  Many innovations are inspired by past ideas in a non-trivial way. Tracing
these origins and identifying scientific branches is crucial for research
inspirations. In this paper, we use citation relations to identify the
descendant chart, i.e. the family tree of research papers. Unlike other
spanning trees which focus on cost or distance minimization, we make use of the
nature of citations and identify the most important parent for each
publication, leading to a tree-like backbone of the citation network. Measures
are introduced to validate the backbone as the descendant chart. We show that
citation backbones can well characterize the hierarchical and fractal structure
of scientific development, and lead to accurate classification of fields and
sub-fields.
