Dog Domestication Changed Both Our Brains

Humans have altered dogs through artificial selection, by breeding them. Dog domestication changed humans also, making us more effective hunters and herders. Domestication reduced the brain size of humans as well as dogs.

How Domestication Worked

Dogs were the first domesticated animals (1. p. 126). This fact may reflect the social similarities between dog ancestors (wolves), and humans. Humans and canines both function as cooperative hunters, and both operate within a dominance-based family breeding system.

In wolves, as in people, only the dominant pair gets to reproduce, and the sexual behavior of other members of the group is suppressed.

Among wolves, this means that the dominant female harasses other sexually mature females in the group so that they fail to ovulate. In traditional human families, the sexual behavior of all except the parents was suppressed by strong taboos and social sanctions.

This unusual dominance-based social system may help explain why dogs proved so adept at fitting in with human families from the beginning and are still regarded as family members wherever they are kept as pets.

Even so, domestic dogs are very different from wolves, and it likely took many generations of hanging around campfires and scrounging on scraps before the earliest dogs got integrated into human lifestyles (2).

Why Dogs Are So Different From Wolves

Modern dogs are evidently descended from extinct Paleolithic wolves. If surviving wolves are any guide, then the ancestors of the dog would have been highly aggressive animals that avoided contact with humans (2). Presumably, the attraction of easy pickings from the scraps discarded by our Ice Age ancestors helped them to overcome their hostility.

Moreover, individuals that were best at overcoming their fear of humans would have obtained more scraps and would have survived and reproduced at higher rates. In this scenario, wolves would have been selected for tameness. At a certain point, they became tame enough that pups were adopted as pets and integrated into human families.

While the earliest dog ancestors benefited from free food around the campsites of megafauna hunters, what did humans get in return? This seems a strange question given the many jobs that dogs currently do for humans, ranging from rescue animals, to sentries, to guide dogs, companion animals, and herders.

These varied functions may have had limited relevance to the way of life of Ice Age hunters. While dogs are very good at finding game hidden in cover, they would not have been required to spot a herd of woolly mammoths traversing their migratory pathways over tundra. Indeed, they could have been a nuisance by alerting large prey to the approach of humans (1, p. 128). They might have served as beasts of burden carrying loads of meat back to a home base.

Whatever their primary initial function, dogs have always been extremely helpful to humans as faithful servants that do all they can to please us. This close alliance went beyond mere mutual toleration (or commensalism). It crossed the line into inquilinism, or the emergence of an evolutionary mutual reliance.

The Inquilinism Angle

The earliest dogs were useful to human hunters on account of their superior senses of hearing and smell that were useful at detecting prey hiding under cover. By helping their human companions find prey, dogs were guaranteed a steady supply of food for themselves.

This mutual reliance worked out so well that it molded the evolutionary trajectory of both species. Relying upon dogs to find prey, human sensory ability declined, which manifested in a shrinkage of the thalamus that acts as a waystation for sensory information (3).

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Dogs were also molded by the experience of domestication. One remarkable adaptation was in the dog's digestive system that allowed them to consume cereals that would not have been present in the diet of their ancestral wolves.

When domestic dogs are released into the wild, they make very poor hunters. Analysis of the diets of feral dingoes in Australia found that they consume much the same diet as domestic dogs (4). Their food is mainly scavenged from dump sites and domestic garbage. Dingoes have apparently been unable to shake their reliance on humans for food. Although dingoes are separated from human companionship, we remain their best friends!

What Domestication Means for the Brain

Domesticated animals manifest reduced fearfulness and often have substantially smaller brains. These traits may be linked. In the safer environment created by proximity to humans, domestic animals do not need to be hyper-vigilant after the fashion of wild animals. Reduced vigilance requires less brain activity.

This means that feral dogs, such as dingoes, may be poorly adapted to life in the wild. As large animals, they have few, if any, predators. So, their adaptive failure is mainly reflected in their inability to hunt prey for food.