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September 19, 2018

The Insight Show Notes — Season 2, Episode 2: The Greatest Human Journey

Filed under: Genetics,hawaii,Podcast,science — Razib Khan @ 8:10 pm

This week on The Insight (Apple Podcasts, Stitcher and Google Play) we touched upon arguably one of the greatest human journeys of humankind, the expansion of the Polynesians across the Pacific.

Bishop Museum

Spencer discussed his visit to the Bishop Museum in Hawaii.

We discussed broadly the interesting confluence of biology, geology, and history one can see in Hawaii. The book The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life discusses the biogeographic characteristics of many islands, including Hawaii.

We discussed the context of Polynesian languages and culture as part of the broader zone of Austronesian language and culture.

The extent of Austronesian languages

Austronesian societies spread over the last 6,000 years from Taiwan to the far west in Madagascar, and far east in Easter Island. The expansion into Polynesia was prefigured by the expansion of the Lapita culture between 1500 BC and 500 BC.

The Lapita culture is defined by its unique pottery. But curiously the usage of pottery disappeared among the Polynesians, the likely later descendants of the Lapita people. Razib mentioned how there is some evidence that cultural bottlenecks and small populations can result in loss of skills such as pottery.

On the other hand, Spencer pointed out that the Polynesians also did not practice rice agriculture, unlike other Austronesian societies. Instead, they expanded with a cultural toolkit of taro, which likely was adopted from the peoples of Near Oceania, New Guinea, and Melanesia.

Sweet Potato

Additionally, Spencer brought up the fact that the cultivation of sweet potatoes in Polynesia likely indicates contact between Polynesians and the peoples of South America. The genomic evidence that Polynesian sweet potatoes derive from South American ones is conflicted. Spencer mentioned that the word for “sweet potato” in Quechua, the language of highland Peru, is kumar. In Hawaiian, it is ku ala.

We mentioned in passing Thor Heyerdahl’s view that there was a South American migration to Polynesia. But the genetic, cultural, and archaeological evidence does not support this.

The Polynesian mtDNA motif was mentioned. With a high frequency in Polynesia, the mtDNA lineage seems to have spread from the west, in line with the idea of a migration to the east. In contrast, the Polynesian Y chromosomes show a mix of Asian and Melanesian heritage.

Much of the arguments hinge on the argument of whether the expansion of Austronesians into the Pacific was via the “slow boat” or “express train” model. The slow boat model suggests widespread cultural and genetic mixture gradually with the Austronesian expansion through Melanesia. The express train model implies a more rapid migration with far less interaction. Culturally the adoption of taro cultivation aligns with the slow boat thesis. As does the existence of Melanesian Y chromosomes across the range of Polynesians. But the overwhelming Asian nature of Polynesian mtDNA lineages fits the express train model.

One way that scholars have reconciled this is that there was a slow expansion of the Lapita people, but that they only assimilated Papuan and Melanesian men into their matrilineal communities. This broad framework was reinforced with the publication of genetic results from native Hawaiians, which showed a minority ancestry from a Papuan-like population.

But wait, there was a twist! Ancient DNA now shows that the Lapita people had almost no admixture with Melanesian people! Follow-up results from Vanuatu and Tonga confirm that the Lapita people had no admixture from Melanesians. Rather, in Vanuatu 2,500 years ago the Lapita people are replaced by an almost entirely Melanesian population, and the Melanesian ancestry begins to show up in Polynesians after this period. The conclusion then is there were multiple migrations into Polynesia!

Spencer and I concluded that the broad sketch is now established, but a lot of complicated details need to be worked out. Instead of express trains or slow boats, some researchers now wonder if Polynesia was more like a subway network.

Interested in learning where your ancestors came from? Check out Regional Ancestry by Insitome to discover various regional migration stories and more!

The Insight Show Notes — Season 2, Episode 2: The Greatest Human Journey was originally published in Insitome on Medium, where people are continuing the conversation by highlighting and responding to this story.

Hawaii: complicated a journey to paradise

Filed under: anthropology,Genetics,hawaii,science — Razib Khan @ 7:11 pm
The extent of Austronesian Diaspora

Ask any American what they think when you say the word “Hawaii,” and certain words will no doubt reoccur from person to person. That’s because certain images, feelings, come to mind. A gentle breeze, beaches, and volcanoes. The 50th state has been the byword for paradise on the mainland. A certain sense of Hawaii is part of American popular culture.

But Hawaii is a real place with real people. It isn’t a dreamland. Rather, it is one of the most isolated large islands in the world. Over 2,500 miles from the nearest continent, there is only a single terrestrial mammal native to the islands: predictably, a bat!

Obviously, the island is crawling with mammals today. Nearly 1,000 years ago voyagers from the lands of the western Pacific landed on the Society Islands, which includes famed Tahiti, and then sailed northward to the Hawaiian archipelago. When the ancient Polynesians settled Hawaii they did not arrive alone. They brought with them pigs, chickens, and dogs. Naturally, rats tagged along as unwanted passengers.

Humans arrived in Hawaii in catamarans

But the settlement of Hawaii by humans was the end of a long journey which began thousands of years earlier in the mists of prehistory. Six thousand years ago a small group of stone-age seafarers, who we call Austronesians, journeyed south from Taiwan and settled the northern Phillippines.

But they did not stop there. Over a period of thousands of years, these ancient mariners spread out over Southeast Asia, sometimes introducing intensive forms of rice agriculture and their distinctive language. But they did not stop there. For whatever reason, these were a people who wondered what was over the horizon, even if it was the deep blue ocean. They moved on west and east. Over 1,000 years ago their descendants reached the western Indian Ocean, mixing with the Bantu farmers of eastern Africa and occupying the island of Madagascar. In the other direction, Austronesians moved into Oceania, abandoning rice and adopting taro from Melanesians. Less than 1,000 years ago the Pacific expansion finally crested, as Polynesians settled in New Zealand, off the coast of Australia, Easter Island, 2,300 miles west of South America. And of course, they ventured north to Hawaii, an isolated ecologically rich and unique jewel in the midst of the Pacific.

In Southeast Asia, the Austronesians merged with native populations of farmers which migrated out of southern China earlier. But as they moved west and east they encountered very different populations, whether it be African farmers and pastoralists, on the one hand, or Melanesians in the case of the ancestors of the Polynesians.

Citation: Kim SK, Gignoux CR, Wall JD, Lum-Jones A, Wang H, Haiman CA, et al. (2012) Population Genetic Structure and Origins of Native Hawaiians in the Multiethnic Cohort Study. PLoS ONE 7(11): e47881

And just as the people of Madagascar, despite speaking a language closest to those spoken in Borneo, have a blended with nearby populations. Polynesians carry signatures of interactions with the peoples of Near Oceania, which includes New Guinea, Australia, and Melanesian islands in the western Pacific, such as the Solomon Islands and New Caledonia.

As genomics began to illuminate all the relationships between human populations, in 2012 a paper was published that surveyed the genomes of many native Hawaiians. The results were clear: the indigenous peoples of Hawaii had a dominant signature of ancestry shared with mainland Asian peoples, but also a minority component that had more affinities with the peoples of Near Oceania.

Lapita culture sites

This result was relevant to what traditionally had been termed the “express train vs. slow boat” models of the settlement of Polynesia. The “express train” hypothesis implies that the Austronesian Lapita culture rapidly pushed out of maritime Southeast Asia, with minimal interaction with local Papuans and other Melanesians. In contrast, the “slow boat” model meant that the expanding proto-Polynesians mixed with Papuans and Melanesians as they spread eastward more gradually, creating a fused culture which pushed onward into the far Pacific.

The results above, along with maternal and Y chromosomal lineages seem to support the “slow boat” model. Not only are all Polynesians, including Hawaiians, descended from Southeast Asian farmers, but their ancestors also include the people who first pushed to the edge of the Pacific. These were the ancestors of Oceanians who settled New Guinea, Near Oceania, and Australia more 40,000 years ago with the first “Out of Africa” migration.

Citation: Skoglund, P., Posth, C., Sirak, K., Spriggs, M., Valentin, F., Bedford, S., … & Fu, Q. (2016). Genomic insights into the peopling of the Southwest Pacific. Nature, 538(7626), 510.

So case closed? Not exactly. Science and history are often more complex than our elegant human imaginings. Over the past few years, the field of ancient DNA has come upon the scene to disturb hypotheses and provoke the development of new ones. Now researchers can see snapshots of the past with much crisper detail than would have been the case in the past.

Two papers have helped reshape our understanding of the peopling of Polynesia. First, a 2016 paper showed that samples of ancient Lapita people don’t show any admixture from Melanesians. This is in accordance with the “express train” model, which the genetic heritage of modern Polynesians presumably refuted!

An immediate solution to this conundrum is that the old models were too simple. That there wasn’t just a simple migration outward, but rather several, and that Melanesian ancestry arrived later. Within the last 2,000 years.

A paper published in 2018 added more nuance and clarity to what may have been going on. Today the island of Vanuatu is considered to be Melanesian and is settled by people of predominant Oceanian heritage. But ancient DNA from 3,000 years ago yielded individuals of nearly total Asian heritage. But by about 2,000 years ago these people were replaced, by the ancestors of modern Melanesians, as later samples show overwhelming Oceanian heritage.

Poke is a melange of flavors and ingredients from the four corners of the world

Where does this leave us? Appropriately, a paper appeared with the title “Human Genetics: Busy Subway Networks in Remote Oceania?” was penned as a response to all this uncertainty and confusion. The title says it all, doesn’t it?

These findings may actually be consonant with recent archaeological results that eastern Polynesia and New Zealand were subject to a massive demographic expansion and radiation beginning around ~1,000 years ago.

Today modern Hawaii is a melange of peoples, reflected in its cuisines, such as Poke, which has been inflected and modified by new ingredients brought by immigrants from the mainland and Asia. And yet perhaps this was always so, as paradise was never as serene and eternal as we may dream in our imaginings. Rather, Hawaii and the Hawaiians were products of daring voyages generation after generation, and the waxing and waning of peoples and cultures, bringing together diverse and disparate threads of the human expansion out of Africa.

Interested in learning where your ancestors came from? Check out Regional Ancestry by Insitome to discover various regional migration stories and more!

Hawaii: complicated a journey to paradise was originally published in Insitome on Medium, where people are continuing the conversation by highlighting and responding to this story.

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