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Reading3-2 (dragged) 1

Reading3-2 (dragged) 1 - i n s i g h t r e v i e w a r t ic...

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Unformatted text preview: i n s i g h t r e v i e w a r t ic l e s H o rs e Z e bra Sheep N o r t h A m e ri c a n big h orn s h e e p G oat R o c k y M o u n t a in g o a t E lk R e in d e e r C ow A f ri c a n b u f f a l o Alm o n d O ak Figure 1 Comparisons of domestic a ted wild spe cies (le ft of e a ch pair) and their never-domestic a ted close rela tives (right) reve al the subtle fa ctors tha t c an derail domestic a tion. game, plus plant foods requiring much preparation, such as grinding, leaching and soaking14,16. Eventually, people transported some wild plants (such as wild cereals) from their natural habitats to more productive habitats and began intentional cultivation17. The emerging agricultural lifestyle had to compete with the established hunter–gatherer lifestyle. Once domestication began to arise, the changes of plants and animals that followed automatically under domestication, and the competitive advantages that domestication conveyed upon the first farmers (despite their small stature and poor health), made the transition from the hunter–gatherer lifestyle to food production autocatalytic — but the speed of that transition varied considerably among regions18,19. Thus, the real question about the origins of agriculture, which I consider below, is: why did food production eventually outcompete the hunter–gatherer lifestyle over almost the whole world, at the particular times and places that it did, but not at earlier times and other places? Changes of wild species under domestication These changes are particularly well understood for southwest Asia’s Fertile Crescent, the site of domestication that was earliest in the world and that yielded what are still the world’s most valuable domestic plant and animal species. For most species domesticated there, the wild ancestor and its wild geographic range have been identified, its relation to the domesticate proven by genetic and chromosomal studies, its changes under domestication delineated (often at the gene level), those changes traced in successive layers of NATURE | VOL 418 | 8 AUGUST 2002 | www.nature.com/nature the archaeological record, and the approximate time and place of its domestication identified9. For example, wild wheats and barley bear their seeds on top of a stalk that spontaneously shatters, dropping the seeds to the ground where they can germinate (but where they also become difficult for humans to gather). An occasional single-gene mutation that prevents shattering is lethal in the wild (because the seeds fail to drop), but conveniently concentrates the seeds for human gatherers. Once people started harvesting those wild cereal seeds, bringing them back to camp, accidentally spilling some, and eventually planting others, seeds with a non-shattering mutation became unconsciously selected for rather than against9,17. Individual wild animals also vary in traits affecting their desirability to humans. Chickens were selected to be larger, wild cattle (aurochs) to be smaller, and sheep to lose their bristly outer hairs (the kemp) and not to shed their soft inner hairs (the wool). Most domestic animals, including even recently domesticated trout20, have smaller brains and less acute sense organs than do their wild ancestors. Good brains and keen eyes are essential to survival in the wild, but represent a quantitatively important waste of energy in the barnyard, as far as humans are concerned3,21. Especially instructive are cases in which the same ancestral species became selected under domestication for alternative purposes, resulting in very different-appearing breeds or crops. For instance, dogs were variously selected to kill wolves, dig out rats, race, be eaten, or be cuddled in our laps. What naive zoologist glancing at © 2002 Nature Publishing Group 701 ...
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