The Ishango bone, found in 1950 near what is now the border between Uganda and the Democratic Republic of the Congo, is a baboon leg bone more than 20,000 years old. Carved along its length are groups of notches arranged in three columns. Some researchers have noted that one column contains the prime numbers between 10 and 20 (11, 13, 17, and 19). Whether the patterns reflect deliberate calculation or chance remains debated, but the bone is among the oldest known objects that may record numerical thinking.