资料题：According to the passage, world population today survives on（　）。

Prehistory men and women enjoyed a more varied diet than people do now, since they ate species of plant and several hundred thousand types of living creature.But only a tiny percentageof these were ever domesticated.Modern shops have accelerated(加速)a trend towards specialization which began in the earliest days of agriculture.The food of the rich countries has become cheaper relative to wages.it is speedily distributed in supermarkets.But the choice annually becomes less and less great.Even individual foods themselves become more standardized.We live in the word of the carrot specially blunted in order to avoid making a hole in the bag,and tomato grown to meet a demand for a standard weight of eighteen tomatoes to a kilo.Someone asks:"Only the three major cereals and perhaps ten other widely cultivated species stand between famine and survival for the world's human population and a handful of drug plants has served Western civilization for several thousand years.A rather obvious question arises: are we missing something?" After all, there are 800,000 species of plant on earth.