks, instead of ploughs, draught animals, and irrigation.

By 7000 BP, there is linguistic evidence that Ethiopia was an independent centre of domestication of numerous crops including species such as tef, noog, and ensete, that never became staples outside the immediate region.

Further south, in the more forested, equatorial regions, yams and oil palm were possibly being cultivated by 6000 BP. But, in general, hunter gathering and nomadic pastoralism were predominant lifestyles for subequatorial African cultures, especially in the far south of the continent. 

Europe:

The development of agricultural societies in Europe was influenced by several factors. First, much of the region was virtually bereft of indigenous food plants suitable for domestication into crops. Second, as the north-westerly projection of the main Eurasian landmass, Europe was adjacent to the major Near Eastern cradles of cereal and legume agriculture. Third, the climatic conditions of continental Europe were sufficiently different from the Near East to preclude the rapid dissemination of most early crops, without the selection of hardier varieties and changes in agronomic practice, such as spring-sowing of grains.

The data are consistent with the migration of small groups of pioneers originating from the Near East. These migrants initially introduced cereal farming into Central Europe via the Balkans by following the rich loess soil deposits and by moving along the alluvial plains of the major river valleys of the Danube, Elbe, Rhine, and Oder. However, the migrants were always in a minority and once they intermarried with the indigenous population, many aspects of their culture were incorporated into this larger group. The end result, which has been seen in many other places and times in human history, was the dissemination of many new aspects of material and farming culture by small groups of incomers without radically changing the genetic makeup of the resident population.

The settled farming communities formed a ribbon-like development along the heavy

soils of the major and minor river valleys that extended deep into the heart of Europe.

Here the early development of the wooden ard plough, and the later invention of metal ploughheads, enabled an individual farmer to work more land and hence generate higher overall yields (although not necessarily on a per hectare basis).

By 6000 BP, the development of hardier cereals, most notably new cold-adapted varieties of wheat and barley, allowed the resumption of agricultural expansion as far as north-west Europe.

6000-3500BP:  For the next few millennia, European agriculture was still based on low-intensity cultivation of temperate cereals supplemented by pulses. Despite the rise of social elites and technologically complex societies in many parts of Bronze and Iron Age Europe, there was no move as yet towards the kinds of highly urbanized states as established by similar, cereal-based cultures in the Near East and the Indus and Nile Valleys.

MesoAmerica:

A particularly innovative form of intensive maize-based agriculture, called chinampas, was developed to the north of the Mayan region in the Toltec, and later Aztec, heartlands of Central Mexico.

It is estimated that chinampas produced between half and two thirds of the food requirements of the mighty city of Tenochtitlán. This metropolis eventually had a

population of half a million people, making it by far the greatest urban centre in the world at that time.

So efficient was their farming that it is estimated that only 20% of the Aztec population was required for agriculture, freeing up the remainder for other state-enriching occupations ranging from craftsmanship to warfare.

State-organized, collective farming on such a heroic scale was not seen again until its much more poorly organized, and thankfully ephemeral, imposition in the Soviet Union and China in the twentieth century.

North America:

One of the key factors in tipping the scales towards farming and eventual urbanization was the appearance soon after 1400 BP of higher yielding, earlier flowering varieties of maize, called maiz de ocho.

Classical and Medieval Periods:

Although a few new mutations in existing crops and some new crops were discovered during this period, the major agricultural innovations in the ancient world were related to technology and agronomy, rather than crop genetics and breeding. Inventions such as the plough and seeder were complemented by the organizational skills of the intensive cereal-farming cultures, with their complex networks of irrigation canals, chinampas, etc., and their acute grasp of the logistics of crop management, from field to granary. However, as agriculture was steadily disseminated from its several centres of origin, there was relatively little diversification of crop production in these central regions.

Babylonia and Assyria:

Despite the impressive degree of their micromanagement of most of the state economy, not even the Sumerians appear to have given anything approaching a corresponding degree of thought to crop production per se. Overwhelmingly, written records show that state bureaucrats were preoccupied with the minutiae of shipment, receipt, storage, and distribution of agricultural goods, but that this was to the virtual exclusion of recording how the crops were actually produced.

Across the Near East, the basic pattern of agriculture, based largely on barley/wheat cultivation and sheep/goat pastoralism, and the division into rainfed and irrigated

cropping, continued with very little change into the Classical Period and beyond. However several new crops were introduced, especially sesame.

Sesame became the third most important crop after barley and wheat, and sesame oil was a greatly valued trade commodity across the Mediterranean and Far East.

During the second millennium BCE, farming was still heavily dominated by the state, and especially by the temple institutions, which in Babylonian cities such as Sippar owned huge tracts of agricultural land, two-thirds of which was worked by temple dependents.984 These ‘temples’ should not be regarded as equivalent to later institutions of the Classical period that share the same name. Temples in ancient Mesopotamia, and to some extent in Pharaonic Egypt, were complex and powerful bodies that carried out many functions of a modern civil service, as well as running their own commercial agricultural and industrial enterprises. These functions were in addition to, but informed by, the religious and ideological roles played by temples and their elites.

In the area of crop management, the Babylonians made several significant technical innovations, including the invention of the seed plough, and the increasing use of draught animals such as oxen and horses to increase crop productivity per worker. Ploughs were made more effective by the use of iron, which became increasingly available during the Iron Age of 1200–300 BCE.

Hellenistic:

However, the synthesis of Greek and Near Eastern knowledge was to have important consequences for the study of botany, and eventually for the improvement of agriculture.

The major Greek innovation was a more systematic approach to both scholarship and experimentation. And perhaps just as important, the Greeks recorded their findings as written texts that were not mere eulogies of the achievements of mighty kings.

Romans:

With the advent of the Roman imperium, the short-lived Greek venture into science came to a standstill that was to last for a further 1500 years. The Romans tended to reinterpret and republish Greek texts rather than uncover new knowledge about agricultural and crop processes. However, they were also more systematic and rigorous than the Greeks in their focus on practical agronomy.

Byzantine and Arab:

For most of the medieval period, the practical agronomists of Al-Andalus were the supreme botanists of Europe, not least in their enlightened and open-minded attitude to wider knowledge and learning.

Unfortunately for the progress of plant science during the later medieval period, the enlightened agronomists and botanists of Al-Andalus (called Andalusia today) tended to be the exception rather than the rule, even in the generally more enlightened Islamic world. In general, there was only a limited degree of cultural intercourse between the major societies and traditions of Eurasia.

However, following the Reconquista of Al-Andalus by Spanish Christians, much of its agriculture sank back into a dark age.

Ironically, while some of these backward Europeans were about to embark on a period of immensely successful agricultural and scientific progress, the hitherto more enlightened Islamic world subsequently entered a long postmedieval period of agricultural, scientific, and cultural stagnation from which it has yet to fully recover.

Europe:

In general the medieval period in Europe was a time of relatively little progress in matters agricultural. In particular, Western Europeans largely went about in sublime ignorance of most of the Classical, Byzantine, and Arabic texts for hundreds of years.

During the High Middle Ages, from the tenth to the thirteenth centuries, there was a limited economic and cultural renaissance in Western Europe. This coincided with the so-called Medieval Climatic Optimum of high temperatures and adequate rainfall.

Throughout this period, farming in Europe was overwhelmingly extensive rather than intensive, following centuries-old traditions as regards crops and agronomy.

Rise of Crop Breeding:

The immediate postmedieval period of the sixteenth and seventeenth centuries marks a transition from an earlier paradigm of empirical efforts at improving agronomy and crop breeding that were largely erratic, localized, and often ineffective in the long term.

In particular, crop improvement became a key objective of a new generation of private entrepreneurs, many from modest backgrounds. Such people were the principal engines of agricultural progress until the rebirth of public sector interest and the professionalization of plant breeding in the early twentieth century.

Many early crops were also used as raw materials to generate new products via a process now know as biotechnology, but formerly called fermentation. This process can be used to make a whole range of alcoholic drinks, as well as for baking leavened bread and cheese manufacture.

One of the major factors that held back progress in the ancient and medieval worlds was the lack of methods for systematic recording and disseminating knowledge of new discoveries…

Unfortunately, such knowledge as existed was all too vulnerable to the depredations of vandals who destroyed cities such as Ur and Persepolis, or burned the irreplaceable libraries of Alexandria and Granada, and ruined the irrigation systems of Al-Andalus and Sumer. The repeated loss of knowledge and infrastructure was partially responsible for the lack of progress in many aspects of agriculture during the almost 4000-year period between the fall of Ur and the ‘English revolution’ of 1600 to 1800 CE. The so-called scientific revolution of the post-Renaissance period was not just a change in our world view; after all, curious and inventive people had been making discoveries since Paleolithic times. Perhaps even more important than the creation of knowledge itself, was the way it was disseminated more widely between scholars and practitioners of science and technology. It was this new access to information, coupled with a willingness to share the

fruits of one’s knowledge, that really stimulated the study and manipulation of the biological world and underpinned the agricultural achievements of succeeding centuries.

It is important to stress here the surprisingly high degree of literacy in England at this time.

Hence, between one-third and a half of male adults would have had at least a degree of familiarity with the written word. This literacy was linked with what has been termed an ‘educational revolution’ in England during this period.

Imperial Botany:

It was in the Netherlands and England that many of the most significant developments in agronomy and plant breeding came about during the seventeenth and eighteenth centuries.

During the eighteenth century, an increasing proportion of the British population was urban and required ever-more efficient agricultural production for its sustenance. Already, by 1700, the population of England was 13.4% urban, in contrast to an average of 9.2% for the rest of Western Europe. By 1800, the English urban population had almost doubled to 24.0%, while that of the rest of Europe remained virtually constant at 9.5%. It is now becoming clear that this industrial revolution was largely based on a previous, and hitherto largely ignored, agricultural revolution. Key elements of increased agricultural productivity during the eighteenth century were the use of new crop types and varieties; the development of new and more effective crop rotational systems; and the recruitment of more productive land by drainage and woodland felling.

Many innovative farming techniques in eighteenth century Britain were imported from the Netherlands, which was probably the most agriculturally advanced region in Europe from c. 1650 to 1750.

The earliest, truly systematic collection and cataloguing endeavours were undertaken in the mid-eighteenth century by the Dutch East India and Dutch West India Companies, both of which established a series of formal botanic gardens throughout their respective colonies in the tropics. The British soon emulated their North Sea neighbours as they became involved in their own burgeoning imperial project around the globe.

The earliest botanical gardens in Europe were probably established in Italy (in the south of which Muslim rulers had held sway for many years), in Salerno by Sylaticus in 1310 and in Venice by Gualterius in 1330. It was not until after the Renaissance that similar gardens were set up in other cities and universities: Pisa in 1543, Padua, Parma, and Florence in 1545, Bologna in 1568, Leyden in 1577, Leipzig in 1580, Königsberg in 1581, Paris in 1590, and Oxford in 1621.

During the mid-eighteenth century, there was a veritable explosion of botanical interest in the highest social circles of Europe.

The promotion of economic and applied botany soon became an integral aspect of government policy, most notably in Britain and the Netherlands.

Modern Times:

After more than ten millennia of rather slow, fitful progress via empirical crop improvement, the period from the Renaissance to the Enlightenment witnessed a much faster transition to a radically different, more scientifically informed process. This transition was fuelled, in part, by the conjunction of new forms of commercial opportunism that were relatively unfettered from state control, and by the related process of imperial expansion that opened up new vistas for crop exchange across oceans and continents. It was also driven by the identification of new or improved techniques of cultivation from other regions of the world. These developments occurred in parallel with the emergence of new science- based methods of knowledge creation. Perhaps more important than the mere creation of additional knowledge, however, was its improved dissemination, thanks to innovations such as printing. An international community of scholars, researchers, and technologists emerged, who communicated more freely and effectively with one other as part of a new tradition of the sharing, rather than the hiding, of knowledge. It was this more ready availability of new evidence-based knowledge, coupled with greatly improved opportunities to harness such knowledge, that launched a quantum leap in our ability to both understand and manipulate the biological world, including how plants develop and reproduce.

At the same time as the burgeoning of new and more reliable knowledge about how plants worked and how they might be manipulated, voyages of discovery and colonization resulted in the (re)discovery of hundreds of new, potentially exploitable plants.

By the late nineteenth century, the more ready availability of an ever-growing corpus of new knowledge, and the realization that it could be exploited for the radical improvement of agriculture, led to the establishment of a professional cadre of trained plant breeders in new forms of public sector institutions. These developments constituted something of a break with the previous paradigm of crop improvement, which in Europe had been mainly carried out by individual husbandmen and private entrepreneurs, rather than public functionaries.1154 Over the past 150 years, this public-sector paradigm has served agriculture and society very well indeed. Virtually all the new plant breeding technologies of the twentieth century, from mutagenesis to mass propagation, were developed within the public sector milieu. Later in the twentieth century, public sector plant breeders

were responsible for the Green Revolution.

The near trebling of global food production in the two centuries from 1700 to 1900 was sustained largely by the cultivation of new land that was won either by reclamation or the establishment of overseas colonies.

After the mid-twentieth century, the even more dramatic population increases of the next five decades were mainly sustained by increased plant productivity on existing land, rather than by expansion into new arable cultivation. This improvement in crop output was due to a combination of the better use of inputs (e.g. fertilizers, herbicides, and insecticides), and the breeding of higher yielding crops in a process that was increasingly informed by the science of genetics.

What is most impressive is that food production has actually trebled since 1950.

On the scientific side, new or improved inputs were developed, including chemical fertilizers, and a variety of crop protection agents ranging from herbicides and pesticides to fungicides and antiviral formulations. On the engineering side, on-farm mechanization was accelerated thanks to powered tractors and combine harvesters. Grain-processing units also became larger and more efficient and storage conditions were greatly improved. Finally, agriculture moved from being a relatively small-scale, family or community-centred, operation to the large-scale agribusiness venture prevalent today in many of the most productive crop-growing areas of the world.

To a great extent, the advances in agriculture over the past century or so have been due to the establishment of effective public-sector networks of research and dissemination of knowledge to farmers. Probably the most influential of these developments occurred in the USA in the late nineteenth and early twentieth centuries.

Nitrogen availability is one of the most important limitations on crop growth, ranking in importance with such key inputs as sunlight and water. Prior to the use of chemical fertilizers, farmers had been forced to rely on biological sources, such as farmyard manure or bird guano. Plants cannot assimilate the organic (carbon-linked) forms of nitrogen in these biologically derived fertilizers, and can only use the inorganic breakdown products, such as nitrates, which are gradually released from the organic compounds. Due to their high cost, limited availability, and their slow rate of nitrogen release, use of organic fertilizers on a large scale can significantly limit food yield from crops.

Second only to nitrate as a yield-limiting mineral for crops is phosphate

Food production was rescued by the arrival of the inorganic fertilizers. In particular, the introduction of chemical forms of nitrate and phosphate fertilizers greatly improved crop yields after the late nineteenth century and these are still mainstays of conventional farming across the world today.

After fertilizers, the other major class of inputs contributing to high yields are the crop protection agents. Every year, between one-third and a half of most crops used to be lost due to competition from weeds, to diseases caused by viruses, bacteria, and fungi and from damage caused by insects, rodents, and other pests. In bad years, an entire crop could be wiped out by a sudden outbreak of disease or pest infestation. For many centuries, and with mixed success, farmers experimented informally with hundreds of treatments against the pests and diseases that regularly ravaged their crops.

See page 267 for diagram mechanism of modern crop breeding.

It is difficult to overestimate the importance of quantitative genetics for practical breeding, and it has been described as ‘the intellectual cornerstone of plant breeding for close to 100 years.’ The foundations of quantitative genetics were laid by British geneticist, Ronald Fisher in his seminal paper of 1918, in which he showed that continuous variation between members of a population could be as a result of Mendelian inheritance, albeit involving many genes, plus an environmental component.

Conclusion:

Over the past two million years, human numbers have oscillated considerably, with often dramatic, local booms, extinctions, and migrations. Population tends to be a function of available resources and, like any other species, humans tend to respond to changing resource levels by adjusting their numbers. At a global level, there have been at least three large-scale, incremental increases in population that coincide with: (i) late-Pleistocene migrations from Africa; (ii) early to mid-Holocene development of agriculture;1222 and (iii) post-1700 CE agroscientific developments.

It has been suggested that the latest global spread of H. sapiens, and the increase in hunting efficiency due to inventions such as the harpoon, bow and arrow, and spear thrower, may have led to an increase in the population to about six million by the time of the final Neanderthal extinctions shortly after 30,000 BP.

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