Carl Zimmer reported recently on some interesting genetic research on dogs that tells us a great deal about their divergence from wolves, but also reflects back on the social mind of people.
Carl Zimmer, From Fearsome Predator to Man’s Best Friend
As they [Ya-Ping Zhang and others] reported on Tuesday [14 May 2013] 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.”
Turns out that the Clan of the Cave Bear theory is wrong. We didn’t nab a mating pair of wolves and domesticate them. Instead, lots of dogs — large packs — hung around with hunter gathers, who gradually culled the more aggressive members of the packs, leaving those that were more docile and pretty soon, those that fared best were those more willing to cooperate with humans. After a short time, the proto-dogs are learning to behave, and perhaps being rewarded by squirts of serotonin in the brain when interacting with humans. This chemical leads to binding and trust when present in human brains.
The second part of the story also comes from Zimmer. Following the release of Ya-Ping Zhang’s paper, other researcher noticed that some of the genes being tracked in the study are related to serotonin production, but also on the list is MET, which is linked to cancers, but that’s not all.
Carl Zimmer, Another Link Between Dog Brains And Our Brains
One of the six cancer genes is called MET. “However,” Levitt wrote to me, “in 2006, my laboratory published a paper in the Proceedings of the National Academy of Science on a mutation in the MET gene that increases risk for autism.” (Here’s the paper.) In fact, a variant of the MET gene is now recognized as one of the strongest genetic risks for autism.
Levitt and his colleagues have continued to study the gene to understand how it plays a role in autism. My fellow Phenomena blogger Virginia Hughes wrote last year about how Levitt and his colleagues discovered that it shapes the wiring connections between neurons. Not just any neurons, however. It’s most active in circuits in the brain that are involved in social and emotional behavior.
“I don’t believe it is a coincidence that both the serotonin transporter and MET are on the list,” says Levitt.
It’s not exceptional for a gene to be active in different parts of the body and to have different functions. Natural selection can spread a gene because one of those functions boosts survival and offspring, while the other function gets carried along for the ride. So scientists who want to know why MET evolved in both us and dogs will need to figure out how its protein changed in each species, and how that change affected its different incarnations. It’s conceivable that MET evolved as a defense against early cancers in both humans and dogs. It’s also conceivable that its transformation was crucial for the emergence of sociable people and dogs alike.
So, then, we have found that the same genes are linked to the foundation of human and canine sociability, although human sociability evolved earlier. But, perhaps there is a tantalizing glimpse of the evolution of how the social mind evolved in people, based on looking at what apparently happened with dogs. One group — nearly modern human hunter-gatherers — was decidedly social, and encounters a relatively non-social species — wolves — who began to wander with them, eating their leavings. The interaction leads to the wolves becoming dogs, because the humans kill the meanest canines, and exert an evolutionary pressure that selects for human-style sociality.
Imagine two hundred thousand years earlier, when one small band of early humans shared a collection of gene mutations, leading to enhanced sociality, like MET, SLC6A4, and others. (Note that this also led to a higher incidence of autism and cancer, as side effects.) Imagine that other, less social hominins might have trailed those early humans around, and the social humans might have exerted the same evolutionary pressures that we later directed on canines. We may have domesticated ourselves into being human, into being social. And maybe we aren’t finished yet.