Whence the Kanum base-6 numeral system?

Base-6-36 numeral systems, a typological rarity, are found in Kanum languages of New Guinea as testified by Donohue (Linguistic Typology 12: 423-429, 2008). We look at the probable relatives of the Kanum languages and show that the base-6 system must have emerged in the Tonda group specifically. Since there is no evidence of body-part terms in the base-6 forms attested, we speculate that these systems have a different origin. Specifically, we suggest that the base-6 systems arose for counting yams. The ethnographic data for Kanum and other relevant languages are in concord with such a scenario. Whether there is a historical connection with base-6 systems of the Kolopom languages, near, but not adjacent, to the west, remains an open question. If there is a connection, it is areal rather than genetic, but sufficient evidence for a pre-historic areal connection remains to be collected. Equally, if not more, puzzling would be the conclusion that there is no historical connection, given the rarity of base-6 in the world as a whole