There's this great recurring "Saturday Night Live" skit from several years back where Phil Hartman plays an unfrozen caveman who goes to law school. He pontificates (发表武断的意见) on the American judicial system while marveling at modern technology like "the tiny people in the magic box" (a TV). It fits a common stereotype: Human ancestors were, well, cavemen, and not as smart as we are today. A new hypothesis from a Stanford geneticist tries to turn this stereotype upside down. Human intelligence may have actually peaked before our ancient predecessors ever left Africa, Gerald Crabtree writes in two new journal articles. Genetic mutations during the past several millennia are causing a decline in overall human intellectual and emotional fitness, he says. Evolutionary pressure no longer favors intellect, so the problem is getting worse. He is careful to say that this is taking quite a long time, so it's not like your grandparents are models of brilliance while your children will be cavemen rivaling Hartman's SNL character. But he does maintain that an ancient Athenian, plucked from 1000 BC, would be "among the brightest and most intellectually alive of our colleagues and companions." His central thesis is that each generation produces deleterious (有害的) mutations, so down the line of human history, our intelligence is ever more impaired compared to that of our predecessors. Not surprisingly, the hypothesis, published in the journal Trends in Genetics, has several geneticists scratching their heads. "It takes thousands of genes to build a human brain, and mutations in any one of those can impair that process, that's absolutely true. It's also true that with each new generation, new mutations arise...but Crabtree ignores the other side of the equation, which is selection," said Kevin Mitchell, associate professor at the Smurfit Institute of Genetics at Trinity College Dublin. "Natural selection is incredibly powerful, and it definitely has the ability to weed out new mutations that significantly impair intellectual ability. There are various aspects in these papers that I think are really just thinking about things in a wrong way. " Crabtree said he wanted to examine the cumulative effect of generation-to-generation mutation on intelligence, which is thought to be controlled by many genes. Using indexes that measure X-chromosome (染色体)-related mental retardation, he comes up with between 2,000 and 5,000 genes related to human intellectual ability. Using another index measuring average mutations that arise in each generation of children, he calculates that within 3,000 years, "we have all sustained two or more mutations harmful to our intellectual or emotional stability.""There is a general feeling that evolution constantly improves us, but it only does that if there is selection applied," Crabtree said in an interview. "In this case, it is questionable about how much selection is occurring now compared to the process of optimizing those genes, which occurred in the jungles of Africa 500,000 years ago." There's already evidence for this in other areas, he argues: Take our sense of smell. Humans have far fewer olfactory receptors than other animals, he said—we're guided by our intellect now, not by smell. We can think about where a piece of food came from, how it was processed, which plant it's from, who has been around it, and so on. A dog, on the other hand, simply sniffs something and either eats it or doesn't. Similarly, he believes evolution now selects for other traits—namely, the most healthy and the most immune, not the most intelligent. But geneticists took issue with his claims, not to mention his citations and methods. Mitchell took issue with Crabtree's characterization of genes—he describes them as links in a chain, with incredible overall disruptive power. They're like a bulb on a string of Christmas tree lights that suddenly fails to work, taking out the entire strand with it: "It can be concluded that genes related to intelligence do not operate as a robust network, but rather as links in a chain, failure of any one of which leads to intellectual disability," he writes. Mitchell countered that this ignores other genes that don't cause intellectual disability. "Biological systems are robust to degradation of several different components," Mitchell said. "Evolution has gone to a lot of trouble to craft your genome so it's finely honed to do its job, and it doesn't make sense that you would have all this random mutation in your brain cells. Also, you would have a very high rate of brain cancer."
单选题
According to the passage, "Saturday Night Live" skit is a________.