经过几代人的人口增长，人们以如此高的密度挤满了这个地区，以至于公共聚居地的出现可能是一个必要的结果。
After several generations of population growth, people packed the landscape in densities so high that communal pueblos may have been a necessary outcome.
“而且经过几代之后，就会形成一个新物种了，”他说。
And after several generations, you'd have a new species, "he said."
我们今天的粮食作物都是从古老的品种经过几代的育种，将产量最大化得来的。
The food crops we have today have been bred for generations from older varieties to maximize yield.
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