Human Genetic Research in Wallacea and Sahul: Recent Findings and Future Prospects

Abstract

Genomic sequence data from worldwide human populations have provided a range of novel insights into our shared ancestry and the historical migrations that have shaped our global genetic diversity. However, a comprehensive understanding of these fundamental questions has been impeded by the lack of inclusion of many Indigenous populations in genomic surveys, including those from the Wallacean archipelago (which comprises islands of present-day Indonesia located east and west of Wallace's and Lydekker's Lines, respectively) and the former continent of Sahul (which once combined New Guinea and Australia during lower sea levels in the Pleistocene). Notably, these regions have been important areas of human evolution throughout the Late Pleistocene, as documented by diverse fossil and archaeological records which attest to the regional presence of multiple hominin species prior to the arrival of anatomically modern human (AMH) migrants. In this review, we collate and discuss key findings from the past decade of population genetic and phylogeographic literature focussed on the hominin history in Wallacea and Sahul. Specifically, we examine the evidence for the timing and direction of the ancient AMH migratory movements and subsequent hominin mixing events, emphasising several novel but consistent results that have important implications for addressing these questions. Finally, we suggest potentially lucrative directions for future genetic research in this key region of human evolution.

Keywords: Sahul; Wallacea; ancient DNA; human migrations; phylogeography.

Figure 1

Map of Wallacea and Sahul with palaeo-geographic reconstructions at lower sea levels Wallacea lies between the former continents of Sundaland (to the west of Wallace’s Line) and Sahul (lying east of Lydekker’s Line), requiring incoming human migrants to undertake a series of interisland movements across the archipelago before arriving in Sahul (historical coastlines at multiple time periods are indicated by different grey shades). Two main routes through the Wallacea have been identified [19]. Migration along the Northern Route (red arrows) would have brought AMH from Borneo through Sulawesi, from which there were three potential entry points into Sahul: (1) through Halmahera before arriving at the Bird’s Head peninsula of West Papua (route 1A); (2) through Seram before arriving at Misool (route 1B); and (3) moving south into Aru (route 1C). Notably, both Misool and Aru were part of the Sahul continent based on sea level reconstructions at 65 kya (i.e., the oldest dated AMH-associated site in Australia [10]). In contrast, Southern Route migrants would have moved across the Lesser Sunda Islands (present-day Nusa Tenggara) and continued either north through Moa and Tanimbar before arriving on Aru (route 2A), or south directly to Australia (route 2B). Palaeo-geographical models comparing the two routes have favoured the Northern Route hypothesis, with route 1B having the most support based on analyses of island intervisibility [16] and a least-cost model [17]. Additionally, a study combining island intervisibility and oceanic drift modelling indicated that the Southern Route was also a viable option [12,18].

Figure 2

Schematic representation of putative AMH population movements into ISEA and Sahul inferred from regional genetic data. Genetic inferences suggest that the initial peopling of the region occurred around 50–60 kya, with the separation of Aboriginal Australian and New Guinea populations occurring around the same time (red arrows; noting that this split may have occurred prior to the settlement of Sahul). Later migrations associated with the arrival of Austronesian seafarers introduced Ami-related ancestry into ISEA (blue arrows), and possibly also introduced MSEA-related ancestries at the same time (though the latter may have come from a separate movement; green arrows). Recent studies have emphasised that the presence of Papuan-related ancestry across Wallacea and the southern Philippines likely represents the movement of Papuan lineages into these regions (light blue dashed arrows), possibly in two distinct movements around 3 kya (i.e., potentially tied with Austronesian arrival) and 15 kya [39,43]. Not shown is an additional, albeit relatively minor, South Asian ancestry component in western Indonesia [38,45], which appears to coincide with the establishment of trading between these regions around 2000 years ago.