"The primary factor that humans selected for was tameness, or low levels of aggression. The main mechanism through which this was accomplished was neotenization, or retention of juvenile low aggression into adult life. We also selected animals who paid attention to us." -- Nigel Barber, Ph D.
In a previous article here, "The Love Hormone: The Key to Canine Evolution," I proposed the idea that oxytocin probably played a major role in the development of the wolf's social instincts, instincts which for years have been misunderstood and misinterpreted as being about the formation of a social, dominance hierarchy, based on rank and status[1].
My model of wolf evolution is based on the idea that oxytocin can effect social behaviors in mammals on a significant scale. In "The Love Hormone" I wrote: "It might sound improbable that a simple neuropeptide, even though it has some pretty fancy tricks up its sleeve (it acts as a hormone and pheromone) could be responsible for such complex changes in behavior. But research by Insel & Young (2002) shows that in monogamous prairie voles, when the effects of both oxytocin and vasopressin (which are closely related), were disrupted -- by derailing their connections to the reward circuits in the nucleus accumbens -- the formerly monogamous voles quickly became promiscuous. This was a huge reversal [which] took place immediately.
"In contrast, Lim et al (2004), did a study with promiscuous meadow voles, and found that the addition of oxytocin and vasopressin (again, in direct connection with the reward circuits in the brain), produced long term pair bonding where no such behavior had previously existed. And again, the results took place immediately, in real time."
Lim et al conclude: "A change in the expression of a single gene in the larger context of pre-existing genetic and neural circuits can profoundly alter social behavior, providing a potential molecular mechanism for the rapid evolution of complex social behavior." (Lim et al, 2004.)
Meanwhile, there are numerous studies showing that many (if not all of) the emotional connections made between dogs and their owners are based at least partially on increases in the production of oxytocin. Just looking into your dog's eyes can increase oxytocin. (Miho Nagasawa, et al, 2009: "Dog's gaze at its owner increases owner's urinary oxytocin during social interaction," Hormones and Behavior, 55, 434-441.)
Given all this evidence, I respectfully (and partially) disagree with Nigel Barber's statement (above) about how dogs became domesticated. I don't think early humans were deliberately selecting for tameness, per se, or for neotenous characteristics, though I agree that we probably liked animals that paid attention to us (particularly if by looking at us, those animals increased positive feelings in us). It's more likely that the major thing we were selecting for (albeit quite unconsciously) was oxytocin.
In an article in the August issue of Current Anthropology, University of Pennsylvania anthropologist Pat Shipman has proposed that there were three distinct phases to what eventually became the domestication process and offers a testable hypothesis that "human adaptive changes were causally linked to the animal connection."
Shipman says the first phase took place over 2½ million years ago, and was probably based on shared hunting patterns between humans and wolves (both species are social hunters who target animals that are larger and more dangerous than themselves). This would've been a time when humans and wolves probably had a respectful, symbiotic relationship much like the one described in the folklore of many native cultures, and which may have echoed the way killer whales reportedly shared hunting duties with the Australian aborigines well into the early 20th Century.
This was not a period of humans domesticating, caring for, breeding, or selecting wolves (or proto-dogs) for any purpose, but the two species probably did interact very closely for a very long period of time (probably up until about 500,000 years ago).
Shipman says the second period probably began about 200,000 years ago, which is when homo sapiens emerged and we see the first examples of art depicting animals. Neolithic cave art (circa 10,000 B.C.), discovered at Tasilli-n-Ajjer, in Algeria, shows what appear to be dogs or doglike animals (with curly tails) surrounding a wild ox or wildebeest, while a human hunter, carrying a spear, stands at a distance, ready and waiting to throw his weapon. This suggests that our Neolithic male ancestors probably used dog-like animals for hunting purposes.
The actual domestication process -- where dogs became part of our families and our daily lives -- began about 40,000 years ago, and continued through the last ice age. (Dogs may have kept us warm!)
In her book The Lost History of the Canine Race, Mary Elizabeth Thurston suggests that females of the tribe may have been the ones caring for the first dogs. "Women early on assumed the task of gathering plant matter and collecting small game because the slower pace of these tasks blended more easily the need to carry and care for very young children. For this reason, prehistoric women might have been primarily responsible for forging the first intimate relationships with canids. (pg. 5)"
A major role for women in prehistoric times was nurturing the young. Oxytocin is sometimes called the "nurturing hormone." Women naturally produce more oxytocin than men. So it seems to me that the primary things our ancestors selected for in their domestication of dogs was not tameness or neotenous characteristics, but for hunting skills and oxytocin. (Not to be sexist, but even today women tend to be more nurturing while men are more likely to roughouse with their dogs.)
The Belyaev silver fox experiment (mentioned by Dr. Barber in his blog article, and also in two of my most recent blogs[3]) shows that tameness happens concurrently with the reduction of adrenaline.
Adrenaline is a fear hormone, oxytocin is a trust hormone. I know of no studies showing an inverse relationship between the two, but it has to exist; you can't be full of adrenaline and trusting at the same time. And since -- according to the Russian fox researchers -- the genetic reduction of adrenaline is on the same pathway as the genes that code for neotenous morphological traits, it seems more than likely that there's a correlation between oxytocin and neoteny.
So rather than "thinking" about what they were selecting for, I believe our ancestors went by what felt good to them. And dogs make us feel good, partly (or perhaps even primarily) because of oxytocin.
In "A Woof at the Door," Pat Shipman discusses canid teeth, worn as ornaments by our ancestors. "Did someone who wore a perforated canid tooth 33,000 years ago proclaim him- or herself to be one of the group that domesticated dogs? Possibly. Domesticating dogs was a remarkable human achievement that doubtless provided a definite selective advantage to those who accomplished it successfully. They might well have had reason to brag about their accomplishment by wearing canid teeth."
