[question]Our current ability to precisely engineer crop genomes was preceded by a long history of genetic manipulation in agriculture. Human impact and its accompanying effects began early in our history at many tropical and subtropical sites around the globe. Our ancestors were omnivores, consuming whatever plant or animal material they fortuitously encountered. Even then, humans had considerable effects on the environment, reducing and even driving to extinction populations of the animal species they hunted and expanding the distribution of plants by accidentally distributing seeds as they migrated.
Humans probably first realized that seeds could yield a stable food supply through agriculture when they observed plants arising from refuse or wasteland, perhaps fruit trees growing along forest and jungle paths from discarded or defecated seeds or else vegetables sprouting in garbage dumps at temporary settlements. A more organized approach to agriculture began about eight to ten thousand years ago coincidentally at a number of locations around the globe. The most diverse farming developed in the Near East, with legumes, cereals flax, sesame, and fruit trees. At about the same time, New World residents were growing beans, maize, squashes, and potatoes, and Asian farmers were beginning to cultivate rice.
[question]These early domesticated crops foreshadowed the overwhelming changes contemporary agriculture has wrought in plants. Humans soon learned to separate varieties that could be grown as crops from wild types in order to prevent characteristics undesirable for cultivation from mingling with those selected for farming. Continued selection of crops with desirable characteristics increased the separation between feral (wild) and managed plants and accelerated the diminishing diversity and more limited variation found in today's crops.
[question]The simplest way to select crops is to save seeds preferentially from plants with beneficial traits, and the first farmers selected for large seeds and fruit, increased seed production, lack of dormancy, faster germination, higher annual yield, and reduced seed scattering. The success of this early selection resulted in an accelerating impact of agriculture on crop diversity and feral plants. Crops quickly became commodities, moved and traded over a rapidly widening area, so that many plants were distributed well beyond their previous ranges, and some throughout the globe.
Three phenomena have characterized the more recent impact of agriculture on Earth.[question] The first was the increase in human population, which has doubled at shorter and shorter intervals over the last thousand years.[question] The result was increased acreage under cultivation and a fundamental remodeling of the globe toward managed rather than wild ecosystems.[question] By 1998 there were 3,410,523,800 acres of land under cultivation worldwide, an area larger than the United States.[question] Entire ecosystems have disappeared, others remain but are threatened, and the sheer volume of people and area of farmland have been major forces of biological change.
[question]The second event through which agriculture modified our planet was European colonization. Previously, migration and trade had moved crops between countries and continents, but the Europeans inaugurated an unprecedented dispersal of biological material worldwide. Maize, tomatoes, and potatoes were transported from the New World to the Old; wheat, rye, and barley were carried from the Old World to the New; and rice, soybeans, and alfalfa were moved from their Asian sources to every arable continent. Each of these and innumerable other introductions conveyed not only unique material but also assemblages of introduced plant pests and diseases that today cause the majority of pest-management problems around the world.
[question]The third factor shaping the nature of agriculture and the environment alike is the increasing precision with which we have selected and bred crops. This acuity stemmed from many advances, but at its heart lies the work of two men-one, the English naturalist Charles Darwin, and the other, an Austrian monk, Gregor Mendel. The concepts of evolution and genetics were not their work alone, but both of them were decades ahead of their colleagues in synthesizing the companion concepts of natural selection and inheritance that are at the core of all contemporary biological science and that form the substrate upon which biotechnology grew.