From Fearsome Predator to Man's Best Friend

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Imagine a wolf catching a Frisbee a dozen times in a row, or leading police officers to a stash of cocaine, or just sleeping peacefully next to you on your couch. It's a stretch, to say the least. Dogs may have evolved from wolves, but the minds of the two canines are profoundly different.

Dog brains, as I wrote last month in The New York Times, have become exquisitely tuned to our own. Scientists are now zeroing in on some of the genes that were crucial to the rewiring of dog brains.

Their results are fascinating, and not only because they can help us understand how dogs turned into man's best friend. They may also teach us something about the evolution of our own brains: Some of the genes that evolved in dogs are the same ones that evolved in us.

To trace the change in dog brains, scientists have first had to work out how dog breeds are related to one another, and how they're all related to wolves. Ya-Ping Zhang, a geneticist at the Chinese Academy of Sciences, has led an international network of scientists who have compared pieces of DNA from different canines. They've come to the conclusion that wolves started their transformation into dogs in East Asia.

Those early dogs then spread to other parts of the world. Many of the breeds we're most familiar with, like German shepherds and golden retrievers, emerged only in the past few centuries.

Meanwhile, back in China, those early dogs lingered on for thousands of years. Today, they're known as Chinese native dogs. "The Chinese native dogs live in rural villages, helping humans to guard homes," Dr. Zhang explained in an e-mail.

Dr. Zhang and his colleagues see Chinese native dogs as the key to better understanding how dogs evolved. Recently, they sequenced the entire genome of Chinese native dogs and compared them with the genomes of Asian wolves and modern breeds like German shepherds. By comparing the mutations in the genomes, they've been able to estimate when wolves and dogs diverged.

As they reported on Tuesday in the journal Nature Communications, they found that the split started 32,000 years ago. Those early dogs would have encountered small bands of hunter-gatherers. People didn't settle in villages to farm in East Asia until about 10,000 years ago.

After dogs split from wolves, their genes began to evolve in a new direction. Dr. Zhang and his colleagues were able to identify some of these evolving genes. A number of them, it turned out, are active in dog brains. (Dr. Zhang and some of his colleagues published some of these results last week in the journal Molecular Biology and Evolution.)

Some of the genes that evolved early in dog evolution are involved in smell or hearing. Others are active in a region called the prefrontal cortex, where mammals make decisions about how to behave. Some genes are involved in growing connections between neurons. One gene, called SLC6A4, transports a neurotransmitter called serotonin into neurons.

The results offer some tantalizing hints about how wolves first turned doglike. "The conventional view is that the hunter-gatherers go out and get a puppy," said Chung-I Wu of the University of Chicago, an author of the Nature Communications study. If humans actually did breed early dogs this way, then dogs would have descended from a very small population.

That's not what Dr. Wu and his colleagues have found, though. Instead, it appears that a large population of wolves started lingering around humans -- perhaps scavenging the carcasses that hunters left behind.

In this situation, aggressive wolves would have fared badly, because humans would kill them off. Mellower wolves, by contrast, would thrive. If this notion turns out to be true, it means that we didn't domesticate wolves -- they domesticated themselves. SLC6A4 may have played a crucial part in this change, because serotonin influences aggression.

To test these ideas, Dr. Zhang and his colleagues are gathering DNA from more dogs and wolves. They also hope to collaborate with cognitive scientists to see how variants of genes like SLC6A4 affect the behavior of dogs today. Their results may also help explain human evolution, because Dr. Zhang and his colleagues found that some of the same genes that evolved in dog brains, such as SLC6A4, also experienced natural selection in human brains.

"Humans have had to tame themselves," said Adam Boyko of Cornell University, one of Dr. Zhang's collaborators on the Molecular Biology and Evolution study. "The process is probably similar to dogs -- you have to tolerate the presence of others."

Carl Zimmer's "Matter" column appears on Thursdays. Follow him on Twitter: @carlzimmer.

science

This article originally appeared in The New York Times.


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