18．
The orangutan （红毛猩猩），the most inactive of the great apes，has unusually stable DNA，too．Researchers have just completed the sequencing （序列） of the entire genome （基因组） of our orange-haired relative，and they have found to their surprise that its DNA has changed much less dramatically over time than has that of humans or chimpanzees．"The orangutan is very unique，"says Devin Locke，a structural geneticist heading the orangutan sequencing project．
The orangutan genome had one other big surprise．Locke and colleagues sequenced six Sumatran and five Bornean orangutans，which are classified as different species．The apes have been physically separated for at least 21，000years-the last time land bridges between the two islands existed-and earlier studies estimated that they became distinct species more than 1million years ago．But the new analysis，reported online today in Nature，rewrites history：it appears they parted ways just 400，000years ago．"Most previous studies used small sets of markers and a limited amount of DNA sequence，"says Locke．"The statistical power is so much greater when you have the whole genome available．"
The orangutan now joins chimpanzees and humans as the third great ape to have its genome sequenced．"The orangutan genome is a wonderful resource，"says evolutionary geneticist Svante P??bo of the Max Planck Institute for Evolutionary Anthropology in Leipzig，Germany．"It will help clarify how each part of human and African ape genomes are related to each other and evolved．"
Such insights are already coming in．Orangutans originated some 12million to 16million years ago，giving their genomes much more time to evolve than those of humans and chimpanzees，which split into their own lineages （血统） 5million to 6million years ago．But a comparison of the three genomes shows that humans and chimpanzees lose or gain new genes at twice the rate of orangutans．
The reason may have to do with stretches of DNA called retrotransposons．These key drivers of evolution jump around the genome，creating new genes，damaging existing ones，or altering gene regulation．The new data reveal that common retrotransposons known as Alu elements have moved around the orangutan genome much less than they have in the human and chimpanzee genomes．"I don't want to say that‘Alu retrotransposition events'are shut off in orangutans，but they've been covered up，"says Locke．
The researchers also discovered that，over time，the structure of orangutan chromosomes （染色体） has changed little，which may be linked to the Alu element finding．Other researchers have suggested that the strong and healthy structural variation in humans and chimps may have stimulated increased intelligence．But Locke notes that orangutans are also highly intelligent．"If orangutans have had very little structural variation，maybe this decouples structural variation from intelligence，"he says．
A separate but related study published today in Genome Research reports yet another unexpected finding from a comparison of the three great ape genomes．A team led by Mikkel H．Schierup and Thomas Mailund of Aarhus University in Denmark （both co-authors of the Nature report） discovered that some regions of the human genome more closely resemble the orangutan than the chimpanzee．This reflects the fact that at the time humans split off from a common ancestor with chimps，both species had the same ancestral orangutan DNA．But humans and chimpanzees have evolved separately for millions of years．In the process，chimps for mysterious reasons lost some orangutan DNA that humans kept possession of．
More surprises are sure to come as researchers compare the genomes of even more apes．Projects to sequence the other two great apes，gorillas and bonobos，are under way．
74．Orangutans from two islands，Sumatran and Bornean，became different species sinceD．
A．at least 21，000years ago
B．over 1million years ago
C．some 12million to 16million years ago
D.400，000years ago
75．Which of the following statements is TRUE according to this passage？B
A．Compared with the DNA of orangutan，chimpanzee's is less changed over time．
B．Only three apes'genomes have been sequenced up to now．
C．Humans'ancestors stepped on their way of evolution 12to 16millions years ago．
D．Chimpanzees gain new genes faster than orangutans do in evolution．
76．The reason of orangutan's little change in DNA is thatB．
A．this species is not as active in intelligence as humans or chimpanzees．
B．the newly identified Alu elements is believed to have played a special role
C．Alu retrotranspositions in orangutan are shut off during the course of evolution
D．orangutan's low intelligence fails to stimulate the change of its DNA
77．Some region of human genome is more similar to that of orangutan than chimp's becauseC．
A．human and orangutan share the same ancestor，but chimp doesn't
B．humans and chimpanzees have evolved separately for millions of years
C．chimps failed to hang on to orangutan DNA for some unknown reason，but human didn't
D．chimps didn't act as actively as humans，resulting in their losing some critical orangutan DNA．
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