Shamanism is an ancient form of mysticism. Although there are many localized variations, the basic tenet is that all things are sacred and alive. Shamans are experts in this perspective and act as priests, holy persons, guides, researchers, explorers or professionals. Most have the ability to enter trance or altered states to access information related to their work.
The shaman acts as a bridge. While in contact with life in other dimensions of the universe, they also use what is available in the local environment as an aid in returning a person, place or situation back into balance.
The historical paths to becomming a shaman are many and varied. Before we had instant mass communication, and global travel, a would be shaman lived in a small village like all our ancestors. These small tribal settings were located in different ecosystems, some tropical, some cold, mountainous, valley, desert, forest, etc. Each place had it's own unique energy, with different plants and animals. You won't find a whale in the desert, so each potential shaman had very different life experiences and local resources to work with. Also, mythological systems vary a great deal.
Individuals were called to become shamans for varied reasons. Some were simply born sensitive. Some individuals had a near-death experience, mental break, disease or accident which caused them to search for a method for their personal recovery. Through trial and error they found a mineral, plant, location, ceremony or something to help cure them. Apon their recovery, them became shamans specialized to that problem. People came to them for their secret knowledge when in a similiar situation. Thus, a new shaman was born into the community.
Still others had a big dream, went on a vision quest or entered an apprenticeship with another shaman. Some became shamans because it was a family tradition, much like doctors tend to run in families. Some were forced to become shamans because of a need in the community. Sometimes the shamans and village elders watched the children, searching for a distinguishing sign or omen; they looked for the troublemaker, the curious one, the adventurer, the leader or the shy one. When they found what they felt was the right personality, they often placed that child in training...even against their wishes, since the needs of the tribe were considered more important.
Today, there are still traditional shamans in every culture, but there are also modern shamans. There are people today who have reawakened an interest in this ancient profession and are in training.