We like to think that as humans, we are somehow above biology—capable of self-restraint and subverting base urges. But professor of history Daniel Lord Smail argues that for humans and animals, life is by and large a quest to feel good. We may be responding to biological urges that are complex and sophisticated, but they are biological urges all the same.

In his most recent book, On Deep History and the Brain (University of California Press), Smail posits a new view of human history in which physiology and culture evolve symbiotically in a process driven by brain chemistry and psychotropic effects. Seen from his perspective, we are all hooked on the hormones and neurotransmitters that signal pleasure and relieve stress.

In popular usage, the term psychotropic refers to drugs—prescription or recreational—that radically alter consciousness. But the technical definition includes all substances that act on the central nervous system—including caffeine, alcohol, and anesthesia. Smail has coined an even wider definition, referring to psychotropic “mechanisms”—not only substances that we ingest, but also actions that affect the balance of hormones and neurotransmitters in our bodies. The list includes exercise, sex, gossip, even skydiving.

He argues that the way we define addiction is arbitrary and artificial. We stigmatize addicts, considering them on the wrong side of a fallacious dividing line that changes location from one culture and one epoch to the next. Why do we wholeheartedly endorse pursuit of a “runner’s high,” equivocate about caffeine and alcohol, and condemn cocaine use? In eighteenth-century Europe, Smail points out, the list of addictive substances to be used with caution included books. With the rise of the novel and the spread of literacy, a new fear of “reading mania” gripped the populace. He quotes one scholar’s account that young women were seen as particularly vulnerable, because they might “grow addicted to the pleasures induced by novels…have their passions awakened, and form false expectations about life.”

Perhaps, then, what differentiates humans from animals is the very ability to intentionally self-medicate? Not so fast, says Smail, who notes multiple examples of animals doing the same thing, including birds that “get drunk” by gorging on fruit that has begun to ferment as it decays. He argues that evolution pays attention to psychotropy, noting that in breeding dogs to enjoy human contact, we have created a class of animals “addicted” to petting, in the sense that they crave and seek out the hormonal response it generates.

Evolution attaches pleasing chemical signatures to behaviors that confer advantages in survival and reproduction; it is obvious enough why sex and eating would be pleasurable. But when we find ways to trip the same circuits with other activities (for instance, by activating the exhilaration of the fight-or-flight response through skydiving), those behaviors have their own evolutionary and historical consequences. Smail does not think it mere coincidence that the increasing accessibility and popularization of products such as coffee and spirits in Europe (the proliferation of coffeehouses, the influx of rum from the Caribbean and Brazil, the gin craze of the mid-eighteenth century) coincided with declining attendance at religious services, and in some places, transitions from monarchical states to democracy. “Where individuals once relied on religion and ritual as sources of dopamine and other chemical messengers,” he writes, “they turned increasingly to items of consumption, giving up God in favor of Mammon.”

He divides psychotropic mechanisms into two categories: autotropic (altering one’s own mood) and teletropic (altering someone else’s mood). In eighteenth-century Europe, he writes, there was “a tectonic shift away from teletropic mechanisms manipulated by ruling elites toward a new order in which the teletropies of dominance were replaced by the growing range of autotropic mechanisms available on an increasingly unregulated market.” Here he sees the seeds of our modern consumerism: the coffee, chocolate, and spirits habits of earlier centuries stemmed from the same ethos as today’s “retail therapy.”

To support his argument that pre-modern societies’ rigid class structure, as oppressive as it might have been, was also soothing, Smail draws evidence from evolutionary biology: researchers have found that baboons feel less stress (as measured by their hormone levels) when they live in groups with clearly delineated rank order. If rank order is less clear, the rules that govern interactions—such as which one should step out of the way when two baboons cross paths—require constant negotiation, and the uncertainty seems to induce anxiety.

The book is less a thorough exposition of ideas than an open invitation to consider them. Smail (a medievalist by training) began writing it as the introduction to another book—a “deep history” of humanity, on which he is now working. Both projects grew out of his frustration with perceived shortcomings in the way history is taught and studied. Historians, he complains, tend to act as though history begins with the first written texts. He aims to write an authoritative, unified history that reaches all the way back to the Paleolithic era and integrates evidence from anthropology and evolutionary biology. And it isn’t a one-way street: Smail, who also teaches a course on the history of emotion, believes biologists might gain insights from analyzing the descriptions of emotional states found in ancient texts—traditionally historians’ domain.

In this scholarly space where disciplines merge, “we can finally dispense with the idea, once favored by some historians, that biology gave way to culture with the advent of civilization,” he writes. “Civilization did not bring an end to biology. Civilization enabled important aspects of human biology.”