We report on the use of rare mutations to tackle biosocial questions such as kinship and differential burial practices from past human populations. To do this, we have inferred nucleotide position 73 of HVS-II in individuals classified as belonging to haplogroup H from 76 human dental samples from the necropolis of Aldaieta (Basque Country, Spain, 6th-7th century) by means of PCR and restriction enzyme tests. The same analysis has been performed for 146 extant individuals from the northern Iberian peninsula. A combination of haplotype H and 73G in HVS-II, rare in extant populations (0.5-3%), has been found at a frequency of 20% in the ancient population of Aldaieta. These data can be explained in terms of the existence of different burial practices associated with a variety of factors, mainly social status and kinship. This hypothesis is also supported by archeological data. These results indicate that caution should be taken when making phylogenetic inferences from extinct populations, because an uncharacterized kinship can significantly bias allele frequencies.