This question is the subject of intense study at this time. In many places around the world, the first organisms found as fossils are trilobites. These creatures, at first appearance were exceedingly complex members of Phylum Arthropoda, to which modern insects and crustaceans belong. They had compound eyes, antennae, fully articulated appendages, gills, and all of the complexity of modern members of that phylum.
Increasingly, immediately to somewhat below the occurrence of the first trilobites we are finding what are commonly called the "Small Shelly Fauna." This fauna consists of small shelly macrofossil forms: brachiopods, molluscs and a number of forms of uncertain affinities.
2010 Arthur V. Chadwick, Ph.D.