The oldest known Chinese characters appeared about 1600 B.C. but quantity and niveau suggest that they already passed a longer development. Just like Egyptian hieroglyphics they derived from natural illustrations, which got more and more conceptional with the time.
The Chinese writing came to japan in the 400th century together with much more Chinese culture. Since the Japanese people had got no own writing, the Chinese characters were adopted very fast. First these characters were used as syllablesigns just like hiragana and katakana. With this method the Japanese could write the phonetics of all their words but those characters represented just one syllabel each, that's why a lot of complicated characters had to be used for longer words. Therefore Japanese people began to use these characters independently from their pronunciation but accordant to their meanings., thus Japanese words could be represented with just one single Chinese kanji.
The Japanese language developed over the years and there were a lot of adjustments from the representive governments. The last profound reform of the language took place in 1946 right after WW II. The number of official kanji was limited to 1850 (tōjō kanji). A lot of kanji were simplified and the writing was standardized.
A well educated Japanese knows about 3000 kanji. Approximately 4000 signs are used in Japanese literature today. The Largest encyclopaedias contain overall ca. 10000 kanji.