The early evolution of eukaryotes has remained a realm of mystery and surprise. The qualitative leap from the prokaryotes to cells, that bear complex and dynamic compartments, is still one of the hardest nuts to be cracked for evolutionary theory. The concept of “symbiogenesis”, postulated by Mereshkowski at the beginning of the last century and much later solidified through molecular evidence by Margulis emerged as central concept to understand, how qualitative and complex traits could be acquired by endosymbiosis. However, Mereshkowski and Margulis left as a burden to evolutionary biology the insight that the conventional model of phylogenetic trees has to be replaced by more reticulate concepts of descendence. This working program is still far from being completed—we have got only glimpses into the complexity of early eukaryotic life. Since the underlying mechanism, endosymbiosis, is cellular, the approach to distangle this complexity must be cell biological in the first place—this remains valid even in the age of molecular genetics. This means that we need ultrastructural research to understand the early evolution of eukaryotes. Three contributions in the current issue perpetuate a long history of this journal on ultrastructural insights on early eukaryotes.
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