During precambrian time, simple life forms existed in the oceans of earth. this was followed, during the paleozoic era, by the "great explosion" when many new life forms appeared on earth in a very short period of time.

The term that fits the blank is CAMBRIAN and this refers to the Great Cambrian Explosion or Radiation that occurred in some 541 million years ago. This was considered as one of the most significant evolutionary event because it marked the appearance of the most major animal phyla. 

Active site is where they touch.

It would be the equator.