A 2007 paper published in "PNAS" put forward DNA and archaeological evidence that domesticated chickens had been introduced into South America via Polynesia by late pre-Columbian times. These findings were challenged by a later study published in the same journal, that cast doubt on the dating calibration used and presented alternative mtDNA analyses that disagreed with a Polynesian genetic origin. The origin and dating remains an open issue. Whether or not early Polynesian–American exchanges occurred, no compelling human-genetic, archaeological, cultural or linguistic legacy of such contact has turned up.