From the magnificence of a towering redwood to the simple elegance of a tiny dandelion, seed-bearing plants abound on planet Earth. The sheer diversity of plants thriving today is largely thanks to the evolution of the seed, as this made plants resilient to environmental changes by enabling them to await optimum conditions for growth before springing to life. In a time of declining biodiversity, studying seeds is now helping scientists preserve this plant diversity for future generations.
With Seeds, Carolyn Fry offers a celebration of these vital but unassuming packages of life. She begins with a sweeping tour through human history, designed to help us understand why we should appreciate and respect these floral parcels. Wheat, corn, and rice, she reminds us, supply the foundations of meals eaten by people around the world. Countless medicines, oils, clothing materials, and building supplies are available only because of the versatility and variety of seed-bearing plants. Fry then provides a comprehensive history of the evolution of seeds, explaining the myriad ways that they have adapted, survived, and thrived across the globe. Delving deeper into the science of seeds, she reveals the fascinating processes of dormancy, reproduction, germination, and dispersal, and showcases the estimable work conservationists are doing today to gather and bank seeds in order to prevent species from going extinct.
Enriched by a stunning array of full-color images, Seeds offers a comprehensive exploration of some of the most enduring and essential players in the natural world.
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A Natural History
By Carolyn Fry
The University of Chicago PressCopyright © 2016 The Ivy Press Limited
All rights reserved.
THE IMPORTANCE OF SEEDS TO HUMANITY
SEPARATING HUMANS FROM THE MONKEYS
Without seeds, Western civilization might never have arisen, for these energy-giving capsules of life have underpinned our evolutionary history over millions of years. Specifically, it was annuals, those flowering plants that produce seeds once in the course of a year and then die, which fed the earliest settled communities of Mesopotamia, the cradle of Western civilization.
Today, the three most important crops in the world, providing about 60 percent of the world's food energy intake, are all annuals from the grass family: maize, wheat, and rice. These plants' seeds are packed full of the essential carbohydrates, protein, vitamins, and minerals that humans need to survive, while their fast life cycles make them easy for us to cultivate in large numbers. Although tubers, such as manioc and potatoes, have also played an important role in developing human societies, without seeds to eat, Earth might not have been able to support such a large human population.
WALKING TO A BETTER DIET
The animals that gave rise to humans did not initially eat seeds, however, our far-distant ancestors lived an apelike existence in forests, foraging leaves and fruits from trees, shrubs, and herbs. But 3.5 million years ago (Ma), these early hominids evolved from being tree climbers to upright walkers and moved onto the savannas. As they did so, their diet changed to reflect these new surroundings, becoming richer in grasses and sedges, and supplemented occasionally with meat, as demonstrated by evidence from the analysis of teeth taken from the remains of early species. Being able to eat a greater variety of foods in turn enabled these hominids to live in a wider range of ecosystems, so they were no longer competing for food with other primates. This advantage helped set humans on the path to becoming the most successful species of the animal kingdom.
FOOD FACTORS IN EVOLUTION
Studies into the nutritional quality of foods eaten by apes and hunter-gatherers have concluded that the leaves of wild plants are low in energy but high in minerals and vitamins, while seeds, nuts, roots, and tubers have a net energy yield that is higher than leaves but lower than meat. Today, apes and monkeys primarily eat leaves, which are of lower quality and easy to acquire, while hunter-gatherers mostly eat higher-quality seeds, nuts, tubers, and meat, which are harder to come by.
The shift from the ape diet to the hunter-gatherer diet 3.5 million years ago represents a move from lower-quality foods to higher-quality ones. Brains and guts both require considerable energy to function, and while apes have small brains and large guts, modern humans have large brains and small guts, an evolutionary transition that scientists believe would have been facilitated by the shift to eating higher-quality food.
COOKING CONTRIBUTES TO HUMAN DEVELOPMENT
How hunter-gatherers prepared their food, such as by grinding seeds and cooking them, might also have contributed to their evolution, as breaking down then cooking seeds and nuts would have made them quicker to eat and easier to digest. This would have enabled the early hominids to consume more calories and spend less time chewing, which could have been a contributory factor in the evolution of larger brains, bigger bodies, and smaller teeth. The brain of our ancestor Homo erectus, who first appeared 1.6 to 1.9 million years ago, was 50 percent larger than that of its predecessor, Homo humilis, and its teeth were also much smaller. A bigger brain would have enabled members of the species to process more complex information as they traversed locations rich in food sources. Meanwhile, a bigger body would have helped them travel farther on a day-to-day basis; Homo erectuswas the first hominid to have a geographic range extending beyond a single continental region.
EATING NUTS ROTTED HUNTERGATHERERS' TEETH
Acorns, the single-seeded fruits of oak trees, were a staple food of hunter-gatherers as far back as 21,000 years ago. However, they may also have rotted their teeth. A study of charred plant remains from an archaeological site in Morocco showed that the inhabitants were harvesting and processing sweet acorns and pine nuts between 13,700 and 15,000 years ago. The teeth of 52 skeletons buried in a cave in Taforalt exhibited a high level of tooth decay; only three of the skeletons had no cavities. Indeed, grinding the nuts into a sticky porridge before eating them might have helped to promote the disease. Prior to this study, scientists had thought that tooth decay only arose with the emergence of agriculture, when eating sugary wheat and barley became common.
The first convincing evidence for cooking is dated to 790,000 years ago, after the era of Homo erectus but still some 600,000 years before the emergence of modern humans, Homo sapiens. Archaeologists examining the remains of a settlement at Gesher Benot Ya'aqov, in Israel, found a small number of charred remnants as they sifted through thousands of seeds and fragments of wood and fruit, from which they concluded that the proportion of burnt remains would have been greater had the materials been exposed to uncontrolled wildfires. They also found clusters of burnt flint, which they interpreted as primitive hearths. Meanwhile a study of modern-day hunter-gatherers has shown that no current human group exists that eats all its food raw.
FROM HUNTER-GATHERERS TO FARMERS
Hunter-gatherer communities switched independently of one another from foraging for wild seeds to cultivating them in several locations from 3,000 to 12,000 years ago. The earliest transition occurred in the Fertile Crescent area of Western Asia, a wide arc spanning parts of modern-day Israel, Lebanon, Jordan, Turkey, Iraq, and Iran. Early agriculture in this area was based around cultivating barley and wheat as these were the crops that were locally available.
Elsewhere the shift from hunter-gathering to farming was based on whichever plants were growing nearby. In China, this was rice and millet; in Papua New Guinea, root and tree crops were planted; sub-Saharan Africans grew sorghum and pearl millet; in Mesoamerica, maize and beans were the main crops; several seed plants were grown in eastern North America; and South Americans cultivated quinoa and beans. Recent evidence suggests that this process was a long, complex evolution, which involved some people cultivating crops long before settling in fixed locations and others forming settled communities while still actively foraging.
Shifting from a society that ate solely what nature provided to one that had some control over its food supply had considerable implications for early humans. Prime among these was that such a change promoted population growth. Women in foraging societies spend up to four years breastfeeding their children, an act that suppresses ovulation and so prevents more frequent births. This makes sense in the context of a mobile lifestyle, as carrying and feeding more than one child while hunter-gathering would be difficult. Once farming provided a more assured food supply, this enabled large communities to settle in one place, and as a result fertility increased and populations began to rise. At the end of the last ice age, the global population is estimated to have been five million. By 1820, it had swelled to one billion (1,000 million).
WHY DID EARLY HUMANS SWAP FORAGING FOR FARMING?
The shift from foraging to farming spanned the end of the last ice age, some 10,000 years ago. Some scientists believe the change in climates and atmospheric conditions may have contributed to this transition. Evidence from ice cores shows that ice-age climates were dry but also variable over short timescales, and that carbon dioxide (CO2) in the atmosphere was low. Even though populations were relatively sophisticated at this time, the conditions were not conducive to agriculture.
At the end of the ice age, climates became warmer, rainfall increased, and concentrations of CO rose. Temperate grass species, which include the ancestors of many major domesticated crops, would have thrived under these conditions. So, the scientists reason, once the conditions for agriculture became viable, different hunter-gatherer communities quickly exploited the opportunity this presented for taking control of their food supplies. Most present-day hunter-gatherers exist in areas that are either too arid or too cold for farming to be viable.
Historically, as more food was needed to feed a region's growing population, new land was simply brought under cultivation. By the nineteenth century, however, the most fertile land was becoming scarce, so the only option was to cultivate poorer-quality land. Many academics at this time were pessimistic about the world's ability to feed its population in the years to come, particularly in the light of longer life expectancies. The economist Thomas Malthus forecast that: "The power of population is so superior to the power in the Earth to produce subsistence for man, that premature death must in some shape or other visit the human race." Malthus had not bargained on advances in agricultural and seed science, however. In the twentieth century, large public investments in scientific research for agriculture brought about huge yield increases in developed countries.
NEW TECHNIQUES BOOST YIELDS
The rise in English wheat yields exemplifies the advances made at this time. Although it had taken nearly 1,000 years for wheat yields in England to rise from /5 to /5 of a ton per acre (0.5 to 2 metric tons per hectare), it took just 40 years to increase them from /5 to 2/2 tons per acre (2 to 6 metric tons per hectare). The improvements came about thanks to innovative plant-breeding techniques, the development of inorganic fertilizers, and modern pesticides. These advances enabled most industrialized nations to attain food surpluses by the second half of the twentieth century. When India suffered recurring droughts in the mid-1960s, which threatened wide-scale starvation, the same methods were employed to increase rice and wheat yields there. This involved creating varieties that were more responsive to soil nutrients; had shorter, stiffer stems that could support the weight of heavier ears of grain; and could grow at any time of the year, enabling farmers to sow more crops on an annual basis.
Within five years, around 20 percent of wheat areas and 30 percent of rice areas were growing high-yield varieties; by 1990 this was 70 percent of each. Farmers who could make more profit from these varieties expanded their cultivation at the expense of other crops. Cereal production in Asia doubled from 1970 to 1995, while the population rose by 60 percent.
ANCIENT SEEDS AND FRUITS REVEAL SHIFT TO FARMING
Some of the best evidence of the transition from hunter-gathering to agriculture comes from the archaeological site of Abu Hureyra in Syria. This village of several hundred inhabitants was permanently inhabited from 7,000 to 11,500 years ago. The site yielded 118 species of seeds and hard fruits that would have been eaten by hunter-gatherers during the site's early days. However, the array of wild plants declined rapidly around 11,050 years ago, to be replaced 9,860 years ago by a suite of cultivated crops, including einkorn wheat, emmer wheat, and lentils. By 8,500 years ago, Abu Hureyra's inhabitants were relying on a mere eight domesticated plants for their vegetable-based energy foods.
Far from starving, the people enjoyed 30 percent more calories per person than they had previously. A downside to the benefits brought by this "Green Revolution," however, was the environmental damage it caused: fertilizers polluted waterways; poor irrigation practices caused salt to build up in the soil, rendering previously good-quality land unusable; and the heavy dependence on a few varieties reduced biodiversity. Perhaps most importantly, breeding within a limited set of popular varieties that favored high yields reduced their overall genetic diversity.
LOSS OF GENETIC DIVERSITY
A handful of wild wheat seeds is genetically diverse in much the same way as a crowd of people. Just like humans, plants in different wild populations can have markedly different characteristics despite being the same species. When farmers began to cultivate crops, the first seeds they sowed from wild plants would have contained only a small subsection of the genetic diversity present in their local wild wheat population. This, in effect, created a bottleneck in genes at the point at which agriculture first developed. Over subsequent millennia, farmers domesticated plants through a process of selection and breeding. They bred out natural traits, such as the shattering of seed heads and dormancy, which enabled plants to survive in the wild but were not useful for agriculture. On the other hand, they retained and selected for characteristics that were helpful, such as higher yields and pleasant taste. Any individual landrace is therefore the result of breeding work by thousands of farmers over many generations.
Modern cultivars are the result of sophisticated programs of breeding and genetic improvement specifically designed to meet the needs of large-scale commercial agriculture. Monoculture farming, under which vast areas are planted with a few such cultivars, results in the highest yields but the lowest genetic diversity. The Green Revolution is an example of this. At the start of the twentieth century, India was home to over 30,000 varieties of rice; today, just ten varieties are grown in 75 percent of the country's rice fields. Moreover, some crop varieties have been genetically modified to tolerate specific herbicides such as Monsanto's Roundup product, a development that has enabled farmers to easily eradicate weeds but has led to them abandoning traditional landraces and varieties in favor of crops that offer better financial returns.
INTERNATIONAL TREATY HELPS KEEP FOOD ON THE TABLE
Signed by 135 countries, the International Treaty on Plant Genetic Resources for Food and Agriculture came into force in 2004. Known as the International Treaty, it aims to ensure that farmers and breeders have access to the plant genetic resources they need, including seeds of food and forage crops, to overcome future challenges to farming such as climate change and environmental issues. The Treaty recognizes that all countries depend for food and agriculture on plant genetic resources that originated elsewhere. For example, wheat originated in western Asia but is now grown most extensively in China and India; and potatoes came from South America but are widely grown in Europe. The Treaty's key aim is: "The conservation and sustainable use of plant genetic resources for food and agriculture and the fair and equitable sharing of the benefits arising out of their use, in harmony with the Convention on Biological Diversity, for sustainable agriculture and food security."
PLANTS UNDER PRESSURE
How a crop plant responds to stresses, such as extremes of temperature, drought, and attacks from pathogens or pests, is at least in part controlled by that plant's genes. When a field of a particular crop is genetically diverse, that population has greater resilience, as individual plants respond to the stress in different ways. But when an entire population has the same genetic makeup, it does not have the armory to resist a particular stress or attack to which it is vulnerable. With climate change increasing the frequency of droughts and floods and making weather in many locations more variable, the concern is that modern-day cultivars may not be sufficiently resilient to grow the volumes of food required in future. This is of huge concern, as 80 percent of humanity's calorie intake now comes from just 12 plant species, while half of our calories come from the three grasses: maize, wheat, and rice. If these are vulnerable, the impacts of their loss will be huge.
DIVERSITY ENSURES RESILIENCE
There is already evidence that resistance is weakening. Today, the world's fourth most important crop after wheat, rice, and maize is the banana. However, nearly all commercially grown banana plants are a cultivar called Cavendish; they are all clones, which makes them genetically identical. Since 1992, 24,700 acres (10,000 hectares) of Cavendish plantations have succumbed to a strain of Panama disease fungus. With no genetic diversity to fight the disease, the entire global crop is at risk. "Plants with little genetic diversity are very vulnerable because there's no variability there," explains Ruth Eastwood of Kew Gardens' Millennium Seed Bank Partnership (MSBP). "If one field of one crop gets infected by a particular disease and it's not resistant then the whole crop gets infected. And if that's the favorite wheat variety that's been grown in a country that year, you could potentially lose the whole of the country's crop. Avoiding such disasters is about ensuring resilience. And to do that you need diversity."
HELPING FUTURE CROPS THRIVE
Ruth is Kew's Project Coordinator for Adapting Agriculture to Climate Change, a ten-year project being run by Kew Gardens and the Global Crop Diversity Trust, which aims to address the issue of resilience. Specifically, the project aims to locate where the wild relatives of 29 popular modern crops grow, assess how much material from those species is already held in seed banks around the world, supplement existing collections with new seed stocks where gaps exist, and then make this genetic material available to "pre-breeders" (experts who identify desired traits in overlooked plants and begin to incorporate them into modern varieties) for selective breeding trials.
Excerpted from Seeds by Carolyn Fry. Copyright © 2016 The Ivy Press Limited. Excerpted by permission of The University of Chicago Press.
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Table of ContentsContents Seed Banks Around the World Introduction Chapter 1: The Importance of Seeds to Humanity Separating Humans from the Monkeys From Hunter-Gatherers to Farmers How Crop Wild Relatives Have Helped Us Breed Resilient Varieties Human Uses of Seeds Down the Ages The Father of Seed Science The Seed Bank that Survived a Siege Plants and Seeds from The World's Arid Lands Seed Profile: Grass Pea Chapter 2: How Plants Evolved on Planet Earth Tiny Algae Give Rise to the First Plants Spore-Bearing Plants Give Rise to the First Seeds Flowering Plants Quickly Gain Ground The Rise of Annuals Underpins Human Success How Plants Evoloved from Algae to Angiosperms Evolution of Land Plants Culminates with the Dramatic Rise of Angiosperms Plants and Seeds from the World's Rainforests Australia's Plantbank Helps to Grow "Difficult" Rainforest Seeds Seed Profile: Wollemi Pine Chapter 3: How Seed Plants Reproduce Double Fertilization Brings Flowering Plants Great Success How Gymnosperms and Angiosperms Reproduce Pollination is a Must for Successful Fertilization Plants and their Pollinators Saving China's Diverse Flora Plants and Seeds from Antarctica and the Arctic Seed Profile: Yew Chapter 4: Dispersal Takes Seeds to New Pastures The Diverse Ways in which Plants Spread their Seeds Animals Disperse Seeds Far and Wide Floating Down Rivers and Across the High Seas Hitching a Ride with the Wind Dispersal by Gravity and Ballistic Propulsion How Seeds are Dispersed Around the World The Seed Bank Keeping New York City Green Plants and Seeds from the World's Islands Seed Profile: Mongongo Chapter 5: Germination Brings Plants Back to Life The Tricks Plants Use to Survive The Test of Time Preserved for Posterity Inside a Seed Inspiring Seed Banking Around the World Plants and Seeds from the World's Costal Zones Seed Profile: Wood Anemone Chapter 6: Using Seeds to Ensure Humanity's Survival Saving Cultivated and Wild Seeds Healthy Ecosystems are Key to Biodiversity Restoring Biodiversity to Chalk Downlands The Shifting State of the World's Flora Keeping Hunger at Bay in the Tropics Foods of the Future Plants and Seeds from the World's Alpine Habitats Seed Profile: Arabica Coffee Glossary Further Reading Picture Credits Index Acknowledgments