South Americans helped colonise Easter Island centuries before Europeans reached it. Clear genetic evidence has, for the first time, given support to elements of this controversial theory showing that while the remote island was mostly colonised from the west, there was also some influx of people from the Americas.
Easter Island is the easternmost island of Polynesia, the scattering of islands that stretches across the Pacific. It is also one of the most remote inhabited islands in the world.
So how did it come to be inhabited in the first place? Genetics, archaeology and linguistics all show that as a whole, Polynesia was colonised from Asia, probably from around Taiwan. The various lines of evidence suggest people began migrating east around 5500 years ago, reached Polynesia 2500 years later, before finally gaining Easter Island after another 1500 years.
But the Norwegian adventurer Thor Heyerdahl thought otherwise. In the mid-20th century, he claimed that the famous Easter Island statues were similar to those at Tiahuanaco at Lake Titicaca in Bolivia, so people from South America must have travelled west across the Pacific to Polynesia. His famous Kon-Tiki expedition, in which he sailed a balsa wood raft from Peru to the Tuamotu islands of French Polynesia, showed that the trip could have been made. But if it was made, no trace remained.
Now Erik Thorsby of the University of Oslo in Norway has found clear evidence to support elements of Heyerdahl's hypothesis. In 1971 and 2008 he collected blood samples from Easter Islanders whose ancestors had not interbred with Europeans and other visitors to the island.
Thorsby looked at the HLA genes, which vary greatly from person to person. Most of the islanders' HLA genes were Polynesian, but a few of them also carried HLA genes only previously found in Native American populations.
Because most of Thorsby's volunteers came from one extended family, he was able to work out when the HLA genes entered their lineage. The most probable first known carrier was a woman named Maria Aquala, born in 1846. Crucially, that was before the slave traders arrived in the 1860s and began interbreeding with the islanders.
But the genes may have been around for longer than that. Thorsby found that in some cases the Polynesian and American HLA genes were shuffled together, the result of a process known "recombination". This is rare in HLA genes, meaning the American genes would need to be around for a certain amount of time for it to happen. Thorsby can't put a precise date on it, but says it is likely that Americans reached Easter Island before it was "discovered" by Europeans in 1722.
Thorsby says there may have been a Kon-Tiki-style voyage from South America to Polynesia. Alternatively, Polynesians may have travelled east to South America, and then returned. There is already evidence for that: chicken bones found in Chile turned out to be Polynesian, so we know that the eastward journey did happen at some stage.
However, Thorsby's findings don't mean that Heyerdahl's ideas have been vindicated. The first settlers to Polynesia came from Asia, and they made the biggest contribution to the population. "Heyerdahl was wrong," Thorsby says, "but not completely."
The work was presented at a Royal Society discussion meeting on human evolution in London today.