These must have been aquired early in the evolution of eukaryotes. Later, after the divergences that create the various groups of archaeozoans, the ancestor of the rest of the eukaryotes formed an endosymbiotic association with an aerobic alpha-purple bacterium. This association became permanent - neither symbiont can now survive without the other. The bacterial symbiont evolved dramatically, and most of it's genes have been transfered to the nuclear genome - we call them mitochondria. On way to interpret this is that evolution is a force driving 'progress', but this is at odds with Darwin's writing and our current understanding of evolutionary processes.
Microscopic crustaceans are abundant and amazingly diverse in marine sand and sediments.
The vast majority of these have never been formally described as species, and almost nothing is known about them. EM pictures of yeast cells never show more than a few of these scars, and never show overlapping scars, leading some to propose that a yeast 'mother cell' can only produce a certain number of buds and can then no longer reproduce.