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In the crowded agora of Athens, sometime in the mid-fifth century BCE, a barefoot man with a famously unattractive face — a flat, upturned nose, bulging eyes, later contemporaries would compare him unfavorably to a satyr — stops a passing citizen with a simple question.

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According to a tradition recorded centuries after the fact, an old court archivist, weary of the corruption and decay he has witnessed in the declining Zhou royal court, mounts an ox and rides west, toward the mountain pass separating civilization from the wilderness beyond.

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Sometime around 551 BCE, in the small state of Lu, in what is now Shandong province in eastern China, a boy is born into a family of minor, declining aristocratic status. His name is Kong Qiu. History will remember him by the honorific title his later followers gave him: Kongfuzi, "Master Kong" — a name Western missionaries would eventually Latinize as Confucius.

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At the moment of the Buddha's death, sometime in the fifth century BCE, his teaching exists as the practice of a modestly sized community of monks and nuns, concentrated almost entirely within a few hundred miles of the Gangetic plain in northern India.

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In a small kingdom on the edge of the Himalayan foothills, in the region of modern-day Nepal, sometime in the sixth or fifth century before the Common Era — the exact dates, like so much in this era, disputed by decades depending on the tradition consulted — a boy is born into the ruling Shakya clan.

Somewhere on the plains of northwestern India, sometime in the second millennium before the Common Era, a body of sacred hymns begins to take shape — composed orally, transmitted from teacher to student with an accuracy so exacting that scholars today regard it as one of the most remarkable feats of oral preservation in the entire history of the human species.

Sometime in the centuries before 600 BCE, on the plains of ancient Iran, a priest performing a ritual purification at a riverbank experienced what he described as a direct, overwhelming vision. A Being of Pure Light Appeared to Him, Identifying Itself as Ahura Mazda — "The Wise Lord."

Between roughly 800 and 200 BCE, something happened that historians still find genuinely difficult to fully explain. In Four Separate Civilizations, With No Direct Contact With One Another, a New Kind of Thinking Emerged Almost Simultaneously.