According to histories compiled in the 8th century, Emperor Akihito's ancestral line descends from the gods who created the Japanese islands.
The first emperor, Jimmu, is said to have established the throne in 660 BC, although historians argue that the origins of the dynasty were more recent, more mortal, and not so Japanese. Jimmu may have lived, but probably not until the 1st or 2nd century AD. He was more likely a tribal chief than a great-great-grandson of the sun goddess. Scholars theorize that he may well have come from Korean or at least Asian stock, but the early peopling of Japan is unclear.
There is little doubt, however, that Japan's imperial family is durable. For roughly 20 centuries, Japanese emperors have been succeeded by their male relatives - theirs is the oldest hereditary monarchy in the world. Women have occupied the throne, but always to keep it warm for a male heir.
Since the first few centuries of the dynasty, the imperial family has been more a symbolic and religious institution than a source of rulers. Powerful clans and political chieftains either manipulated emperors or relegated them to obscurity for most of Japanese history.
In the 17th century, however, thinkers began to focus on the imperial line as a source of Japanese identity - an intellectual movement that culminated in 1868 when Emperor Meiji was "restored" to power by politicians intent on simultaneously modernizing Japan and bolstering nationalism.
Although previous emperors had embraced imports like Buddhism and Confucianism, Meiji's backers concentrated on Japan's indigenous religious tradition, known as Shinto. A religion without a text, Shinto is an elaborate sort of nature worship focused around rituals and festivals.
The thinkers of the Meiji era used Shinto for political purposes, styling the emperor as a god-king and declaring him in 1889 the nation's sovereign - in name, at any rate. All power stemmed from the emperor, but he exercised no power, wrote the late Edwin Reischauer, a US historian of Japan.
Other scholars wonder just how innocent of responsibility Meiji and his grandson Showa (the posthumous name of Hirohito) were during the military adventures that Japanese governments pursued during their reigns. Critics also question the US decision to leave Showa on the throne instead of trying him as a war criminal.
Showa renounced his status as a divine being in 1946, and the following year a new US-drafted Constitution described the emperor as "a symbol of the State and of the unity of the people." Sovereign power was said to reside with the people.
Reforms stripped the imperial family of much of its wealth, aristocracy, and formal standing as the head of state. Today the emperor carries out some of those functions - representing Japan on goodwill trips abroad, appointing the prime minister, and receiving foreign dignitaries - but he is not the nation's ruler, even in name.
Showa toured the country after the war, uttering vague apologies for the suffering caused by the conflict and its aftermath and encouraging the people to rebuild. Then he more or less retired until his passing in 1989 - an event that prompted a massive and startling outpouring of emotion from the Japanese people.
The thousands who stood at the gates of the imperial palace, weeping over their ailing emperor, proved that the institution still had life in it. But it has been up to Akihito to define what it means to be a powerless, formerly sacred "symbol."