Scientists have long puzzled over how iguanas, a group of lizards(蜥蜴) mostly found in the Americas, came to live in the isolated Pacific islands of Fiji and Tonga. Some scientists used to suppose that they must have traveled there on a raft, a journey of around 5,000 miles from South America to the islands. There are documented cases of iguanas reaching remote Caribbean islands and the Galapagos Islands on floating logs. But new research in January by Brice Noonan and Jack Sites suggested that iguanas may have simply walked to Fiji and Tonga when the islands were still a part of an ancient southern supercontinent.
The ancient supercontinent was made up of present-day Africa, Australia, Antarctica and parts of Asia. If that’s the case, the island species would need to be very old. Using “molecular (分子) clock” analysis of living iguanas’ DNA, Noonan and Sites found that, sure enough, the lineage of iguanas has been around for more than 60 million years—easily old enough to have been in the area when the islands were still connected by land bridges to Asia or Australia.
Fossils (化石) uncovered in Mongolia suggest that iguanid ancestors did once live in Asia. Though there’s currently no fossil evidence of iguanas in Australia, that doesn’t necessarily mean they were never there. “The fossil record of this continent is surprisingly poor and cannot be taken as evidence of true absence,” the authors write.
So if the iguanas simply walked to Fiji and Tonga from Asia or possibly Australia, why are they not also found on the rest of the Pacific islands? Noonan and Sites say fossil evidence suggests that iguana species did once inhabit other islands, but went extinct right around the time when humans settled in those islands. But Fiji and Tonga have a much shorter history of human presence, which may have helped the iguanas living there to escape extinction.
The researchers say that their study can’t completely rule out the rafting theory, but it does make the land bridge theory “far more reasonable than previously thought.”
小题1: What did some scientists previously believe about the iguanas?
A．They were once discovered in America.
B．They traveled by raft to Fiji and Tonga.
C．They could survive in poor living conditions.
D．They moved to Fiji and Tonga from Australia.小题2:According to Noonan and Sites, 60 million years ago ____.
A．the land of the world was a supercontinent
B．Fiji and Tonga were connected to Asia or Australia
C．Africa, Australia and America were a continent
D．iguanas walked to Fiji and Tonga from Africa小题3:The underline word “lineage” in Paragraph 2 probably refers to ____.
A．conditions in which creatures can survive
B．the change in ancient plants and animals.
C．the line of generations of an ancestor
D．the habitat of a type of an ancient animal小题4: What is the main topic of this passage?
A．The life span of animals living on the ancient supercontinent.
B．The two islands being home to several iguana species in the Pacific region.
C．The fossil evidence suggesting iguanas’ ancestors’ swimming to Fiji and Tonga
D．By raft or by land — how did iguanas reach the tiny Pacific islands?