I've reading books by Joseph Campbell, Marija Gimbutas and Merlin Stone (and others) about ancient religions. I'm curious about the concept of Mother Nature and how it was thought of several millennia ago. I'm also interested in what snakes and bulls and some other symbols meant.

According to Gimbutas, the idea of Mother Nature was one group and ideas about the Snake Goddess were another. But it seems to all overlap with Mother Nature worshippers having the snake as various symbols.

It is evident the the snake meant healing by it's use in present day medical symbols even though the snake is pretty much demonized in the Judeo-Christian tradition. I have read things in the past that suggest there may have been some use of venom that was used in ceremonies in ancient Greece (such that a little bit of venom may have helped induce visions in people who were versed in that). 

One book said that there was an old idea from India that the earth is balanced on a giant snake. And that the snake accounted for earthquakes. (In China - snakes are currently used to predict earthquakes.)

Beyond those things, snakes have been associated with water and also with renewal and transformation - what with their skin shedding. Water, of course, being integral to life, was also integral to various Goddesses and was represented in different ways.

The concept of snakes being associated with the divine is common to so many religions around the world that it leads to the idea that the concept was commonly shared - probably when the world population was quite small and the idea was spread as people spread over the earth. There are snakes that adorn the roofs of old Stave Christian churches in Norway  - and also snakes that adorn the roofs of religious buildings in Thailand. There are snakes and dragons on religious buildings in the Americas from before Columbus came. 

Most ancient culture's religions have something about goddesses and snakes. In addition to water, healing and renewal, snakes were associated with the ancient concept of the Tree of Life. A snake rested at the bottom and an eagle was on top.  

Gimbutas studied abstract snake imagery on Goddess figurines dating from 6000-2200 BCE. Some pregnant goddess/women figurines had a symbol of a snake coiled around the abdomen area. Campbell wrote that the Goddess figurines were created during the Aurignacian culture 35,000 - 26,000 BCE through the Old Europe time that Gimbutas wrote about and that there were still pockets of places in the mid-20th century where they were still being made. Often they were made in such a way that they could be stuck in the ground. Wishes for fertility, especially of crops, were often the purpose.