## 1 Introduction
### Endosymbiotic Events of Parasitic Protozoa  
Protozoa were first grouped as a single monophyletic group and were considered ancestors of the animal kingdom [23]. However, molecular studies showed that protozoa are not a single monophyletic group [23]. Eventually, the ancient subkingdom Protozoa was rejected and reclassified with several algae and fungi [24]. The classification includes several phyla, including Sarcomastigophora, Apicomplexa, Ciliophora, Labyrinthomorpha, Macrospora, Asectospora, and Myxospora [23]. However, only Sarcomastigophora, Ciliophora, and Apicomplexa contain major human pathogens [9, 23, 24]. Sarcomastigophora includes important parasitic families such as Endamoebidae (including _Entamoeba histolytica_), Trypanosomatidae (_Trypanosoma_ spp., _Leishmania_ spp. ), Hexamitidae (_Giardia_ spp. ), and Trichomonadidae (_T. vaginalis_) [25]. The phylum _Apicomplexa_ includes parasitic families such as Eimeriidae (_Isospora_ spp. ), Cryptosporidium (_Cryptosporidium_ spp. ), Sarcocystis (_Sarcocystis_ spp. and _Toxoplasma gondii_) and Plasmodiidae (_Plasmodium_ spp.). Finally, the Ciliophora phylum contains the Balantidiidae family, which is mainly represented by _Balantidium coli_. Other classifications include apicomplexan parasites together with dinoflagellates and ciliates within the alveolates group [24, 26]. There is evidence indicating that during evolution, alveolate ancestors underwent two endosymbiotic processes. Primary endosymbiosis occurred between a phagotroph ancestor of the current algae and a cyanobacterium [26, 27] and originated in the Archaeplastida group, which includes Glaucocytophytes, _Rlodophyta_ (red algae), and Viridiplantae (green algae and plants). Secondary endosymbiosis probably occurred between a second alveolate phagotroph ancestor and red algae [21]. Due to these phenomena, several species of photosynthesisizing dinoflagellates that produce chlorophyll and peridinin [27]. Several apicomplexans still harbor a non-photosynthetically modified chloroplast called the apicoplast. The apicoplasts of _Plasmodium falciparum_ and _T. gondii_ seem to be strongly associated with mitochondria [28, 29], and both organelles share metabolic pathways such as heme biosynthesis in _Plasmodium_[29, 30]. Due to the fact of these endosymbiotic processes, apicomplexan parasites possess metabolic pathways that are typically present in photosynthetic organisms or bacteria. Some examples are amylopectin biosynthesis [31], aromatic ring biosynthesis [32], the calcium-dependent protein kinase (CDPK) multigene family [33] and isoprenoid biosynthesis by the methyl erythroid phosphate (MEP) pathway [34]. Due to the presence of these pathways, apicomplexan parasites are sensitive to some herbicides, as described in this review [35; 36]. Recent studies also indicate that trypanosomatids share a common ancestor with euglenids, a group containing photosynthetic organisms [37]. Some authors suggested that this plastid was recently acquired by the photosynthetic euglenid common ancestor [37; 38] while other researchers suggests that the plastids appeared at the beginning of euglenozoic evolution before the bodonids and euglenids diverged [37; 38]. After this event, bodonids may have lost the plastid and subsequently diversified to several phylogenetic groups, including trypanosomatids. Finally, archezoan protists (including diplomonads, microsporidia, and trichomonads) were thought to have separated from eukaryotic organisms before the endosymbiotic event, which originated the mitochondria [38; 39; 40]. However, molecular studies revealed that the _T. vaginalis_ genome encodes mitochondrial proteins such as a chaperone protein (HSP70) associated with a sister group of \(\alpha\)-purple bacteria and proteobacteria [40]. Similarly, mitochondrial enzymes such as pyridine nucleotide transhydrogenase and the chaperonin cpn60, associated with mitochondrial lineage sequences, were identified in _E. histolytica_ and _Giardia lamblia_[38].