Have you heard of Spiritual Literacy?
If "spirit" = "breath" by definition, and animal life facilitates the processes of mechanical aeration and chemical respiration that occur in nature, then animals (and plants) can be understood as earth spirits, literally and scientifically. They are the parts of the global breathing apparatus that supply air, food, and energy within our ecosystem.
While this might not be a very detailed description of what's occuring in nature, it is valid and more than suitable scientific information that our ancient forebears could reasonably pass on to future generations. In fact, narratives about woodland spirits (including The Lorax), apparently have also conveyed far more an understanding of "oneness" and a responsibility to nature than a discussion of scientific mechanics ever has - a global discussion and understanding that we are only getting around to now. It's enough indication that the word "primitive" to describe our ancestors, should probably be replaced by "primary."