Why Are Humans So Smart Compared To Animals – Unlike the human brain, the chimpanzee brain does not undergo a rapid explosion of neural connectivity during the first two years of life, which could explain the superior intelligence of humans. Etsuko Nogami / Primate Research Institute, Kyoto University

Despite sharing 98 percent of our DNA with chimpanzees, humans have much larger brains and are much more intelligent as a species. Now a new study sheds light on why: Unlike chimpanzees, humans go through a huge explosion of growth in white matter, or the connections between brain cells, in the first two years of life.

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The new results, published Tuesday in the Proceedings of the Royal Society B, partly explain why humans are so much smarter than our closest living relatives. But they also reveal why the first two years of life play such a crucial role in human development.

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“What’s really unique about us is that our brains experience rapid reconnection in the first two years of life,” said Chet Sherwood, an evolutionary neuroscientist at George Washington University, who was not involved in the study. “That probably helps explain why those first few years of human life are so critical in setting us on the path to acquiring language, cultural knowledge, and all those things that make us human.”

Chimpanzees While past studies have shown that human brains undergo a rapid expansion of connectivity, it was not clear that this was unique among the great apes (a group that includes chimpanzees, gorillas, orangutans and humans). To prove that this is a sign of humanity’s superior intelligence, researchers would need to demonstrate that it differs from that of our closest living relatives.

But a U.S. moratorium on acquiring new chimpanzees for medical research meant people like Sherwood, who is trying to understand chimpanzee brain development, had to study the brains of decades-old baby chimpanzees lying in the labs of veterinary pathologists, Sherwood told LiveScience. [ Pictures: baby chimpanzees welcome ]

But in Japan, those restrictions didn’t come into effect until later,  allowing researchers to take live magnetic resonance imaging (MRI) brain scans of three baby chimpanzees as they grew up to 6 years old. They then compared the data with existing brain scans of six macaques and 28 Japanese children.

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The researchers found that both chimpanzees and humans had much more brain development early in life than macaques.

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“The increase in total brain volume during early childhood and juvenile stages in chimpanzees and humans was approximately three times greater than in macaques,” the researchers wrote in the journal article.

But the human brain expanded much more dramatically than the chimpanzee brain during the first few years of life; most of this expansion of the human brain was driven by the explosive growth of connections between brain cells, which is reflected in the expansion of white matter. Chimpanzee brain volume grew by about half the human expansion during that time period.

The findings, while not unexpected, are unique because the researchers followed the same individual chimpanzees over time; past studies have instead pieced together brain development from scans of several monkeys of different ages, Sherwood said.

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The explosion in white matter may also explain why experiences during the first few years of life can greatly influence a child’s IQ, social life, and long-term stress response. Human intelligence is one of the most consistent inventions of evolution. It is the result of a sprint that began millions of years ago, leading to ever-larger brains and new abilities. In the end, humans stood up straight, took up the plow and created civilization, while our primate relatives stayed in the trees.

Now scientists in southern China report that they have tried to close the evolutionary gap, creating several transgenic macaque monkeys with extra copies of a human gene suspected of playing a role in shaping human intelligence.

“This was the first attempt to understand the evolution of human cognition using a transgenic monkey model,” says Bing Su, a geneticist at the Kunming Institute of Zoology who led the effort.

According to their findings, the modified monkeys performed better on a memory test involving colors and picture blocks, and their brains also took longer to develop – just like those of human children. There was no difference in brain size.

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The experiments, described on March 27 in the Beijing journal National Science Review and first reported by Chinese media, are still far from revealing the secrets of the human mind or leading to a rebellion of intelligent primates.

Instead, several Western scientists, including one who collaborated on the attempt, called the experiments reckless and said they called into question the ethics of genetically modifying primates, an area in which China has taken a technological lead.

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“Using transgenic monkeys to study human genes related to brain evolution is a very risky path,” says James Sikela, a geneticist who conducts comparative primate studies at the University of Colorado. He is concerned that the experiment shows animal neglect and will soon lead to more extreme modifications. “It’s a classic slippery slope problem and we can expect it to happen again as this kind of research continues,” he says.

Research on primates is increasingly difficult in Europe and the US, but China has rushed to apply the latest high-tech DNA tools to the animals. The country was the first to create monkeys altered with the gene-editing tool CRISPR, and this January a Chinese institute announced it had produced half a dozen clones of monkeys with severe mental disorders.

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Su, a researcher at the Kunming Institute of Zoology, specializes in looking for signs of “Darwinian selection”—that is, genes that spread because they were successful. His research covered topics such as the adaptation of Himalayan yaks to high altitude and the evolution of human skin color in response to cold winters.

However, the biggest puzzle of all is intelligence. What we do know is that the brains of our human-like ancestors grew rapidly in size and power. To find the genes that caused the change, scientists looked for differences between humans and chimpanzees, whose genes are about 98% similar to ours. The goal, Sikela says, was to locate the “jewels of our genome”—that is, the DNA that makes us uniquely human.

— the “language gene” in press reports — became famous for its potential connection to human speech. (A British family whose members inherited the abnormal version had trouble speaking.) Scientists from Tokyo to Berlin were soon mutating the gene in mice and listening with ultrasound microphones to see if their squeals changed.

, that is, microcephalin. Not only does the gene sequence differ between humans and great apes, but babies with microcephalin damage are born with small heads, which is related to brain size. Su once used calipers and wrenches with his students to measure the heads of 867 Chinese men and women to see if the results could be explained by differences in genes.

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By 2010, however, Su saw an opportunity to perform a potentially more definitive experiment—adding the human microcephalin gene to a monkey. By then, China had begun to link its massive monkey breeding facilities (the country exports more than 30,000 of them a year) with the latest genetic tools, turning it into a mecca for foreign scientists who need monkeys for experimentation.

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To create the animals, Su and colleagues at the Yunnan Key Primate Biomedical Research Laboratory exposed monkey embryos to a virus carrying a human version of microcephalin. They created 11 monkeys, five of which survived to take part in a series of brain measurements. Each of these monkeys has between two and nine copies of the human gene in their bodies.

Suo monkeys ask unusual questions about animal rights. In 2010, Sikela and three colleagues wrote a paper called “The Ethics of Using Transgenic Nonhuman Primates to Study What Makes Us Human,” in which they concluded that human brain genes should never be added to apes, such as chimpanzees, because they are too similar to us.

“You just immediately go to the Planet of the Apes in the popular imagination,” says Jacqueline Glover, a University of Colorado bioethicist who was one of the authors. “To humanize them is to cause harm. Where would they live and what would they do? Do not create a being that cannot have a meaningful life in any context.”

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In the e-mail, Su says he agrees that apes are so close to humans that their brains should not be modified. But apes and humans last shared a common ancestor 25 million years ago. For Su, this alleviates ethical concerns. “Although their genome is close to ours, there are also tens of millions of differences,” he says. He doesn’t think that monkeys will become anything more than monkeys. “Impossible by introducing just a few human genes,” he says.

Judging from their experiments, the Chinese team expected that their transgenic monkeys might end up with increased intelligence and brain size. So they put the creatures in MRI machines to measure their white matter and give them computerized memory tests. According to their report, the transgenic monkeys did not have larger brains, but they did better on a short-term memory quiz, which the team found

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