References of References: How Far is the Knowledge Ancestry

Scientometrics studies have extended from direct citations to high-order citations, as simple citation count is found to tell only part of the story regarding scientific impact. This extension is deemed to be beneficial in scenarios like research evaluation, science history modelling, and information retrieval. In contrast to citations of citations (forward citation generations), references of references (backward citation generations) as another side of high-order citations, is relatively less explored. We adopt a series of metrics for measuring the unfolding of backward citations of a focus paper, tracing back to its knowledge ancestors generation by generation. Two sub-fields in Physics are subject to such analysis on a large-scale citation network. Preliminary results show that backward citation generations bear some resemblance to forward ones in the macroscopic aspect, but they do behave differently in several specific details. Citations more than one generation away are found to be still relevant to the focus paper, from either a forward or backward perspective. Yet, backward citation generations are generally smaller in the size of networks but higher in topic relevance to the paper of interest. This is implicational for recommending citations in tasks of searching related literature but further research is needed regarding this question.