Racial migrations and human genetics: The “game changer” in the South Pacific that wasn’t – part 2

John Edward Terrell and Kevin M. Kelly

This is part 2 of a 3 part commentary

Necessary, plausible, and sufficient

Nobody, as far as we know, has come up with a universally accepted checklist of what makes a scientific hypothesis about anything something worth paying attention to. There are three criteria, however, that strike us as items that ought to be on such a checklist. Here is how we see these three applying to the conclusions now being made about the biological origins of the Polynesians.

Visualization by David Eccles of the two popularly assumed racial migrations from Asia out into the Pacific. Source: https://commons.wikimedia.org/wiki/File:Polynesian_Migration.svg
  1. Necessity: What needs to be explained? Both of the hypotheses weighed by the 31 contributors to the paper in Nature (Skoglund et al. 2016) under discussion here are alternative ways of trying to understand certain widely accepted observations about islanders in the Pacific: (a) people in Polynesia speak languages assigned by linguists to the Austronesian (Malayo-Polynesian) family, as do many people in Melanesia and Island Southeast Asia; (b) archaeologists now generally agree that what they have labeled the “Lapita cultural complex”* dating to ca 3300–2800 cal BP (Specht et al. 2016) exhibits a mix of cultural traits, some local to Melanesia and others apparently having roots to the west in Island Southeast Asia (Specht et al. 2014); and (c) the Lapita skulls found in Vanuatu and Tonga are morphologically and genetically sui generis (as the authors of this paper note, in some respects these four individuals are unique unto themselves).
  2. Plausibility: The two hypotheses considered by this consortium of scholars differ in their plausibility. (a) The idea that people traveled directly from Taiwan to Vanuatu and Tonga is basically impossible to assess given that nothing is said about how they might have done so—a striking omission considering the major dimensions of space and time involved. (b) The second hypothesis put on the table is similarly deficient, but it at least acknowledges that the set of material culture traits associated with the four Lapita skeletons in Vanuatu and Tonga wasn’t  imported in toto direct from Taiwan.
  3. Sufficiency: As Richard Levins observed years ago, truth is the intersection of independent lies. (a) Not only are the two hypotheses considered by this consortium of authors basically left unspecified, but (b) no reason is given for limiting the field of possible hypotheses solely to the two considered by these contributors.
The problem of equifinality

In light of #2 and #3 just noted, consider the old cliché “there is more than one way to skin a cat.” If you are a feline lover, there is even another way of saying more or less the same thing. As the biologist Ludwig von Bertalanffy made famous in the last century, you can call it equifinality. However phrased, it is wise to remember there is usually more than one way to get from A to Z, or even just A to B. The corollary relevant to the present discussion is that one cannot just assert that B came from A without offering a sufficient explanation for how that would have been possible. And more to the point, granting for the sake of discussion that B did somehow come from A, the scientific way of doing the job that needs to be done entails offering more than just 1–2 inadequately specified hypotheses.

The absence of evidence is not evidence of absence

According to these 31 authors: “our modelling indicates that Philippine populations (Kankanaey) are the closest outgroup to the First Remote Oceanians [i.e., these 4 skulls], indigenous Taiwanese (Atayal) second closest, and mainland southeast Asians such as the Dai most remote, consistent with models of population movement along a route from Taiwan to the Philippines to Near Oceania to Remote Oceania.” Maybe yes, maybe no.

Recall that only 14 of the 83 places in their modern comparative genetics sample are located in Island Southeast Asia, and none of the other 69 localities included in their analysis is in the region between the Philippines and northern New Guinea except for a single sample of 10 individuals from Sulawesi. Now look at a map of the region in question (see below).

“Figure 1 | Data from ancient and present-day populations. a, Locations of 778 present-day individuals genotyped on the Affymetrix Human Origins Array and 4 ancient individuals (red symbols).” Source: Skoglund et al. 2016: fig. 1a. Note: blue letters A and B added to the original.

Note two things, in particular. First, if it is true, as the song goes, that it’s a long, long way to Tipperary, then it is an even longer way from (A) Taiwan to (B) Tonga—more than 5,300 miles (8,500 km) in a straight line if a bee could fly that way that far. Second, notice the total lack of genetics samples from the big gap between the Philippines and the Bismarck Archipelago east of New Guinea (the few samples from New Guinea don’t count for reasons we will not go into here).

You don’t have to be a grumpy skeptic, therefore, to ask: if the four Lapita skulls from Vanuatu and Tonga look genetically most like people today in the Philippines, what about folks today, say, in the Moluccas and Halmahera off the Bird’s Head region of western New Guinea? And possibly also people living along  the north coast of New Guinea itself? Must we assume these four individuals from Vanuatu and Tonga somehow came all the way from Taiwan or the Philippines to come ashore there?[*]

Part 3: How many immigrants does it take to make a migration?

* The three skulls from Vanuatu were not found with the rest of their skeletons (Skoglund et al. 2016: supplementary notes). How they had been buried as well as their condition as skulls prior to burial suggest they had been cared for as portable heirlooms for an unknown period of time after death: “Ancient DNA was successfully obtained from three skulls from striking mortuary contexts: a jar burial containing a single skull (B17), an alignment of three skulls lying on the chest of a skeleton without a skull (B10B)”. There is a possibility that these individuals might have been long dead before their skulls arrived in Vanuatu. In contrast, with regard to the single individual from Tonga: “Ancient DNA was successfully obtained from the right petrous bone of burial SK10, a single primary interment of an adult female . . .”.


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© 2017 John Edward Terrell and Kevin M. Kelly. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. The statements and opinions expressed are those of the author(s) and do not constitute official statements or positions of the Editors and others associated with SCIENCE DIALOGUES.

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