Using seafaring simulations and shortest-hop trajectories to model the prehistoric colonization of Remote Oceania

Crossings to Palau.

For shortest-hop trajectories departing from the Philippines and Maluku Islands, movement into Micronesia requires crossings to Palau and Yap. Paths departing from the Maluku Islands can arrive at Palau through crossings that do not exceed 360 km, significantly shorter than the longest crossings required to reach Palau from the southern (∼657 km) or central (∼735 km) Philippines. Many crossings departing from the Maluku Islands take place in an area with average annual westerly winds (Fig. 1).

Simulations are in good agreement with wind and shortest-hop results. No downwind voyage from Taiwan, the northern and central Philippines, or the Bismarck Archipelago reaches Palau. Successful crossings to Palau take place from the southern Philippines, the Maluku Islands, western New Guinea, and the Marianas (Table S1). Average crossing times of ∼1 mo are simulated for the first two areas, but Maluku Island crossings have a larger probability of success (5–35%) than those from the Philippines (1–11%). Crossings from New Guinea are faster (15 d), but occur only for simulations started in July and have much smaller success rates (1%). With the exception of the Mariana departures, feasible downwind voyages are simulated for NH summer and fall, with crossings under September conditions showing the highest success rates. Trips departing the Marianas to Palau between December and April have success rates ranging from 1% to 5%, the highest value coming from February departures (Table S1).

Drier conditions associated with El Niño over the Philippines, the Maluku Islands, and New Guinea could act as an incentive for movement away from these areas (Fig. 3 and Figs. S5 and S6). El Niño wind anomalies favor eastward movement out of the Philippines and northeastward displacement from the Maluku Islands, particularly during the already favorable conditions of the NH summer (Fig. 3 and Fig. S6).

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Fig. S5.

ENSO seasonal anomalies. Wind (green vectors) and precipitation (colors, mm/d) deviations from the long-term December–January–February mean under El Niño (Upper) and La Niña (Lower) conditions.

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Fig. S6.

ENSO seasonal anomalies. Wind (green vectors) and precipitation (colors, mm/day). (Upper) deviations from the long-term June–July–August mean under El Niño conditions. (Lower) Deviations from the long-term September–October–November mean under La Niña conditions.

Crossings to the Mariana Islands.

The majority of shortest-hop trajectories departing from the Philippines and the Maluku Islands move east along Micronesia, but when one or two bin-sized islands are ignored (SI Data and Methods), some pathways include crossings to the Marianas (Fig. 1). Simulated downwind voyages from Taiwan and the Philippines are not able to reach the Marianas, but August–October departures result in successful crossings from Palau, the Maluku Islands, New Guinea, and the Bismarcks. Central New Guinea voyages initiated in September show the highest probability of success (25%). Bismarck September crossings also show relatively high probability of occurrence (13%) and have an average duration of 93 d, ∼10% shorter than average central New Guinea crossings (Table S2). La Niña is associated with drier conditions as well as easterly and southeasterly wind anomalies that could have played a part in movement from New Guinea and the Bismarcks toward the Marianas (Fig. 3 and Fig. S6). Crossings from Palau to the Marianas exhibit the smallest probability of success (1–2%) but have the shortest durations (52 d on average).

Crossings to central and eastern Micronesia.

Central and eastern Micronesia are reached by simulated downwind voyages starting from the eastern and western Solomon Islands, Vanuatu, and Fiji, although eastern Solomon and Vanuatu departures result in success rates of only 1% (Table S3). Successful voyages from the western Solomons are observed between July and October. The largest success rate (24%) occurs in July crossings that arrive in the region around Chuuk after an average trip of 84 d. August is the month with the most successful crossings from Fiji (14%), which have an average duration of 124 d and also arrive in the region near Chuuk. Both source areas become drier during El Niño, when wind changes also facilitate movement to the north and east (Fig. 3 and Fig. S6).

Crossings to Samoa.

Shortest-hop departures from the East Solomon Islands generate three general trajectories that reach Samoa: a northern route through Tikopia, Rotuma, and Tuvalu, another that from Rotuma reaches Fiji, and a third that moves through Vanuatu, Fiji, and then Tonga. The wind regime would favor eastward displacement along the northern routes (Fig. 1 and SI Data and Methods).

The eastward distance required in all shortest-hop crossings toward Samoa from the Solomon Islands can be covered by downwind sailing trips making use of 3- and 5-d westerly spells (Fig. 2). Shortest-hop trajectories toward Samoa that start in Palau, move east through Micronesia, and then southeast along Kiribati and Tuvalu contain fewer challenging crossings (long distance compared with wind easting) than trajectories that reach Samoa through West Polynesia (Fig. 2 and Fig. S3), though there is no archaeological evidence to support this advance.

Crossings to the Marquesas and Society Islands.

In all shortest-hop trajectories, movement from West Polynesia into East Polynesia occurs through Samoa (Fig. 1). Several components of our analyses point to the relative difficulty of moving eastward from that area. All shortest-hop trajectories encounter above-average easting distance for crossings from Samoa toward East Polynesia (Fig. 2 and Fig. S3). For present-day sea level, required crossings span ∼400 km, and though this is only 11% larger than the longest Micronesia crossing and ∼50% shorter than the Tikopia–Rotuma hop in West Polynesia, these take place in a region where westerly winds are less frequent than in areas to the west (Fig. 1 and Fig. S4). Downwind sailing simulations did not generate successful crossings in East Polynesia where viable trips only occurred in directed sailing experiments with minimum headway speeds from 1 to 3 kn. However, it seems unlikely that such speeds could be maintained over long distances, largely to windward, or that the target islands could be reached so directly. Under a minimum headway speed of 1 kn, the Marquesas and the Society Islands could be reached from Samoa in trips started during several months. Crossings to the Marquesas generally show higher success rates. For both target island groups, voyages started in April show the largest probability of success and shortest duration (Tables S4 and S5 and Fig. S1). El Niño conditions decrease precipitation over Samoa and make eastward travel easier, particularly toward the Marquesas (Fig. 3 and Figs. S5 and S6).

Crossings to Hawai’i, Easter Island, and New Zealand.

Hawai’i can be reached by directed voyages from both the Marquesas and Society Islands with most successful crossings taking place during the NH fall. Trips from the Marquesas are ∼60% faster, but require more efficient windward sailing capabilities (Tables S4 and S5). La Niña conditions result in drier conditions for the Society Islands. No large change in precipitation is observed over the Marquesas under La Niña conditions, but movement to the northwest toward Hawai’i, especially during the NH fall and in the area north of the equator, is easier than usual due to wind anomalies (Fig. 3 and Fig. S6).

Simulations indicate that Mangareva is a more likely source area for the settlement of Easter Island than the Marquesas. Mangareva crossings have more viable start months (three instead of two), higher probability of success (92–45% vs. 57–6%), and shorter average duration (65–47 d compared with 109–92 d) (Tables S4 and S5). Both crossings occur in an area that experiences northwesterly wind anomalies during El Niño SH winters, which would favor movement toward Easter Island. Directed voyages from the Society Islands to Easter Island are only successful for January departures with a 13% probability of occurrence. El Niño wind anomalies are small and do not clearly facilitate the Society Islands to Easter Island voyage (Fig. 3 and Figs. S5 and S6).

Simulations show that voyages from the Society Islands to New Zealand can occur throughout the year, with a larger probability of success and shorter travel times for trips started between November and March (Tables S4 and S5). During La Niña years, the area near the Society Islands is drier and wind anomalies favor movement to the southwest toward New Zealand (Fig. 3 and Figs. S5 and S6). Paleoclimate reconstructions point to decadal scale wind anomalies between AD 1100 and 1300 that would have facilitated downwind sailing voyages from East Polynesia to New Zealand (22).