That's why it's a theory. No one is "sure" of theories. Because that's what science is about, finding more information that points in a direction and changing ideas about how the world works. That's done by inference, and they have lots of clues that point toward it, like they have lots of clues that point toward the existence of molecules. No one has seen an atom though. The idea that there are atoms is also a theory. Done by inference from mathematical treatment of collected data. Just like evolution.. Does that help? I worry that so many people don't understand how science itself works, it's a pretty common misconception. Science is not like religion, it's not about beliefs, it's about explaining observations, and not being attached to those explanations.. I almost forgot your question-. There are lots of "branches" in the evolution tree. Different animals developed abilities and atributes that enhanced their ability to survive. Humans didn't necessarily evolve from those animals, they evolved from a common ancestor to those animals that may or may not live today.