A lot of the organisms that have existed on earth have left no fossils because of the incredibly specific set of requirements for a fossil to form. First, it must settle into sedimentary rock, instead of being roasted to nothing else but carbon fragments from the formation of igneous rock, or be crushed and burned from the formation of metamorphic rock. Then, the conditions of the surrounding environment must be just right: high sedimentation rates, low soil acidity, lots of minerals and anoxic conditions are best. Also, the organism must have bones or some kind of material that can easily mineralize into a fossil. An outlier would have been the organisms from the Cambrian period, which were fossilized at a high rate despite being mostly millimeter to centimeter long soft-bodied organisms. We have recently found out that this is due to the thick algal mats at the bottom of the Cambrian oceans. You can actually go see the Cambrian fossils at the Burgess Shale.
