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Journal Club: Ancient diet helps tell the story of Easter Island

By characterizing the diet of the mysterious ancient people Easter Island, researchers shed light on whether this extinct population was more capable of adapting to the island than some theories have suggested.  Image: Shutterstock/Amy Nichole Harris

By characterizing the diet of the mysterious ancient people of Easter Island, researchers shed light on whether this extinct population was more capable of adapting to their environs than some theories have suggested.
Image: Shutterstock/Amy Nichole Harris

Famous for its towering stone human figures, Easter Island, or Rapa Nui, has long been shrouded in mystery, with the demise of its people the subject of an intense debate. Now, new findings about what its residents ate hundreds of years ago are challenging commonly held views about the society and its collapse. The results were recently reported in the American Journal of Physical Anthropology.

Eking out a living on Rapa Nui was challenging, according to study author and isotope biogeochemist Brian Popp of the University of Hawai’i at Mānoa. Very little was easily accessible. “It took a lot of effort to grow crops, to fish,” he says.

One controversial view, popularized by Jared Diamond in his book Collapse, suggests that the island people failed to survive because they overharvested trees and drove animals and plants to extinction. By characterizing the diet of the people of Rapa Nui, Popp and his team hoped to reveal whether this population was in fact more capable of adapting to the island than this narrative suggests. The evidence implies that they may well have been.

The team first extracted collagen from the remains of humans, rats, birds, fish, and marine mammals that lived on Rapa Nui around 1400 AD. In hopes of ultimately clarifying the human diet, the researchers then analyzed the isotopic composition of individual amino acids that make up the protein in this collagen for clues about what each organism ate.

In one analysis, the team focused on nitrogen isotope composition. For some amino acids, the proportion of the heavier isotope, 15N, relative to the lighter isotope, 14N, remains mostly unchanged as the amino acid is passed, for example, from a plant to a fish to a bird. Other amino acids, however, lose a disproportionate number of the lighter isotope each time they get metabolized by a new organism. For each sample, the team compared the 15N/14N ratio in the amino acid phenylalanine, which maintains its isotope composition, with that of glutamic acid, which does not. The bigger the difference in ratios, the higher the researchers placed the organism on the food chain.

In another analysis, the team studied carbon isotopes. Plants, algae, and bacteria all contain unique ratios of the heavier carbon isotope 13C to the lighter 12C for each essential amino acid. Because this ratio remains largely unchanged, the researchers used it as a “fingerprint” to document the kinds of foods that higher organisms ate. “The fish obtained a fingerprint of the plants they ate and they transferred that on to humans,” explains Popp. These techniques, then, allowed researchers to glean inhabitants’ diet in unprecedented detail.

The results showed that seafood made up about half of the protein in the humans’ diets, suggesting that they may have been more active anglers than some previous studies showed. The findings also suggest that the inhabitants ate nitrogen enriched plants that could only have been grown by people with a deep knowledge of how to manage soil fertility—a finding that implies better farming skills than some research had previously suggested, and hence challenges the view that the civilization collapsed because the people couldn’t manage their own resources. The discovery also aligns with abundant archaeological evidence that the islanders built agricultural structures, including rock mulch gardens—piles of rocks placed around crops that trapped moisture, leached nutrients, and offered wind protection. It’s possible that the people fertilized their land with food scraps or bird guano, says Popp.

However, the present study only focused on a narrow window of time. Amy Commendador, a biological sciences PhD student at Idaho State University who’s also researched Easter Island diet, would like to see Popp’s technique applied to different time points to create an even fuller picture of exactly what transpired on Rapa Nui. In a previous study, Commendador employed a different form of stable isotope analysis, and found evidence suggesting a primarily land-based diet made up of rats, chickens and plants. Commendador says Popp’s technique “is a more recent and potentially really viable tool for trying to get a little bit more accurate in those interpretations.”

“This is a very good way to look at diet,” says archaeologist Christopher Stevenson of Virginia Commonwealth University, who researches ancient farming strategies on Rapa Nui and was not involved in the study. His working hypothesis has been that starches like sweet potato, yam, and taro made up the vast majority of the diet. He says this new evidence that the people relied heavily on seafood challenges that perspective. “It will make us rethink our research questions going forward.”

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