I believe the best answers are: humans moved out of Africa, and h<span>umans learned how to control fire
But i'm not sure since there are not any options available</span>
Since we don't have the available answers, let's see what were the <u>main causes</u> of the Dust Bowl devastation:
I) Mechanization of agricultural operations. Because of World War I efforts food production had to grow which increased wheat prices. This caused an enormous increase in the size of farms and in the rhythm and size of their crops that weren't accompanied by adequate care of the soil.
II) Farming in the Great Plains. Because of several federal land acts since the 19th century, many farmers were settling in the plains believing rain would come once farming started. This and the belief in Manifest Destiny made people farm in regions where irrigation couldn't reach.
III) The Great Depression. In order to compensate for the decreasing wheat prices, farmers tried to plow up even more land and harvest bigger crops. This led to the plowing of native grassland that kept the soil stable.
IV) Drought. In 1931 started a drought caused by a lack of rain that would end only in 1939. This killed the crops and left the soil exposed. Without grassland and with the winds the soil was blown away in dust storms.
I don't know what are your answer choices, but Rock The Vote is an organization that encourages just that. :-)
