Figure 2

Inferred life cycle and high degree of organellar complexity of the last common ancestor of all extant eukaryotes. This reconstruction assumes that the root of the eukaryotic tree is between Euglenozoa and excavates [7, 8]. If so, every homologous character present on both sides of the neokaryote/euglenozoan split must have evolved prior to the cenancestor, provided that its later lateral gene transfer from one to the other can be ruled out, as it can for the complex characters shown. The major uncertainty is whether there were only one centriole and cilium as shown or more likely two of each [9]. In addition to the pellicular microtubules there would also have been centriolar roots consisting of bands of microtubules (probably two if the ancestor was uniciliate and three if biciliate) and a specialized anterior cytostome and cytopharynx for prey ingestion (all not shown for simplicity). The peroxisome (p) was probably attached to the nucleus and the Golgi was probably attached to a centrin body; centrin would also have been associated with the centriole and intranuclearly at mitotic spindle poles. The mitochondrion (m) was probably actually attached to the centriole and/or nucleus. A branched actin cytoskeleton permeating the cytoplasm was linked to nuclear envelope (NE) via KASH/Sun integral membrane protein complexes and to the plasma membrane via membrane-embedded integrin proteins. Syngamy involved fusion of plasma membrane, NE, and probably mitochondria.