Chile's Easter Island (Rapa Nui) is famous for its giant monumental statues, called moai, built by early inhabitants some 800 years ago. The islanders likely chose the statues' locations based on the availability of fresh water sources, according to a recent paper in PLOS One.

Scholars have puzzled over the moai on Easter Island for decades, pondering their cultural significance, as well as how a Stone Age culture managed to carve and transport statues weighing as much as 92 tons. They were typically mounted on platforms called ahu. According to co-author Carl Lipo, an anthropologist at Binghamton University, you can have ahu (platforms) without moai (statues) and moai without ahu, usually along the roads leading to ahu; they were likely being transported and never got to their destination.

Back in 2012, Lipo and his colleague, Terry Hunt of the University of Arizona, showed that you could transport a ten-foot, five-ton moai a few hundred yards with just 18 people and three strong ropes by employing a rocking motion. Last year Lipo proposed an intriguing hypothesis for how the islanders placed red hats on top of some moai; those can weigh up to 13 tons. He suggested the inhabitants used ropes to roll the hats up a ramp.

Now Lipo, Hunt, and several colleagues have turned their attention to the question of why certain locations were chosen for the statues and platforms. If their purpose was symbolic or ritualistic, one would expect them to be placed prominently, on hilltops, for instance, where everyone could see them. But instead they're located along the coast, and they're not evenly distributed. Over his 20+ years of field work, Lipo noticed there were places that he felt should have been ideal locations for ahu, yet there weren't any at those sites. Clearly there was some other reason the early islanders chose particular sites for these monuments.

While working in Hawaii with a hydrogeologist, Lipo realized that fresh water is such a precious resource on an island, it can't help but have an impact on where people settle—and where they might place their statuary. The volcanic islands of Hawaii have an unusual feature, where fresh water flows down through the volcanic tubes into the ocean. "There's actually places offshore where freshwater will be flowing," said Lipo. "Fishermen will know you could scoop water right from this spot in the ocean and it will be fresh."

Fresh water is also a limited resource on Easter Island. With his graduate student, co-author Tonya Broadman, Lipo decided to investigate whether the same thing might be happening there and discovered it was, based on the conductivity measurements they made of how salty the water was along the coasts. "At low tide, when the saltwater's down, fresh water pours right out at the coast," he said.

Evidence from historical accounts of European visitors to the island verified this was also true at the time the early inhabitants lived.

Lipo and Broadman meticulously mapped out where those fresh water sources were located all around the island, and wherever they found fresh water pockets along the coast, they also found ahu. (Fresh water also pools in craters on the island, forming lakes, but the archaeological evidence didn't support houses and villages in those areas.) They used a technique called quantitative spatial modeling to demonstrate that the pattern they observed was statistically sound, not just a matter of human perception.

"The fresh water location is the strongest component of determining the location of ahu and moai."

They also applied their model to other natural resources to further test the hypothesis. "It's complicated, because everything is located near the coast, so we wondered, is just being near the coast the predictor?" said Lipo. But based on their statistical analysis, "the fresh water location is the strongest component of determining the location of ahu and moai, more so than any of the other factors."

"Many researchers, ourselves included, have long speculated associations between ahu/moai and different kinds of resources, [such as] water, agricultural land, areas with good marine resources, etc.," said co-author Robert DiNapoli of the University of Oregon. "However, these associations had never been quantitatively tested or shown to be statistically significant. Our study presents quantitative spatial modeling clearly showing that ahu are associated with freshwater sources in a way that they aren't associated with other resources."

According to co-author Hunt, the data collected thus far indicates that the early inhabitants of the island survived for more than 500 years by building strong communities around their limited resources and fostering a strong sharing economy. It was the arrival of European colonialists that disrupted their way of life and contributed to their social collapse. Lipo and his team are heading back to Easter Island in May for more field work, since thus far they have only collected comprehensive freshwater data for the western portion of the island. A complete survey will hopefully shed even more light on why the inhabitants of Rapa Nui invested so much time and effort into building the ahu and moai.

"We may never get a [definitive] answer, because people had a lot of reasons for making statues," said Lipo. "It seems counter-intuitive to us. But once we start to see the constraints of the resources, we start to see that communities are tied to those local resources. The people survived by cooperation, and the statues served to mark their communities."

DOI: PLOS One, 2019. 10.1371/journal.pone.0210409 (About DOIs).