The banjo is hundreds of years old with its original origins lost in time, and not an instrument which was bourne in America.  The banjo belongs to the group of instruments that were drums with strings stretched over them. These strings were  plucked or bowed.
       Early banjos spread throughout countries that were engaged in trade and it is quite possible that the Arabs brought it to the West African Coast.  It was imported into America with other trade from that area. The best mention of its appearance in America is that by Thomas Jefferson,  1781.  "The instrument.....is the banjar, which they brought hither from Africa."   
       It is likely that the original instrument had three gut strings, In the late 1850's metal strings were added, although they were not widely available until the 1890's.  The first fretted necks appeared about 1878. Banjos with up to 10 strings were also available.  Joe Sweeney, born 1810, is credited with adding a little fifth string running half way up the neck. By the early part of the 19th century groups of players joined together and toured the country.  A prominent group was the Sweeney Minstrels, led by the aforementioned Joe.During this period the banjo became very popular as people spread west.
At the turn of the 19th century the five string fell out of favour for the four stringed tenor banjo.  This was used with heavier strings.  Bands of anything up to 30 musicians.  With concert virtuosos taking it out of the minstrel shows.  By the 1920's the tenor had had its day. During this period the five string was kept alive in the back country, especially the south.
       In 1945  Earl Scruggs joined Bill Monroe and his Bluegrass Boys.  In his younger days Earl had worked out a highly syncopated three fingered style using the thumb, index, and middle fingers.  Within a short period this style took off and ever since has led the banjo revival.  In the 60's the banjo was once again given a boost with the 'Folk Revival', led by such groups as The Weavers and the Kingston Trio. Since then 'bluegrass' has continued be popular never more so than at the present time.