Human sacrifices, uranium, and corals


Marae Manunu, Society Islands. Image by Veromortillet. Creactive Commons license (Attribution-Sharealike 3.0 Unported)

The development of shrines and temple architecture associated with chiefdoms and early states is thought to be a slow process.  In Mesoamerica, a sequence of architectural evolution took 1300 years, according to archaeological evidence.  However, this may not always be the case.  Researchers from Berkeley and Honolulu have found rapid changes in ritualistic temple and shrine building in Mo’orea (central Polynesia) within the Ma’ohi chiefdoms over less than 200 years.  The socio-political structure of the Ma’ohi revolved around activities conducted at marae (temples).  The scale of the marae reflected the power and importance of its chief.  These were not small affairs; when first contacted by Europeans in CE 1767, the chiefdoms of the Ma’ohi were estimated to be more than 200,000 people. At major temples, human sacrifices were performed as offerings to the war god ‘Oro.

An integral idea within the anthropology here is that changes in the ritual temple architecture can provide clues as to how the Ma’ohi devolved socially and politically over time.

Past attempts to date the construction of the murae have incorporated radiocarbon (14C), but this approach yielded was uncertain, as only organic materials could be dated. For example, charcoal or offering remains that could be dated do not necessarily reveal the true age of the structure—these events could have occurred much later. The researchers here (Sharpe et al. 2010) decided to date the building material itself. The Ma’ohi used corals (Porites spp. and Acropora spp.) in the building of the ahu (the altar platform) within the temples. Assuming that coral colonies were collected while they were living and immediately incorporated into the ahu, the date of the coral’s final growth would reflect the approximate age of the temple. Sharpe et al. employed uranium-thorium dating.  Corals incorporate uranium—which is present in seawater—into their skeletons as they grow.  After the coral dies and stops growing, uranium converts to thorium as it radioactively decays (loses energy and emits radiation).  By working out the half-lives of the specific isotopes of the elements involved, ages can be assigned.

Acropora pulchra. Albert Kok, Wikipedia

20 marae were dated, ranging from CE 1617 to 1761. Older marae are surely present; however, it is only in this time range that corals were incorporated.  By examining temple characteristics along with the known dates, an architectural progression was clearly present.  Early temples incorporated Acropora corals and had a low ahu.  Later structures showcased a raised platform ahu with basalt and cut Porites corals, eventually showing stepped ahu platforms. The marae themselves also increased in size.

These changes, over roughly 140 years, occurred just before contact with Europeans in CE 1767, and are thought to be associated with the ascent of the ‘Oro war cult, which involved human sacrifices for ‘Oro to be kept happy. The marae kept increasing in size, probably related to the ongoing war between Mo’orea and Tahiti, indicating how important they were for the chiefs to assert their power, likely by gaining the gods’ favor. This shows just how quickly architecture can be changed in light of changing political—an ongoing war—and social culture –a new god who demands blood.


Sharp, W., Kahn, J., Polito, C., & Kirch, P. (2010). Rapid evolution of ritual architecture in central Polynesia indicated by precise 230Th/U coral dating Proceedings of the National Academy of Sciences, 107 (30), 13234-13239 DOI: 10.1073/pnas.1005063107


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