Earliest voyages of Polynesian settlement written in current-day Pacific Islander genomes
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Rapa Nui, or Easter Island, is one of many to have massive stone sculptures. Exactly when these islands were settled is still being solved. (Getty Images: David Madison)
The vast ocean voyages of the first people to set foot on Polynesian islands have been teased out of present-day genomes, not only showing where those founder groups travelled, but when they set sail, too.
A study published in Nature also found the groups of people who produced intricately carved, massive stone sculptures, located on Polynesian islands some thousands of kilometres apart, were closely genetically related.
Alexander Ioannidis, a computational geneticist at Stanford University and co-author of the study, said the analysis reinforced the theory that a group left Samoa for the largest of the Cook Islands, Rarotonga, around the year 830.
There they stayed for more than 200 years before groups set sail eastwards, and in just a few centuries, island-hopped across the Pacific to eventually reach Rapa Nui, or Easter Island, some 5,000 kilometres away.
Studying present-day genomes, Dr Ioannidis says, can complement geological and archaeological evidence, and help fill in gaps in the region’s history.
“One of the limitations of the archaeological record is that if you’re trying to date when an island was settled, you need to find the oldest site and the oldest artifact, and there’s no guarantee that you’ve found it,” he said.
“If you’re looking at genomes of islanders themselves, those genomes pass through their entire history.”
But while some of the genetic results were consistent with previous studies, others were at odds.
How to extract history from biology
Polynesia comprises islands scattered across the Pacific Ocean in a rough triangle, with Hawai’i, Rapa Nui and Aotearoa, or New Zealand, as the points.
Some island groups are hundreds of kilometres from their nearest neighbours, but historians and oral traditions tell of Polynesian people traversing these vast distances, often in family groups of 30 to 200, in double-hulled canoes like this vessel unearthed on the New Zealand coast.
To do this required incredibly good navigation skills, using celestial cues such as the stars, and currents and swell patterns, to guide canoes across vast tracts of ocean.
Navigation was also at the core of Polynesian spirituality, said Ian Goodwin, an adjunct climate researcher at the University of Western Australia and Macquarie University, who wasn’t involved in the study.
“The head navigator was considered to be a priest,” he said.
As people progressively sailed to new islands and made them home, they left clues in archaeological remains, such as tools and sculptures.
What they didn’t leave much of was DNA — heat and humidity help break DNA apart, and sandy islands don’t preserve remains too well either.
Searching for rare traits
Dr Ioannidis and his colleagues wondered if they could tap into present-day genomes and trace when and where the first Polynesian settlers arrived at each island.
“With modern samples, if you have the right computational techniques, you can extract ancestry of interest, and then you suddenly have this huge extra power of having lots of samples,” Dr Ioannidis said.
“It lets us do really interesting historical work without needing ancient genomes.”
Their idea was to use ancestry algorithms to search for rare traits hidden in the genome of Polynesian people living today.
It’s a computational technique based on the idea of “genetic bottlenecking”. If a small group of people set sail and settled on a new island, any rare genetic traits they had — say, one that caused their fingernails to grow faster — were passed onto the next generation.
Then, if another group split off from that first group to settle yet another island, they would take that fast-growing fingernail trait with them, as well as other unique traits.
Dr Ioannidis and his colleagues hoped to trace those earliest migration routes by mapping rare traits in present-day Polynesian people, such as those that increase their risk of developing certain conditions and diseases.
They recruited 430 volunteers from 21 Pacific Island populations, and their genome — that is, their complete set of genetic information or DNA — was sequenced.
After stripping away sections of DNA from, for instance, European colonisation, Dr Ioannidis and his crew examined 600,000 individual sites in each genome and looked for rare traits encoded in the DNA.
“From that, we can tell how long ago those [people currently living on] two islands … were the same population living on the same island, before one population left and went and settled a new island,” he explained.
Polynesian voyages between islands
Dr Ioannidis and his fellow researchers found migration didn’t kick off from Rarotonga, the largest of the Cook Islands, until the year 1050 — more than 200 years after the first populations arrived from Samoa.
From there, it was a pretty rapid expansion to Tahiti and the rest of the Tōtaiete mā (Society Islands), the Tuhaʻa Pae (Austral Islands) to the south and the Te Henua ʻEnana (Marquesas) to the north, all the way to Rapa Nui in the east.
One way to explain this flurry of migration, Dr Ioannidis said, was a slight drop in sea level.
Many Polynesian islands, such as some of the Tuāmotu Islands, are coral atolls — very low islands, essentially sand bars on coral reefs, that peek above the waves.
As sea level drops, it exposes some of these atolls.
“The Tuāmotu Islands are believed to have arisen around 950 AD,” Dr Ioannidis said.
“If you imagine that it took a while for vegetation to solidify on these new islands, the migrations happened right about the time that they became inhabitable.
“And it’s not just the Tuāmotus — all of these low-lying atolls, and there’s several intermediate islands in their path.”
Towards the end of these initial migrations, islands known for their stone statues were settled: Nuku Hiva and Fatu Hiva in the North and South Marquesas, Rapa Nui and Raivavae, which is part of the Tuhaʻa Pae (Austral Islands).
“The four island groups that have these megalithic statues are all also most closely genetically related … even though they’re really in different geographical locations,” Dr Ioannidis said.
Conclusions have ‘some inconsistencies’
Some of the study’s conclusions are in line with previous work on reconstructing Polynesian migration history, Patrick Kirch, an anthropologist at the University of Hawai’i, wrote in an accompanying News and Views article.
But Professor Kirch noted “there are some inconsistencies” between the new study and others, such as those that trace how languages diverged and different dialects emerged as populations settled on different islands.
These studies suggest there was quite a bit of contact between islands during those eastward migrations, whereas the migration sequence proposed by the new study suggests there was very little inter-island contact.
Another inconsistency concerns when the island of Rapa Nui was settled.
Through genome research, Dr Ioannidis and his colleagues dated Rapa Nui’s settlement in the year 1210.
But this is somewhat later than dates suggested by archaeological and geological evidence.
“The geological evidence [for settlement] from sediments on Rapa Nui is earlier than 1210,” Dr Goodwin said.
“Much of it most closely aligns with the late 1000s and early 1100s.”
An earlier arrival to Rapa Nui and some of the other outermost Polynesian islands is also consistent with wind patterns of the time.
In a 2014 study, Dr Goodwin and colleagues found the Pacific winds that blow to the west today actually shifted in the other direction around the year 1100.
Polynesian canoes couldn’t sail against the wind. But a wind shift from 1100 allowed groups to set sail eastwards and, carried downwind, they could reach distant islands within a couple of weeks.
Those shifting winds also likely gave early Polynesian people a very good reason to set sail: to find fresh water, Dr Goodwin said.
Winds blowing to the east meant there were fewer cyclones, which made sailing in the open ocean safer, but it also meant less rainfall.
Less rain meant more drought, forcing groups to go looking for new islands to inhabit.
“You have to ask, why would a group of humans decide to voyage into the unknown?” Dr Goodwin said.
“That’s why understanding the climate is so important — they were looking for more reliable water on lots of these islands.”
Around the early to mid 1100s, Dr Goodwin said, “they’d gone as far east as they could go and then the weather changed, and there were really strong trade winds [pushing west again]”.
That’s when Hawai’i and Aotearoa (New Zealand) were settled, between the years 1140 and 1290.
“By the early- to mid-1100s, the climate window for eastward voyaging shuts, and they start exploring back in the other direction,” Dr Goodwin said.
“That’s when we see all the action heading towards New Zealand.”
Tracing traits could improve medical care
Despite these discrepancies in migration patterns, ancestry algorithms do have another use.
Dr Ioannidis hopes identifying rare traits — many of which cause disease — could help provide more personalised medical care.
“If you want to be able to provide useful personalised genetic health results to people on those islands, it’s not good enough just to say, ‘are you a Pacific Islander?'” he said.
“This is not a uniform population. And if you really want to do good, personalised health, you need to zoom into each island.”
Dr Ioannidis and his colleagues are also using ancestry algorithms to identify parts of the genome that are associated with, for instance, higher COVID-19 severity risk.
“So it’s an important thing to be studying these populations to give them good health results, because there’s going to be some unique traits and risk factors that they have,” he said.
Source: Thanks msn.com