享受这个长阅读中的obstacle

这篇文章中有个点，觉得逻辑很模糊，请指教

Rats caught a free ride to New Zealand when they hopped aboard the boats of early Polynesian explorers. Now, their ancient bones may help pinpoint when humans first set foot on the island. Carbon-dating of bones from the rodents indicates that people reached New Zealand around 1280 or later, rejecting previous research that suggested humans may have landed there more than 1400 years earlier. Although most anthropologists think that humans first arrived in New Zealand around 1250 to 1300, a minority holds that people might have set foot on the island as early as 200 B.C.E. That conclusion is based on 1996 research that carbon-dated bones of rats, which are thought to have been brought to New Zealand by humans either as stowaways or for food. But this study has been controversial because there's no evidence of human settlements at that time. Some critics have suggested that the carbon dates were due to a lab error in preparing the bones. To help clear up the confusion, a team led by Janet Wilmshurst, a paleoecologist at environmental research organization Landcare Research in Lincoln, New Zealand, used a different preparation technique that is thought to be more accurate. The researchers obtained 17 bones from the two excavation sites where the oldest rat remains had been found. Carbon-dating with the improved method indicated that the new bones were from 1280 or later. When the researchers tried the new technique on some of the bones from the previous study, all of them dated to later than 1280, indicating that the earlier research was flawed. The researchers next carbon-dated ancient seeds that the rats had gnawed and that came from one of the excavation sites. The results gave a date of 1290 or later, confirming that humans did not arrive until 1280 at the earliest, the researchers report in the 3 June issue of the Proceedings of the National Academy of Sciences. Ian Smith, an anthropologist at the University of Otago in New Zealand, says the finding "provides convincing evidence against the assertion that either rats or people reached New Zealand prior to the 13th century A.D." He adds that the later arrival indicates that humans' devastating impact on New Zealand, which has included deforestation and the extinction of birds and marine mammals, happened in only 600 years, versus more than 2000 years if the initial bone dating had been confirmed. David Lowe, a soil scientist at the University of Waikato in Hamilton, New Zealand, says the findings also indicate that "the destruction caused by the rats in New Zealand has been pronounced and very fast indeed." The rats wiped out several species, including some birds and frogs. Wilmshurst adds that the speed of destruction "makes the risk to currently declining populations of rat-sensitive species more pressing as they could be diminishing faster than previously assumed."