I ended my Shamanism post with a too quick comment on human nature. I'll try to clarify and expand my ideas here.
It would be better to say that language made humans human, thus creating human "nature." By that I mean biologically. Most people probably believe that modern humans have the same anatomy as the first homo sapiens with language, about 50,000 to 100,000 years ago. There have been superficial anatomical changes in some populations, both widespread things like increased height and limited things like increased obesity. But these are very minor in the big picture - they don't change the way we work. The obvious example is the appendix. It might be going away, but that won't change anything about being human.
The difficulty comes when we apply this idea to the brain. Our brains are exactly like the brains of our oldest human ancestors with language. I say exactly very deliberately. I would guess that most people intuitively think that the brain has changed more than other internal organs or any external anatomy over the last 100,000 years. But that has it completely backwards. Any change in the anatomy of the brain over that time is minute compared to changes of the rest of the body.
The reason is there have been strong selective pressures for things like height, lung size, bone strength or the like. There has never been - and won't likely be in the future - strong selective pressure on our brains. In fact, there is a general pressure against developing increased brain functions in that the brain is a high user of energy. Any improvement will come from a new capacity or function that has to provide a big enough benefit that it is worth the extra needed calories. Thus, such a change will be far less likely - and take far longer - than a more flexible wrist for example.
The mammalian brain is somewhere around 100 million years old. It took a very long time for it to develop the mind of an ape or dolphin . In that time, living in groups, becoming social, was the spur that created more brain power. That happened over millions of years. The development of language was a huge leap, in a flash of evolutionary time, because the physiological change wasn't that great. The best theory I've heard is that the larynx of homo sapiens dropped in the throat. It makes choking more likely, but the advantages of improved speech differentiation were very great.
The order is probably off, but a wider range of speech led to labels, symbolic thinking, grammar, language and abstract thought. These changes created new wiring in the brain that gave us the human mind. (Maybe I'll tackle the brain/mind issue sometime) This was the significant change that made us different from all the rest that came before. Homo sapiens then spread across the world, out-competing all other hominids. Our brains still work the same way.
So we're just like the first humans. No "Golden Age" or "After the Fall." However, language was a very unique evolutionary change. It created a whole new kind of evolution. Up until then, all evolutionary change was biological, its information was passed down the generations in our genes. But language allowed non-biological information to be transmitted across time - and it wasn't limited to only your descendants. Language created cultural evolution. Religious or social innovations are cultural evolution. They have caused great changes obviously, but those changes were not to human nature.