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  3. In A short story by Ian McEwan, Reflections of a Kept Ape, a woman takes a pet chimpanzee as her lover. Although truth is often

In A short story by Ian McEwan, Reflections of a Kept Ape, a woman takes a pet chimpanzee as her lover. Although truth is often

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问题     In A short story by Ian McEwan, Reflections of a Kept Ape, a woman takes a pet chimpanzee as her lover. Although truth is often stranger than fiction, a study published this week by scientists in America demonstrates that both can be pretty odd. The research concludes that humans and chimpanzees interbred after the two species first separated, before eventually going their different ways some 5.4 million years ago. Humans are thus much more recently related to their closest relatives than was previously thought. The researchers, led by David Reich of the Broad Institute of Harvard and the Massachusetts Institute of Technology, examined the genetic record of humans and chimpanzees. The sequencing of the human genome was completed in 2001 whereas that of the chimpanzee genome was finished last autumn. The two genomes are alike, differing by only 1.2% over the course of some 3 billion pairs of the genetic "letters" in which the language of genes is written. In fact, almost a third of the shared genes (each of which is several thousand letters long) are identical in the two species.
    Instead of looking at average genetic differences, though, Dr. Reich and his colleagues used the complete sequences to reveal the evolutionary history of the genomes. Scientists have long suspected that some regions of the human (and chimpanzee) genome must be older than others—that some sections trace their origins far back to the common ancestral population that gave rise to humans and chimps.
    Dr. Reich and his colleagues examined these common sequences, mindful of the assumption that genes steadily collect mutations as time goes by. They aligned sections of the human and chimpanzee genomes and identified how much they diverged. At different genes, humans share a common genetic ancestor with chimpanzees at different times. By studying more than 31,000 bits of the genome and measuring how closely humans are related to chimpanzees in different places, the scientists were able to study how quickly the species became different from each other. The results were published online this week in Nature.
    The team found that it took at least hundreds of thousands of years and, perhaps, 4 million years for human and chimpanzee ancestors to stop interbreeding after they began to be differentiated. Humans and chimpanzees were interbreeding for all this time, before finally separating no more than 6.3 million years ago and probably less than 5.4 million years ago. This is more recent than was thought. Moreover, the argument that the two species were interbreeding over such a long time is, to say the least, controversial.
    Most interesting was what the scientists discovered about the X chromosome in humans and chimpanzees. The X is one of two chromosomes that determine a person’s (or a chimp’s) sex. Females carry two copies of the X chromosome while males carry one X and one Y chromosome. The progeny of interbreeding start with a big evolutionary disadvantage. It is thought that if human and chimp ancestors initially became separate species and then started to interbreed, then the hybrid males produced tended to be infertile. (No one knows exactly why males are more affected than females, just that they are in groups ranging from mammals to insects.) A viable hybrid population could only be created if the fertile females mated back to one of the ancestral populations.
    The scientists found that human and chimpanzee X chromosomes are relatively similar. Indeed, their differences are roughly some 1.2 million years younger than the average of all the non-sex chromosomes. This lends weight to the theory that a viable hybrid population was sustained by interbreeding over a long time.
    Further evidence could come from the fossil record. Fossil finds are notoriously difficult to classify—people disagree on which physiological features are important; and each new find represents a class of one. The Toumai skull, found in Chad in 2001 and thought to be the earliest hominid, is between 6 million and 7.4 million years old. However, Dr. Reich points out, if humans and chimpanzees had undergone an initial separation at this time, it could account for why the skull has human-like features, including a relatively flat face without a protruding snout. The interbreeding came after this time. The researchers, along with other scientists across the world, are now working to sequence the complete genome of other close relatives to humans, including gorillas and orangutans. Primate evolution could yet reveal plenty more oddities.
Why does the author mention the Toumai Skull?
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答案The Toumai skull was found in Chad. It bears human-like traits, which serves as a further evidence that the interbreeding happened after then.
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