How did our Solar System, in general, and the Earth, in particular, form? Long inaccessible, the answer to this fundamental question is encoded in the very isotopic composition of meteorites: i.e., the remnants of Solar System and planetary formation. Decoding these telltale signatures provides a means of reconstructing the genealogy and evolutionary pathway of planetary bodies, including the Earth. Yet, despite this formidable potential, the planetary ancestors of the Earth have still not been identified. Here, I propose to leverage novel insights and next-generation instrumentation to conduct the first successful search for Earth’s missing building block. This ambitious program will simultaneously locate the as yet unrecognized planetary ancestors of the Earth, and harness the untapped potential of a new isotopic tracer to illuminate, in an entirely new way, (i) the architecture of the early Solar System, and (ii) the cosmic origins of the Earth, and its water.