Judging from the difference between the parasitic beast and the human being: the parasitic beast is cold-blooded and ruthless, while the human being is full of various emotions, which is most clearly reflected in the exchanges between Shinichi and You.
But with the development of the story, we will develop. Many so-called emotions such as helping others, protecting the earth, having love for small animals, etc., in the final analysis, are just developed by people to satisfy their own psychological needs after their own physical needs are met. The self-deception of needs is just as ruthless when it comes to one's own life. And the parasitic beasts are just because they can't get rid of their own survival instinct when they first arrive on the earth. When they also have the same physiological needs, the corresponding emotions also develop accordingly.
From this perspective, humans and parasitic beasts are of the same origin.
PS: If one day human beings really perish, perhaps to a large extent, it will not be because of the invasion of foreign enemies but because of human beings.
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