I have only seen the show, so I have more limited info than some of you, but here is what I think based on what the show revealed:
First, we know that wesen woge instintively sometimes, and that only other wesen and grimms can see this without a special compound given to the wesen. Second, we know that wesen can force the change to show themselves to normal humans. Third, we know that grimms have a unique property to their eyes that allows them to see wesen when they woge. Fourth, we know that nick has had a unique effect from being "zombified", that is likely to be explored more in the future. Fifth, we know that wesen can in fact have more than 1 woge, and that the ability appears undetectable to humans until they reveal themselves.
What I suspect is that the treasure is in fact what caused the wesen(and that the volcanalis as well as others are just more specialized wesen), specifically by means of a radiation that induced mutations(possibly a third helix that swaps place with one of the first two).
Basically, at some time long into the past, something was found by a tribe that was strange to them, and made them feel strange in its presence(making them proto-wesen). They interacted with it frequently, noticing that they become more fit as they were in its presence longer. Some were bit, scratched, burnt, or otherwise injured, and absorbed DNA or traits as a result. They went on to have children who exhibited some visual traits, but also could appear perfectly human(the first wesen), but one dies and is revived, and leaves the tribe seeking answers, who goes on to have children with somewhat unique eyes, and a deathly stillness(the first grimm).
The tribe, via its strength, managed to gain more territorial control, growing in numbers, and taking members from other tribes. In an attempt to stop this tribe, an alliance of other tribes formed, who attacked and took the object, but fearing it to be a cursed object, hid it, sacrificing their lives in the process(the first time it is lost). The tribe having experienced this, declares themselves gods and rulers of all, and proceeds to expand through the regions; while the grimms, being unable to successfully avoid all contact, and seeing the wesen as demons, fight and kill those that pursue them.
After a long time, the wesen leaders are defeated in battle, and driven from their kingdoms by human rulers, who then declare themselves the rightful rulers of Earth(the first royals), who also witness the wesen destroyed by mighty warriors(grimms), and declare those warriors their guards(while grimms unaware of the conflict live out their lives, sometimes hunting and killing wesen).
Much time passes as these new kingdoms grow, and the free grimms live their lives. The royals dispatch their grimm guards to find new wesen to rule, and a group of them finds the original object(the 7 knights), which they sense to have great power, which they believe is too much for any person to have; finding this object then makes them question their purpose in life, and they abandon the quest they are sent on, choosing instead to live their lives elsewhere in hopes of no longer encountering wesen.
More time passes, leading to the present day, and the events of the show.
As to what all are wesen, I consider all extra-human humanoids(grimms, known wesen, the volcanalis, and others) to be wesen, due to a single origin.(perhaps the volcanalis is a result of exposure to microbes that reside in super high temperatures). Hexenbiest could be the result of contact with medicines and toxins. Manticore could be the result of exposure to scorpions and canines within one life.
Sorry for the length of this post, and thank you for reading(I would appreciate any feedback).