Unique Japanese

Japanese is not unique. As a human natural language it shares many similarities with every other language in the world, some more than others. But Japanese is unique, as it appears to have no common relatives, sitting as it does in its own language group. There have been various theories as to where it comes from, but none has been conclusive.

Numbers often a good way of determining the origin of a language as their social importance means that they cannot radically change from one generation to the next, and Japanese numbers show some unique properties.

In modern Japanese the Sino-Japanese numbers imported from China have all but replaced the native numbers, but the latter is clinging on for dear life as generic counting words up to ten.

ひとつ、ふたつ、みっつ、よっつ、いつつ、むっつ、やっつ、ここのつ、とお

The older forms were:
ひとつ、ふたつ、みつ、よつ、いつつ、むつ、やつ、ここのつ、とお

Believe it or not, there is a hidden pattern.

Going from one (ひとつ) to two (ふたつ）: hito→huta
Three (みつ) to six (むつ): mi→mu
Four (よつ) to eight (やつ): yo→ya
Numbers are doubled by changing the internal vowels.
Five (いつつ) to ten (とお) doesn't follow the rule, unless you take the stem of five to be つ, so it would be tu→to.

The first recorded person to notice this was a chap called 荻生徂徠 (1666-1728), so as far as we know, until that point nobody had realised that such a rule existed.

Finding the rule opens up new questions:

• Why does changing the vowel lead to doubling?
• Why don't five and ten follow the rule exactly?
• Is the same rule seen elsewhere in the language? 
• Are there any other languages that share the phenomenon?

The pattern is too consistent to ignore, but it is not shared by any other language that Japan has historically had ties with. Mysterious....

Actually, as unique as Japanese may be, the answers to some of these questions are more related to how similar the language is to others than how different it is. Similar phenomena can be seen in all natural languages. But it's still quite amazing that the two times table is hidden in the number system, and that hardly anybody notices.

References
数字とことばの不思議な話　窪薗晴夫 岩田ジュニア新書　（p.2～)
日本語　金田一 岩波新書　（上、p.51～)