Cave paintings show instances of human spirtuality. (Patrick Aventurier/SIPA)

Rachel Newcomb is an anthropologist and the Diane and Michael Maher Distinguished Professor of Teaching and Learning at Rollins College. She is the author of “Everyday Life in Global Morocco.”

Author Reza Aslan’s “God: A Human History” is less a biography of God than a study of why and how humans tend to anthropomorphize the divine. As societies developed from small groups of hunter-gatherers to large, specialized populations supported by agriculture, ideas about God changed as well. At the core of all belief systems, Aslan observes a tendency to seek a “humanized God,” which has been “embedded in our consciousness the moment the idea of God first occurred to us.” Studies have shown that although most followers of monotheistic faiths believe God is an abstract force, they will nevertheless describe God “as though they were talking about someone they might have met on the street.” What we say about God, in other words, says more about us than about what God might actually be.

The basic religious impulse, Aslan suggests, is an evolutionary response to environmental stimuli. People tend to attribute agency to natural events (think lightning emanating from the fingertips of a gray-bearded man in the clouds), and our hunter-gatherer ancestors might have imagined that faces seen in trees meant that trees possessed spirits. Cave paintings found throughout the world, some dating as far back as 41,000 years, represent the earliest evidence of human spirituality and demonstrate our ancestors’ interdependent relationship with nature. The Trois-Frères caves of southwest France, for example, feature elaborate paintings of floating animals, devoid of hunting imagery, with a mystical creature at their center: part human, part stag and part owl. Aslan believes that these images do not represent actual animals but rather are “symbols meant to represent ‘the other world’ — the world beyond the material realm.”

As people formed agricultural societies, they began to see humans as dominating nature, and they envisioned gods who did likewise. Perhaps, Aslan argues, agriculture was invented as a response to religion. The elaborate stone temple of Göbekli Tepe in Turkey dates to the end of the last ice age, about 13,000 years ago, yet humans had not begun living settled agricultural lifestyles at this point, leading Aslan to conclude that agriculture was invented to support organized religion, not the other way around. A tremendous workforce would have been needed to build such massive structures, and those workers had to be fed, causing people to turn away from the slender offerings of a hunter-gatherer lifestyle and toward the production of crops that could sustain larger populations.

“God,” by Reza Aslan (Random House)

With settled societies came writing, and cultures such as the ancient Sumerians, Egyptians and Greeks left more detailed records of their beliefs. “The act of writing about the gods,” Aslan says, “of being forced to describe in words what the gods are like, not only transformed how we envision the gods; it made conscious and explicit our unconscious and implicit desire to make the gods in our own image.” As people increasingly attempted to harness the forces of nature, the deities themselves became powerful and temperamental humanlike gods who vied with each other for control over their environment.

At some point, the gods became so humanized that the ancient Greeks began to question their legitimacy. Did gods, as depicted in Greek statuary, truly look like people? Creeping doubts led to a different proposition: maybe the gods were not many but one. Aslan lists the philosophers, pharaohs and prophets who first proposed this radical concept; however, none of these early monotheists found a willing audience since “one god conflicts with our universal compulsion to humanize the divine.” For instance, how could one god be capable of both good and evil? People were, however, beginning to accept the idea of one more-powerful god above a panoply of lesser gods, which mirrored the political hierarchies they experienced in society. “As more authority is vested in a single individual on earth, more authority is given to a single god in heaven,” Aslan notes.

It took a radical event for true monotheism to take root: the expulsion of the Jewish people from Babylon. Historical and linguistic evidence indicates that in the ancient land of Canaan, people worshiped two different supreme gods, El and Yahweh, as well as other, lesser deities. Early Israelites, including the prophets Abraham, Isaac and Jacob, would have followed El. But the burning bush that the prophet Moses encountered in the desert is the first appearance of Yahweh, the god followed by the tribe of the Midianites, whose territory was believed to be south of Canaan (Moses had married into a Midianite family). As ancient Israel united into a nation, Yahweh prevailed, yet a defeat at the hands of the Babylonians led many Israelites to conclude that the Babylonian god, Marduk, was stronger. A small group of exiles continued to profess their faith in Yahweh, believing that “perhaps Yahweh was punishing the Israelites for believing in Marduk in the first place.” At this moment, Aslan says, we see “the first expressions of unambiguous monotheism in the entire Bible.” As Yahweh proclaims, “I am the first and last; besides me there are no gods.” To move from the idea of different deities responsible for good and evil to “a single vengeful god full of contradictions” required a great cognitive leap.

Although God was now without peers, many found Him to be too wrathful. When Jesus appeared, Aslan argues, He embodied those human characteristics of the divine that people still needed, while at the same time preaching the message that God was loving and forgiving. At first, not everyone agreed that Jesus Himself was divine, and it was not until 325 at the Council of Nicea, convened by the Roman emperor Constantine, that God and Jesus were declared to be “of one substance.”

Enter Islam, which arose in the 7th century “out of the deserts of Arabia to confront Christianity’s conception of the humanized God.” Yet even as Islam denounced the idea of the trinity and insisted on a God with no image, substance or form, there are still parts of the Koran that are “replete with anthropomorphic descriptions of God.” And some Muslims still had questions: How could God be one with creation yet separate from it? Such theological conundrums led to the rise of Sufism, the mystical branch of Islam that concerns itself with a direct and personal experience of the divine. It is this belief system that Aslan finds most sympathetic. “God is everything that exists,” he writes.

Aslan devotes significant time to the “big three” monotheistic religions but does not explain why other cultures have been able to follow religious systems, such as Buddhism, without a deity at their center. If the human tendency is to want a humanlike God, what explains the success of these other religions? Nonetheless, Aslan’s fluid writing style makes the reader inclined to drop any lingering questions and accept his assertions on faith alone. His use of scholarly sources from fields ranging from archaeology to neuroscience will introduce many readers to information that otherwise would be relatively inaccessible, and he combines these disparate sources in compelling ways. Whatever God may be, at the very least Aslan shows us the long history of how humans have made Him in our image, and not vice versa.

A Human History

By Reza Aslan

Random House. 298 pp. $28