Therefore, all present forms of life arose from ancestral forms that were different. Birds arose from nonbirds and humans from nonhumans. No person who pretends to any understanding of the natural world can deny these facts any more than she or he can deny that the earth is round, rotates on its axis, and revolves around the sun. One heck of an assumption. If you let me make the rules, define the terms, state as a fact the very postulate I'm attempting to prove and slander anyone who disagrees, I can 'prove' anything. Got anything a little less presumptuous and more authoritative and sientific?
Actually, it's not an assumption. It's a FACT. It is also a fact that you managed to start a few sentences shy of the good stuff when you chose your quote.