Dogs have a special chemistry with humans that goes back many tens of thousands of years. Researchers investigated this special evolutionary relationship from a number of different angles. Their results are surprising.

The social unit

Domestic dogs are descended from wolves so recently that they remain wolves in all biological essentials, including their social behavior. Wolf packs have some intriguing parallels with human families:

They are territorial.

They hunt cooperatively.

Pack members are emotionally bonded and greet each other enthusiastically after they have been separated.

In a wolf pack, only the alpha male and female are sexually active even though other pack members are sexually mature.

The social adaptations of dogs and humans are similar enough that dogs can live perfectly happy lives surrounded by humans and vice versa. Dogs are pampered with the best of food and medical care, frequently in their owners' comfortable beds.

A family member

Why do people lavish so much care on a member of an alien species? A short answer is that on an emotional plane, families do not see the dog as alien. According to John Archer (1) of the University of Central Lancashire, who has conducted a detailed study of dog-human relations from an evolutionary perspective, about 40% of owners identify their dog as a family member reflecting social compatibility between our two species.

Dogs are extraordinarily attentive and have an uncanny ability to predict what their owners will do, whether getting the dog a meal or preparing to go on a walk. Experiments show that dogs and wolves can be astute readers of human using the direction of our gaze to locate hidden food (2) a problem that is beyond chimps.

Dogs also seem attuned to the emotional state of their masters and express contrition when the owner is annoyed, for example. Otherwise, the capacity to express affection, unconditionally, makes the dog a valued "family member."

Domesticating each other?

Dogs were the first domestic animal with whom we developed a close association. Mitochondrial research suggests that most domestic dogs have been genetically separate from wolves for at least 100,000 years so that we have associated with dogs for as long as we have been around as a species (Homo sapiens). Indeed, some enthusiasts, including Colin Groves of the Australian National University, in Canberra, believe that our success as a species is partly due to help from dogs (3).

According to Groves: "The human-dog relationship amounts to a very long-lasting symbiosis. Dogs acted as human's alarm systems, trackers, and hunting aides, garbage disposal facilities, hot water bottles, and children's guardians and playmates. Humans provided dogs with food and security. The relationship was stable over 100,000 years or so and intensified in the Holocene into mutual domestication. Humans domesticated dogs and dogs domesticated humans."

Relying on dogs to hear the approach of danger and to sniff out the scent of prey animals, our ancestors experienced a decline in these sensory abilities compared with other primates. This conclusion is confirmed by shrinkage of brain regions devoted to these senses (the bulb and lateral geniculate body).

During the long period of our association, dogs' brains have shrunk by about 20 percent, typical for animals such as sheep and pigs who enjoy our protection. Domesticated animals undergo tissue loss in the cerebral hemispheres critical for learning and . If we relied on dogs to do the hearing and smelling, they evidently relied on us to do some of their thinking. If Groves is correct that dogs have domesticated humans, then the human brain would also have gotten smaller. Surprisingly, human brains have actually shrunk, but by only a tenth, suggesting that dogs got more out of the deal than we did.