The Pacific Rat Race to Easter Island: Tracking the Prehistoric Dispersal of Rattus exulans Using Ancient Mitochondrial Genomes

Abstract

The location of the immediate eastern Polynesian origin for the settlement of Easter Island (Rapa Nui), remains unclear with conflicting archeological and linguistic evidence. Previous genetic commensal research using the Pacific rat, Rattus exulans; a species transported by humans across Remote Oceania and throughout the Polynesian Triangle, has identified broad interaction spheres across the region. However, there has been limited success in distinguishing finer-scale movements between Remote Oceanic islands as the same mitochondrial control region haplotype has been identified in the majority of ancient rat specimens. To improve molecular resolution and identify a pattern of prehistoric dispersal to Easter Island, we sequenced complete mitochondrial genomes from ancient Pacific rat specimens obtained from early archeological contexts across West and East Polynesia. Ancient Polynesian rat haplotypes are closely related and reflect the widely supported scenario of a central East Polynesian homeland region from which eastern expansion occurred. An Easter Island and Tubuai (Austral Islands) grouping of related haplotypes suggests that both islands were established by the same colonization wave, proposed to have originated in the central homeland region before dispersing through the south-eastern corridor of East Polynesia.

CLAFS, a Holistic Climatic-Ecological-Anthropogenic Hypothesis on Easter Island's Deforestation and Cultural Change: Proposals and Testing Prospects

“…This idea is based on the frequent occurrence in soils and archeological sites of endocarps of palm fruits with rat gnaw-markings (Hunt, 2006(Hunt, , 2007Hunt and Lipo, 2011). Rats would have been of Polynesian origin, likely transported by the first settlers (West et al, 2017). In this scenario, the timing of deforestation does not differ considerably from that of the ecocide hypothesis.…”

Commensal Rats and Humans: Integrating Rodent Phylogeography and Zooarchaeology to Highlight Connections between Human Societies

“…Pacific rat lineage III may have diverged early (Figure 1a), where this sample was collected in Far Oceania [ 26 ] yet the center of diversity was described westward on Flores. [ 23 ] While competitive exclusion in Near Oceania of lineage III haplotypes may explain their Far Oceania distribution, it is unsatisfying that the pre‐expansion range remains unknown.…”

“…a) Neighbor‐joining mitogenome phylogenetic tree ( n = 15 Rattus species and one Mus musculus outgroup (not shown [ 79 ] ); NCBI accessions NC023347, NC029888, and those found within published papers [ 26,80,81 ] ) where interior colored branches denote possible evolution of commensalism, and grey squares indicate rat species able to live in artificial habitats (27 of 64 Rattus taxa listed by IUCN). We note identifiers of within species diversity in parentheses based on Aplin et al.…”

“…[ 81 ] for R. norvegicus . Inferred natal ranges (filled polygons) and range expansion routes for b) R. rattus (green); c) R. tanezumi RrC II (lime green) and RrC III (hunter green); d) R. exulans lineage I (grey; not shown in phylogeny), Thailand/mainland SEA (light blue), lineage II (dark blue), and lineage III (medium blue); [ 23,26 ] and e) a distilled subset of R. norvegicus clades highlighting patterns from the nuclear genome. [ 27 ] Dashed arrows in panels b, c, and e represent expansions to the western hemisphere.…”

“…[ 24,25 ] In Pacific rats, lineage II is found in the Philippines, New Guinea, and Santa Cruz Island, while lineage III extends throughout Far Oceania including New Zealand, Hawaiian Islands, and Easter Island. [ 24,26 ] Finally, lineage I and an un‐numbered clade expanded to Borneo and the Philippines, and southeast Asia (SEA), respectively (Figure 1d), [ 23,26 ] under the hypothesis that Flores is the ancestral range.…”