The Yamato Dynasty has very few parallels in world history (perhaps barring the former Ethiopian monarchy) in terms of an unbroken lineage from a single family (at least 1500 undisputed years). Even considering Japan's neighbours in East Asia (China, Korea, Vietnam, etc.), no dynasty comes close to the longevity of the Japanese. The closest I can think of are the Zhou (perhaps 700 years of real and nominal control) and the Joseon (500 years).
How come this office was never usurped by another ruling house (like the Zhou were removed finally by the Qin) - was it due solely to prestige and ceremonial factors? Secondly, how was it possible for the dynasty to sustain itself in terms of providing viable heirs for such a long period (thinking perhaps of the various Capet branches in France)?
I understand this question may be a bit broad in scope, but I'm looking really for general factors in the Japanese political culture that would explain the phenomenal continuity in the Imperial Office.
It definitely helped, that for quite some time, the Emperor did not have any real power. From 1192 until the Meiji restoration in 1868, their political influence was very limited. The shogunal dynasties, that also exerted various degrees of control in this time, changed three times during this time.
The main reason that ensured an issue is definitely polygamy, that allowed emperors and other nobles to have concubines and secondary wives. Sometimes there were even two empresses. Children that in medieval Europe would have been considered bastards had a place in the line of succession, if the primary wife did not produce an heir.
An interesting fact: The Portuguese, when they arrived in Japan, compared the Emperor to the Pope and the Shogun or similar to secular rulers, like the Holy Roman Emperor. They referred to Toyotomi Hideyoshi as Emperor. Just like the Pope in medieval Europe, the Emperor was often used as legitimization for your own claim.
I have always wondered the same thing, and as a followup question, how legitimate is the house of Yamato's claim? Obviously the claim to descent from Amaterasu is not provable, but how far back is the lineage claim plausible? Is there a point in the succession where the only record of that monarch's existence is the lineage claim? Are there any documented cases where succession was disputed, and the acknowledged heir may not have been truly legitimate, causing a break in the direct male succession?