Sino-Tibetan (Trans-Himalayan) is one of the typologically most diverse language families in the world, one of the few comprising all gradients of morphological complexity, from isolating to polysynthetic. No consensus exists as yet on whether the rich morphology found in some languages, in particular person indexation, should be reconstructed in the common Sino-Tibetan ancestor or whether it is a later innovation confined to and defining a particular “Rung” subgroup. In this article, we argue that this question is fundamentally a problem of phylogeny, and that the results of recent works on the phylogeny of Sino-Tibetan, supplemented by a more refined investigation of shared lexical innovations, provide support for the idea that person indexation morphology is not a recent innovation and that the languages lacking such a feature are thus innovative.