In the two refernces cited by Jan below, I suggest that polyandry was brought to India by Iranian speakers appearing as “PaaNDavas” in the Mahaabhaarata. I cited Herodotus on the Iranian tribe of Massagetae. But the polyandry of the Toda tribe of the Nilagiris is further evidence for the Iranian origin of polyandry in India. Archaeological evidence suggests that a batch of Iranian speaking horsemen - Sakas - came to the Nilagiris in the early centuries CE, while the phonology of the Toda language, which is radically different from related Dravidian languages can only be explained by assuming that it reflects a Dravidian language (Pre-Tamil) adopted by Iranian speakers. See
Parpola, Asko, 1984. The Todas of the Nilgiris: Hypothesis of a twofold origin -- Pre-Tamil and Saka*. Pp. 319-336 in: Joshi, S.D., (ed.), Amṛtadhārā: Professor R. N. Dandekar felicitation volume. Delhi: Ajanta Publications International.
Best regards, Asko