Cambrian
At the start of the Cambrian, the earlier animals (the Ediacaran biota) may have become extinct. It seems these older life forms were affected by new types which burrowed into the sea floor. This changed the conditions of life for the older types. The burrowing must have disturbed the mat of bacteria and algae which covered the sea-floor. About this time we find the first examples of many phyla. There are also some fossil traces on what was land at the time, so perhaps a few Cambrian organisms did leave the water.