The first sentence support the thesis statement by using a metaphor: it compares a settler cleaning weeds from a corn field with men killing all types of life to comfortably settle in a land.
The second sentence expresses how men have devoted years and centuries developing and improving different types of weapons that would make them better killers.
And the third sentence shows that man has taken his desire for killing so far that he even wants to kill the microorganisms that live in their homes.
