Manter Laboratory of 6-1-2008 Parasites as Probes for Prehistoric Human Migrations ?

Host-specific parasites of humans are used to track ancient migrations. Based on archaeoparasitology, it is clear that humans entered the New World at least twice in ancient times. The archaeoparasitology of some intestinal parasites in the New World points to migration routes other than the Bering Land Bridge. Helminths have been found in mummies and coprolites in North and South America. Hookworms (Necator and Ancylostoma), whipworms (Trichuris trichiura) and other helminths require specific conditions for life-cycle completion. They could not survive in the cold climate of the northern region of the Americas. Therefore, humans would have lost some intestinal parasites while crossing Beringia. Evidence is provided here from published data of pre-Columbian sites for the peopling of the Americas through trans-oceanic or costal migrations. Ancient migrations to the Americas The timing of the arrival of humans in the New World has been the focus of a long-standing and intense debate with different scientific disciplines contributing their own perspectives to the question. Some challenge the widely accepted hypothesis that populations of prehistoric humans crossed into the Americas when climate conditions created the Bering Land Bridge at least 13,000 years ago. However, people could have reached the Americas more than 7,000 years ago using alternative routes, thus avoiding the extreme cold of the Arctic. From the parasitological perspective, unequivocal evidence shows that populations of pre-historic humans were infected by several host-specific intestinal parasites found in both North and South American archaeological sites [1]. The life cycle of these exclusively human parasites could not have been maintained among human groups during the crossing from Siberia to Alaska via Beringia. Heirloom parasites and souvenir parasites Some infectious agents of human disease have coevolved with species in the Homo lineage for over 400,000 years [2– 5]. It is clear that our most common parasites had a concomitant origin with earlier species in the human evolutionary line and these coevolutionary events certainly date to an early common ancestor of humans and apes. These long-term, coevolved parasites are sometimes called ‘heirloom parasites’ [2,6] or, in the parlance of evolutionary biology, parasites that are biological symplesiomorphies [7]. As humans evolved, their heirloom parasites evolved with them. This appears to be the case for both Trichuris trichiura (whipworm) and Enterobius (pinworm) [2–4]. The evolution and diversification of Enterobius resulted in two species in humans, Enterobius vermicularis and Enterobius gregorii. As pointed out by Cameron in 1958 [8], Ascaris lumbricoides (intestinal roundworm) and Ancylostoma duodenale (hookworm) are also human heirloom parasites [9,10]. Humans have acquired a myriad of other parasites during their long biological and social history. Sometimes called souvenir parasites [6,11], these include species that occur in humans commonly, arriving through host-switching from other animal species. Animal domestication might have provided opportunities for parasites to colonize humans. This was considered the origin of two species of taeniid tapeworms in humans that use cattle and pigs as intermediate hosts. However, phylogenetic systematic studies show that association of beef tapeworm in humans (Taenia saginata) pre-dates the development of the domestication of cattle (Bos spp.) or swine (Sus scrofa) [12,13]. Many heirloom parasites evolved with humans in the Old World tropical and subtropical regions. Therefore, it is not surprising to find whipworm, pinworm and hookworm eggs in ancient Old World archaeological sites [1]. These parasites accompanied early Homo groups as they migrated out of Africa to other tropical regions, persisting in their hosts as long as environmental conditions enabled transmission. Climate and soil temperature are limiting factors for human hookworm and whipworm, being hostspecific and without intermediate hosts. High soil moisture and moderate temperature are necessary for eggs or larvae to reach the infective stage. By contrast, eggs of E. vermicularis are infective as soon as they are expelled from female worms; in the external environment, they can infect humans immediately through contaminated hands, soil, water, any kind of object or as an aerosol. Therefore, pinworm eggs are not dependent on climate conditions [4]. All three nematodes have been found in North and South American archaeological material. However, because they are dependent on a habitation for transmission, only pinworms could have accompanied humans during the crossing of Beringia [14]. Hookworm and whipworm have biological requirements that could not have been met in Beringia Published in Trends in Parasitology, vol. 24, no. 3 (2008): 112-115. doi:10.1016/j.pt.2007.11.007. Copyright 2008, Cell Press/Elsevier Ltd. Used by permission.