The Ishango bones were found in the 1950s by Belgian archaeologist Jean de Heinzelin near a Palaeolithic residence in Ishango, Africa. The inscriptions in the bones, which can be interpreted as numbers, are unique in their complexity for the Old Stone Age. Interestingly, on one of the two Ishango bones, we also find the six consecutive prime numbers 5, 7, 11, 13, 17 and 19. Did Stone Age people already know the secret of the prime numbers? This question is explored in my mathematical essay “The Secret of Ishango Bones”, an adventurous journey around the world from Basel in Switzerland to Erode in India. The presumed connection between the numbers on the Ishango bones and the structure of the prime numbers is illustrated by a sketch at the end of the text. Are the prime numbers organized as a double helix like DNA? Where did the people of Ishango get this knowledge? Did they perhaps have visits from aliens? As the physicist and mathematician Freeman John Dyson said so beautifully: “For any speculation which does not at first glance look crazy, there is no hope.”