Sentences 3 and 4 are the following:
The dinosaurs are the best known example of this. Today, man has populated the Earth to such an extent that habitat destruction and animal poaching are now the primary causes of species decline.
Are they a good thesis statement? Let's break it down a bit. A thesis statement needs to be a single statement. Sentences 3 and 4 are two sentences, so you may want to combine them in order to form a thesis statement in a single sentence. Additionally, a thesis statement is often the final sentence of the opening paragraph.
Your thesis statement could be "The dinosaurs are the best known example of this." This statement can be improved by changing the final word. Don't assume your reader knows what "this" is. A better sentence is the following sentence. "The dinosaurs are the best known example of natural selection leading toward species extinction." This would work better, but based on the rest of the paragraph, I don't believe that this is the argument that you are trying to make. From the looks of it, I believe that your main argument is that individual people can help in small ways to prevent further extinction of existing species. If that is the case, then the dinosaur extinction and natural selection part is evidence that could be used to support your initial argument. The goofy part that I see thus far is that the argument is about how people can work toward preventing extinction, but much of the paragraph is about extinction events that happened long before people could have done anything. On the other hand, the paragraph also discusses poaching which doesn't fit nicely with natural selection. I do think both topics can be covered in a single thesis, and that will ultimately lead to a more well balanced and robust paper. For example:
"Although species have gone extinct through the process of natural selection, humans are a very real threat as well and can proactively find ways to stave off species extinctions."