Nine out of ten pet owners say that they consider their pets to be members of the family, and more than half of the respondents in an American Animal Hospital Association survey say that if stranded on a desert island, they would prefer the company of their pets to any human companion.

But as Meg Daley Olmert argues in Made for Each Other, our four-legged friends offer much more than companionship. Recent studies indicate that caring for pets brings a host of health benefits, from lower levels of stress to reduced blood pressure and risk of heart disease.

Pet owners make fewer visits to health care providers than non-pet owners and seem to enjoy higher survival rates following coronary heart disease. And therapy with animals has been shown to help in a range of conditions, from autism to Alzheimer's to depression.

Made for Each Other sets out to explain the science behind these remarkable findings, bringing together research from a range of fields, from evolutionary biology and neuroscience to psychology and anthropology, to reveal "the physiological reality of why animals can love us, why we can love them, and why that love is so good for everyone it touches" (xvii), as Olmert puts it.

Arguing that the ability to bond with animals may have offered such a survival advantage that it actually entered into the gene pool, Olmert traces the evolution of human-animal relationships from the dawning fascination with animals recorded in literally millions of cave paintings by our Stone Age ancestors to the full-blown attachments and mutual ties of affection that formed when humans moved beyond merely hunting or hiding from animals and began breeding them, living with them, and caring for them.

Perhaps as long as 400,000 years ago, judging by bone remains, wolves started to wander into hominid settlements for the first time, lured by the wafting aroma of cooked meat. Realizing that their presence warded off more dangerous predators, our perpetually wary ancestors, outnumbered and often outmatched by other animals, may have tossed some scraps of meat to the lurking wolves, encouraging them to remain.

It would be hundreds of thousands of years before dogs evolved out of wolves and not until 10,000 BC or so that full-scale domestication of animals began, but according to Olmert, these first tentative interactions between wolf and human, cautious alliances of convenience, set into motion a powerful chemical shift that made the human-animal bond possible. "The fact that wolves stopped stalking us and we took them into our caves proved to be a miraculous leap of faith that changed our world forever," she writes (ix). And the key to the story is a powerful neurochemical known for its affiliative properties: oxytocin, the same hormone that bonds a mother to her newborn.

Best known for its role in labor and lactation-- its synthetic form, pitocin, is given in childbirth to induce contractions-- oxytocin is produced by all mammals, male and female, and is thought to facilitate a wide range of social behaviors, from the nurturing behaviors of new mothers to the acquisition of social recognition skills and the development of feelings of trust, affection, empathy, and attachment.

Manufactured in the hypothalamus and released into every key region of the brain involved in behavior and emotion, oxytocin functions as a sort of natural tranquilizer, lowering stress hormone levels, heart rate, and blood pressure, decreasing feelings of aggression and anxiety, and increasing feelings of relaxation, contentedness, and social receptivity-- physiological processes that quell the body's defensive fight-or-flight mechanisms and make social interaction and bonding possible.

The crucial discovery underpinning Olmert's book is that oxytocin production can also be triggered in us by caring for animals. One study, for example, showed that after friendly interactions between dogs and their owners, oxytocin levels almost double in both dogs and humans. "These dramatic results could explain not just why we love our pets like children, but also what biological forces are producing the wide range of mental and physical therapeutic effects reported in animal therapy studies," Olmert writes. "These studies show that caring for pets can lower our heart rate, blood pressure, and production of stress hormones. For almost twenty years, we've known that oxytocin can reduce and regulate all these physiological factors" (74).

At the same time, Olmert argues, our emerging understanding of the science of oxytocin helps us to piece together a plausible theory about how the human-animal bond evolved. Drifting on the margins of a hostile world, prey as much as predator, our hominid ancestors may have experienced an increase in oxytocin levels as their behavior became more social and cooperative in response to the demands of Ice Age life.

(Ice Age mothers likely played a pivotal role in this process, she speculates, their cooperative childrearing practices establishing an oxytocin-rich environment for social bonding.) Because the same neurochemical processes that help to override the brain's fear circuitry also unleash strong affiliative powers, this surge in oxytocin may have emboldened our early ancestors to approach the animals they feared while also encouraging them to form emotional attachments to these animals.

The result is that "we inadvertently created a powerful chemical feedback system that changed our hearts and minds" (xvii), as Olmert puts it-- oxytocin stimulated feelings of affection and trust toward animals, and these in turn triggered the release of more oxytocin, perpetuating the cycle.

Wide-ranging and well-researched, Made for Each Other is an entertaining and insightful book crammed with interesting science presented in a thoroughly accessible way. Olmert convincingly shows that the urge to connect with animals is deep in our nature, and she livens up her writing with engaging stories and intriguing tidbits of information that make for fascinating reading.

For example, we learn that attempts to domesticate the silver fox produced animals that barked, looked, and acted like dogs, that coyotes and badgers team up to hunt squirrels, and that ancient Egyptians revered cats so much that household members would shave their eyebrows in mourning when the family cat died. (Apparently, they also honored deceased felines with full-scale mummification; over 19 tons of mummified cats were discovered in one excavated tomb.)

At times, however, Olmert falls into an overly reductive style of explanation, giving the impression that complex behaviors and intricate networks of effects can be explained simply in terms of a single hormone or chemical process. For example, in language that seems to endow oxytocin with its own peculiar agency, we're told that the hormone "makes us smarter, calmer, friendlier, healthier, even more attractive" (191), that it is "capable of creating monogamous social bonds" (xii), and that it "sent a wave of trust signals around campfires throughout the Pleistocene world" (61).

In describing the experiences of animal behaviorist Barbara Smuts, Olmert notes that "it was oxytocin that made Barbara Smuts want to live with baboons in Africa" (119), as if a sudden surge in oxytocin levels would be all it takes for any of us to be off to the wilderness to live with the baboons.

Elsewhere, Olmert cites the anthropologist Evans-Pritchard's observation that a young Nuer man's grooming of his ox includes removing ticks from its belly and scrotum and picking adherent dung from its anus. "Gross? Of course, unless you are under the influence of oxytocin" (169), she remarks-- reducing a vast web of cultural influences to the action of a single chemical. (Smitten as I am, I'm pretty sure that interacting with my cat keeps me flush with oxytocin. But, probably like most people in my own culture, I still can't imagine myself picking adherent dung from her anus.)

The impulse to oversimplify complex phenomena is perhaps most evident toward the end of the book, where Olmert comes close to turning oxytocin into a magic super-chemical that holds the secret to well-being and social progress, with a whole slew of modern ills-- from skyrocketing rates of ADHD to increased levels of depression and anxiety-- chalked up to the fact that we are suffering from oxytocin deprivation brought about by our growing detachment from animals and nature. "We are smack in the middle of a modern meltdown brought on by a lack of oxytocin" (209), Olmert warns. Luckily there's an easy solution to the afflictions of modern life:

we can all get pets, she suggests. But strangely for a book meant to celebrate our connection to the natural world, Olmert also takes seriously the idea that pharmaceutical companies may one day be able to offer salvation in the form of an oxytocin pill. Given that we are profoundly social creatures with a deep need to connect with other living beings, it makes sense that relationships with other people and animals are essential to our health and happiness.

But concluding that we can distill the complicated reality of these relationships into a single chemical component that can be isolated from the whole-- perhaps even bottled and sold-- seems to be an example of overly reductive thinking narrowing our focus to the point that we miss the big picture.

Nonetheless, Made for Each Other is a thoughtful and thought-provoking book full of interesting ideas about the nature of the human-animal bond. Although Olmert does not directly address ethical concerns, the book raises issues that prompt serious questions about our treatment of animals today.

We're gaga over our own pets, shelling out over $35 billion a year on their care and pampering them with specialty foods, luxury products, and even trips to the spa. Meanwhile, however, we increasingly segregate ourselves from the animals we actually depend on for our sustenance.

We've turned farms into factories where animals are treated like living machines in a vast industrial operation. We breed millions of animals who exist simply to be pumped full of chemicals and mutilated in laboratories so that we can test out the drugs and products we consume.

Yet as Olmert shows, through most of our very ancient history as herders and breeders and farmers, our relationships with the animals we domesticated were drastically different. Though we used them for labor and food, we did not treat our animals merely as tools. We lived among them. We lavished care on them, like the Nuer people with their oxen. And bound by a shared biology that drives us to connect with other living creatures, we inevitably developed mutual ties of affection and respect. Seen from this perspective, the way we treat farm and lab animals today is not just cruel and inhumane, but profoundly unnatural.

One of the most important points in Made for Each Other is that our relationships with animals were never simply one-sided, a straightforward matter of humans exerting their power over animals and over nature. Instead, our relationships with animals also changed us in crucial ways, so that in domesticating animals we also ended up civilizing ourselves. As Olmert puts it, "the very act of taming plants and animals tamed us too" (221). But if so, then it is a sad irony that our treatment of animals today is marked by such barbarity.