The Ishango bone was found in the 1950s, in a fishing village on the Congo-Uganda border and dates to around 20,000-30,000 BC. It is an exhibit today at the Royal Belgian Institute of Natural Sciences at Brussels. The bone is the fibula of a baboon, and what makes it interesting is the set of notches that were deliberately marked on its surface, presumably by the people who lived in the region then.
The number of these notches and their grouping seem to imply a knowledge of mathematics and calculation - of prime numbers, multiplication and division - quite advanced for its time and unheard of elsewhere. If indeed the markings were inscribed with these concepts in mind, the Ishango bone would be the earliest evidence of "mathematical thinking". But is it just a coincidence that the notches are uniquely grouped and have certain mathematical properties? Or do they represent a calendar, as has been proposed by some? The problem with these interpretations, of course, is that they use modern day notions to explain something from a very, very long time ago. Nevertheless, here’s a interesting link that talks more about the markings, and also points to other such examples, including what might be an ancient calendar in the form of dots (just a little younger than Ishango), in the famous caves of Lascaux in France.