Mutation - Twenty Four Fingers and some even with a tail.

In a few rare cases tribal members have been even more restricted endogamically, as in the inbred clan called Foldi in the Hyabites tribe of Arabia, all of whose people have six toes on each foot and five fingers plus thumb on each hand and who have come to value their 24 digits so highly they will kill any 20-digit baby born to them as the illegitimate issue of an adulterous mother. The same goes for the monkey-tailed clan of Kali who traditionally refuses to keep any infant who doesn't have a tail.

Inter marriages  can correct this after generations.