The sentence above requires a "connector" in order to connect <em>two independent clauses</em>.
First clause: <em>Douglass believed that education was important</em>.
Second clause: <em>He taught other slaves to read the New Testament at a weekly church service.</em>
A connector in the form of "conjunctive adverb"<em></em><em>(thus/therefore)</em> was used because the clauses show a <em>transitional expression. </em>It meant that Douglass wanted people to become educated and to value education. This is the reason why he taught the slaves to read.
The author is saying that because of humans and the ways they are impacting earth is causing irreparable changes like causing animals to go extinct, etc.