Little evidence is available on the distribution and host specificity of the early Digenean trematodes ^[1][16]. It is hypothesised that, before adaptation to vertebrate hosts, these trematodes parasitised molluscs ^[2][13]. Some excavations revealed that Digenean trematodes can be found in dinosaur coprolites and mollusc fossils from the Upper (Late) ^[3][17] and Lower (Early) ^[4][5][18,19] Cretaceus periods; nevertheless, Cretaceous-Paleogene Mass Extinction (KPgME) around 66 million years ago (MYA) swept away most host species with their parasites ^[1][16].

The most probable cause of KPgME was the Chicxulub asteroid, which ejected enormous amounts of dust, ash, soot, and other aerosols at its impact. The outcome was prolonged sunlight screening, which resulted in months of global blackouts. The direct consequence was the extremely cold “impact winter,” lasting for a decade. The exaggerated cooling destabilised all trophic levels in the biosphere, which resulted in the extinction of most species ^[6][7][20,21]. For parasites, host switching might have been an option to survive KPgME, as it is confirmed in the feather lice of birds ^[8][22]. In these hardly tolerable climatic conditions, freshwater microhabitats with higher thermal inertia could provide refugia for certain species. The detritus of these swamps and ponds supplied nutrients to the survivors. In freshwater habitats, the biodiversity loss was barely 10%, contrary to the terrestrial 90% ^[6][20]. The recovery from this climatic catastrophe took about 30 years ^[6][7][20,21].

The seismic effect of the asteroid impact generated changes in the volcanic activity of the Deccan Traps, resulting in increased CO₂ outgassing ^[9][10][23,24]. Due to this extensive volcanism, the greenhouse effect augmented, and a drastic global warming began, which contributed to the formation of the early Cenozoic ecosystem ^[7][11][21,25]. The massive carbon release to the atmosphere finally led to the Paleocene-Eocene Thermal Maximum (PETM) around 56 MYA, characterised by global warming, reduced latitudinal temperature gradients, ice-free poles, and ocean acidification ^[12][26].

The emergence of the Fasciolidae family dates back to 90–100 MYA, after the fragmentation of Gondwanaland into Africa and South America ^[13][27]. Based on molecular genetic analysis, it is confirmed that the lineages of Fasciolopsis and Fasciola diverged cca. 88.1 MYA, long before the KPgME ^[14][28]. The recent intermediate hosts of the Fasciolidae family are gastropod species of the Basommatophora suborder, which evolved in the Carboniferous period. Based on the internal transcribed spacers of the ribosomal DNA (ITS-1 and ITS-2) and the mitochondrial 16 S ribosomal DNA, it is confirmed that Lymnaeidae snails, the most important intermediate hosts of the Fasciolidae family, originated during the Jurassic period ^[15][29], while the Lymnaeidae and Planorbidae families diverged 250 MYA ^[16][30], long before KPgME. Therefore, these snails are also survivors of KPgME.

Both Fasciolidae and their basommatophoran snail hosts evolved before the extreme winter caused by the Chicxulub asteroid impact. They also avoided extinction later, during the extreme hot climate of PETM. Presumably, their characteristic habitat, freshwater ponds and swamps, mitigated the temperature extremes and provided nutrients for the snails and thus their parasites. The harsh climate change caused the aridification of inland habitats; therefore, surface water cover decreased, resulting in habitat fragmentation ^[6][20].

Among these circumstances, isolated subpopulations, living in ponds, creeks, ditches, and shallow lakes almost worldwide, sustained the survivor species in small subpopulations ^[17][31]. Extreme habitat fragmentation jeopardises the sustainability of the subpopulations, thus the whole population itself, because if a population decreases below the effective population size, its maintenance is questionable. The ability of self-fertilisation reduces the effective population size, whereas hypothetically only one mature individual can multiply itself ^[2][18][13,32]. Several taxa of Basommatophora possess the ability of self-fertilisation, which assures the survival of the population after large-scale losses and contributes to the invasive ability of certain species ^[19][20][33,34].

The gene flow between the subpopulations of the habitat fragments is indispensable to the survival of the species. Lymnaeid snails are very small animals with limited moving ability; therefore, they cannot travel very far under their own power. However, these species are maintained during the rapidly changing KPgM; moreover, they spread worldwide from their fragmented refugia ^[15][29]. The historical spread of Lymnaeid snails is confirmed by the finding of a fossil Galba sp. from the Cretaceous period on an island 1500 km from the mainland. The migration route cannot be reconstructed; however, the authors presume that an ancient bird should have transported it on its body surface or within its intestinal tract through the Indian Ocean ^[21][35]. The same phenomenon is observed in the European distribution of F. magna, whereas ungulate wildlife can transport snails by mud stuck on hair ^[22][36]. This route can contribute to the translocation of a limited number of snails. Population-level spread of these small gastropods can occur in lotic habitats in the flow direction ^[23][37], which is generated by strong currents, e.g., stormwater runoff ^[24][38]. The phenotypic plasticity that helped these mollusc taxa survive and spread during the climatic extremities of the Cretaceous-Paleogene Boundary (KPgB) has been conspicuous recently. These snail taxa can be found in temperate zone floodplains ^{[22][25][26][27]}[3,5,7,36], high-altitude mountain lakes in the Alps ^[28][39], in the Andean areas of South America ^[29][40], and in the tropics ^[30][31][32][41,42,43].

Within the intensely changing environment between KPgB and PETM, the Fasciolid parasites should have met their ancient final hosts, the early proboscideans ^[13][33][1,27]. After the loss of the late Cretaceous megafauna, the ancient mammals began to diversify and fill the ecological niches left by the extinct non-avian dinosaurs ^[34][44]. The order of Proboscidea was one of the earliest known modern placental orders in the Late Paleocene, cca. 60 MYA in Africa ^[35][45]. The oldest fossils of ancient elephant relatives can be traced back to 45–60 million years ^[35][36][37][45,46,47]. Due to the drastically increasing temperature, tropical and subtropical forest environments became the dominant habitats globally, and the primitive proboscideans lived in swamp ecosystems and fed on freshwater vegetation in riverine or swampy settings ^[16][34][30,44] such as the Lymnaeid snails at that time ^[17][31].

The following 40 million years in the Cenozoic Era witnessed the evolutionary history of proboscideans. During this time period, the order diversified and spread almost all over the world through three major radiations ^[14][36][28,46]. The first occurred during the Palaeocene Eocene Boundary (56 MYA), with the diversification and intra-African spread of primitive proboscideans ^[35][36][45,46]. The second radiation took place during the Oligocene and early Miocene periods, when the supervening connection of Africa to Eurasia could facilitate the faunal interchange between the two continents. This eventuated 25–20 MYA and was termed the Proboscidean Datum Event (PDE), which resulted in rapid global dispersal of the order, by which it reached the Americas ^[36][38][46,48]. After the PDE, the proboscideans became one of the most diverse taxa of the paleofauna ^[38][48]. The last radiation took place during the late Miocene/early Pliocene and resulted in the diversification of Elephantidae, the ancestors of the extant two genera, Loxodonta and Elephas ^[36][46].

The phylogenetic analyses of the Fasiolidae family support the hypothesis that proboscidean dispersal spreads these parasites globally. The basal extant member of the taxon, Protofaciola robusta, parasitises the small intestine of the African bush elephant (Loxodonta africana) and uses Planorbid snails as intermediate hosts. The next most basal member, Fasciolopsis buski, is also an intestinal parasite with a planorbid intermediate host ^[33][1]. The whole genome analysis of Fasciolidae revealed that the clade comprising Fasciola and Fascioloides genera split from the ancient lineage between 65 MYA and 55.9 MYA during the rapid diversification of the Proboscidea order. This split also meant a switch between both habitats (from the intestine to the liver) and intermediate hosts (from Planorbidae to Lymnaeidae) ^[13][14][33][1,27,28].

The closest relative of F. magna is Fascioloides jacksoni ^[39][49], which parasitises the liver of the Indian elephant (Elephas maximus). This fact suggests that these sibling species were spread by their primary definitive hosts, which were proboscideans ^[40][41][50,51]. A comparative study of karyotypes and chromosomal locations of rDNA genes in F. magna and F. hepatica suggested that Fasciola is the younger genus within the liver pathogen clade of Fasciolidae ^[42][52]. Based on molecular genetic investigations, fossil records, and recent epidemiological and pathological findings, it is very probable that the Fasciola genus evolved as an adaptation to the emerging taxon of ruminants ^[13][27]. Though a recent phylogenetic investigation hypothesised that the oldest member of the Fasciola genus is F. nyanzae, the first hosts of this genus were hippopotamuses, and subsequent fasciola species jumped to ruminants by an intermediary host, which could be the wild boar (Sus scrofa) ^[43][53].

After the PETM, about 34 MYA, the temperature fell and the aridification process started. During the Miocene (23.03–5.333 MYA), the climate began to cool drastically, which induced changes in the ecosystem. The cooler and, in mid-latitudes, dry and seasonal climate supported grasslands to spread. The early fossil record of grasses dates back to the Late Cretaceous; however, until the Miocene, grasses did not form extensive grassland ecosystems ^[34][44]. The previously woodland-dominated vegetation turned to more open, savanna-type environments ^[44][45][54,55].

This period coincided with the Messinian Event 6.0-5.3 MYA, when the Mediterranean dried up and the world’s oceans became less saline ^[46][56]. Just after that, the Miocene-Pliocene Boundary (MPB, 5.3 MYA) marks a transition from a late Miocene cooling trend to the early to middle Pliocene warm period, when the Northern Hemisphere was largely ice-free and atmospheric CO₂ concentrations were comparable to present-day levels ^[47][57]. This interim warming during the global cooling trend turned to the subsequent glacials and led to the unstable Pleistocene with extended arid glaciations, sea level changes, fluctuating atmospheric carbon dioxide, and shifting vegetation ^[46][56]. During this volatile geological time of MPB, the two sister species of the genus Fasciola, F. hepatica and F. gigantica, diverged ^[14][28].

The alteration in vegetation brought about a dramatic change in the nutrient supply of the megafauna. Those taxa could reach evolutionary success, which adapted to the use of grasses as nutrient sources (grazers, GR); while those herbivores, which depended on woodland plants (browsers, BR), began to decline ^[38][48]. The diversification of GR and the decline of BR were initiated during the Miocene and accelerated through the MPB. This process was more rapid in North America than in the rest of the world ^[45][55].

Among these conditions, ruminants began to diversify extensively through adaptation to more heterogeneous habitats and seasonal climates, and they colonised a large range of biomes ^[44][54]. In the meantime, specialist biodiversity declined due to reduced primary production and the loss of forest habitats. Proboscideans are mostly browser herbivores, which means that they depend on woody and non-woody dicotyledonous plants, leaves, and barks of trees as food sources ^[34][46][44,56]. Owing to the enhanced competition with other ungulate clades that evolved specialist grazing ecomorphs, the extinction of the proboscidean species accelerated around 8 MYA, after the onset of grassland-dominated habitats ^[38][45][48,55].

The global temperature and the consequential vegetation shift cannot explain the replacement of proboscideans with grazing-adapted ungulates ^[34][44][44,54]. A paleontological study confirmed that adaptation to grazing lifestyles could be detected in late gomphotheres, a proboscidean taxon that went extinct in the late Pleistocene ^[48][58]. However, another study highlighted that proboscidean fossils from the middle Miocene to the late Pleistocene showed enamel hypoplasia, which can be regarded as a sign of chronic stress, though the reason cannot be specified ^[49][59].

Multiple extinction events occurred during the Late Pleistocene, mainly over the last 100,000 years, with a peak between 12,000 and 10,000 years ago ^[34][45][44,55]. The most severely affected fauna was large-bodied herbivores above 1000 kg body weight. Especially in the Americas, where 100% of this sized species became extinct ^[45][55]. This era of natural history is characterised by the human presence. A consensus is emerging that climate change and early human activities went together in contributing to the biodiversity loss of megaherbivore fauna ^[34][44]. The top-down effects of human predation, landscape modification, and especially the controlled use of fire affected the proboscidean populations of the palearctic regions ^[45][48][55,58].

In contrast with proboscideans, the ruminants proved to be the real winners of the climatic fluctuations of the last 45 million years. After the Eocene-Oligocene cooling and aridification event 34 MYA, and especially during the Miocene Climatic Optimum cca. 18–15 MYA, the ruminants diversified and successfully adapted to the heterogeneous habitats and climate of the Earth ^[44][54].

Within the Ruminantia suborder, the Bovidae and Cervidae families evolved in Asia during the Miocene and started colonising a large range of ecosystems ^[44][50][54,60]. By analysing two mitochondrial protein-coding genes and two nuclear introns for 25 species of deer, an investigation estimates that the Cervidae family originated in the Late Miocene, 7.7–9.6 MYA, and the common ancestor of the American Odocoileini is dated between 4.2 and 5.7 MYA. The fossil records of American cervid species also supported the molecular findings that Odocoileini entered the American continent during the MPB, while other cervid taxa reside in America probably as a result of a recent dispersal event, cca. a million years later, during the Early Pleistocene ^[50][60] or at a much later time, during the presence of the last landbridge between Eurasia and America between 10,000 and 70,000 years ago ^[51][61].

The host switch of F. magna from proboscideans to cervids presumably eventuated in North America ^[33][1]. In Eurasia, a large number of potential host species were available, and the proboscidean loss was not as severe as in the Americas ^[13][44][45][27,54,55]. It is very probable that a drastic population collapse of the American proboscidean host species forced the host shift to the sympatric cervid species ^[33][1], which could be the ancestor of the recent white-tailed deer (Odocoileus virginianus).

On the North American continent, white-tailed deer were the most abundant host species for thousands of years. It is estimated that before the arrival of the European settlers, between 23 and 40 million white-tailed deer inhabited North America. Due to its extensive hunting, cca. 500,000 animals remained by the late 1800s. During the 20th century, 41 states made a law for the protection of the species; therefore, the currently estimated population is between 14 and 20 million individuals in the United States ^[25][3].

On the North American continent, F. magna has five focuses: (1) the Great Lakes region; (2) the Gulf coast, lower Mississippi, and southern Atlantic seaboard; (3) the northern Pacific coast; (4) the Rocky Mountain trench; and (5) northern Quebec and Labrador ^[52][62]. Though potential habitats and appropriate intermediate and final host species exist even between the main focuses, considerable expansion cannot be detected ^[27][7]. Phylogenetic analysis of F. magna strains originating from different American focuses confirmed that most populations share haplotypes with low genetic differences; however, the eastern and western populations show genetic distance ^[53][54][63,64]. The patchy distribution and the contradictory genetic overlap of the geographically separated subpopulations are suggested to be the consequence of the final host’s population loss, which forced the partial switch to alternative hosts belonging also to the Cervidae family. This phenomenon is frequently detected in host-parasite systems during host extinction when the parasite prefers host-switching to species that are phylogenetically close to its original host ^[55][65].