The greatest migration of humanity was the spread of the Polynesian culture to the Pacific. The evidence suggests that the Polynesians originated in Taiwan, and traveled to the Americas. They also settled on various islands throughout the journey, including New Zealand to Hawaii. Many of these islands maintained trade routes for centuries, even though they were small and hard to find in the vast Pacific.
It has been difficult to reconstruct the Polynesians' route. In the hot, humid environment of the tropics, very little ancient DNA survived. Although artifacts can be dated, it is not clear how closely they are related to the arrival of a population on an island. They also don't often indicate where this population originated. Postcolonial travel has made it difficult to find the genetics and linguistic evidence necessary to sort out these issues.
A large international group of researchers has now developed a new method of analysing the genomes of modern Polynesians. It is based on the effects that long-term settlement events could have on genomes. It provides a map showing which islands were settled in which order. The results also provide an estimate of the date when Polynesians arrived.
A standard analysis is not applicable
It is possible to compare two genomes and make an estimation of their relative relationships. This is done by comparing the number of variations they have, and the groups of variants that are sometimes inherited together on one chromosome. You can sample the members of two different populations to get a similar idea of the time they have been apart, as well as the chance that each of them will pick up new variants over time. It is possible to compensate for mixing with other populations, although it can make this analysis more difficult.
We have developed sophisticated software to analyze these genomes, given the number of them available. All of this software works because of the behavior of populations when they move between and within continents.
This analysis is based on many assumptions, but the Polynesian expansion has thrown off many of these assumptions. Polynesians settled many new islands groups in a short time span, rather than waiting for thousands of years to acquire new mutations that differentiate them. The founder effects of small settlements that created new genetic variations are responsible for most genetic differences. After settlement of new islands, trade routes kept people in touch. The colonial era ensured that most of the current population has some DNA from other parts of Polynesia.
Advertisement
This is enough to show that the methods designed for humanity won't work in understanding the Polynesian expansion. The new research team devised new methods to analyze Polynesian genomes.
Methods that match reality
The new approach is not difficult in some cases. We have enough genomes from other parts of the world at this point that it is fairly simple to identify non-Polynesian genes and exclude them from our analysis.
The remainder of the approach involves thinking through what would happen when new islands are settled. A settlement typically consists of a small group, usually a few dozen people, who are able to move away from the larger island of origin. Chances are that settlers will inherit some of the rare genetic variants found in large populations. The settlers may not have some variants that are more common in large populations.
Two key points are required to use this method to identify the process of settling Pacific islands. First, it is important to note that if one island population sends settlers to different destinations, the variants affected would differ at each destination. One settlement could lose variants A, C, and B while the other might lose variants X, Y and Z with very few overlaps.
This second key point is that these events will happen in series. There will be enough people on many settled islands to support new settlers. When that happens, all the variant changes that their ancestors made when they were settlers will be passed on to the new population. They will also inherit a new set. Let's return to the hypothetical. The island is settled by the population who lost A, B and C. It then sends out its own settlers. Although the new settlers would still be without A, B and C, they would now lose a new set of settlers by chancesay H I and J.
This allowed the researchers to determine which island's population was responsible for which settlements.
This produced patterns that you would expect. The island chains were almost always grouped together and shared an ancestry that could be traced back from a single island source. These patterns suggested that certain island groups were hubs for expansions. For example, Palliser and Rarotonga were the sources (directly or via intermediary islands) of all the Eastern Polynesia's population.
Advertisement
This exception may be instructive. All of the Austral Islands, located in south French Polynesia, were settled by one source, except Raivavae. It seems that the population arrived from Tuamotus or Mangareva islands, which are much further east in eastern French Polynesia. This is the same population that settled all of the islands where we have found Native American DNA. This suggests that Polynesians may have made some of the longest journeys by this group.
Who got there when?
Researchers also used their data for estimates of when Polynesians arrived on different islands. However, this involved an entirely different method. The idea behind this method was that each generation the segments of the chromosomes our father and mother inherit from us swap in a process called "recombination". This will result in the fragmentation of the variants that have been inherited from ancestral populations. The ancestral segments will become smaller over time.
By comparing the lengths of the ancestral segments, it is possible to estimate when different populations split. Combining the timing data with the tree from the other analysis will give you a complete picture of the date each island sent settlers.
It's not exact as trade or later contact could change the date of separation. However, the chronology produced by dating archeological findings is consistent with that of the results. It even matches the average time between human generations in cultures that have followed the ancestry and settlement of an island.
It is likely that this method's results are in line with other chronologies. However, those who prefer alternative chronologies will be keen to find flaws in them. The study provides a complete picture and gives people many opportunities to test it. The work can also be used to show that there is no universal way to understand human ancestry. It might encourage people think about other situations where more specific approaches are required.
Nature, 2021. DOI: 10.1038/s41586-021-03902-8 (About DOIs).
