Final answer:
A scientist observing shared physical characteristics between an extinct species and a contemporary one can likely conclude that they share an evolutionary relationship, implying the extinct species may be an ancestor of the living one.
Step-by-step explanation:
The scientist studying the physical characteristics shared between the extinct Species X and the living Species Y can most likely conclude that Species X is an early ancestor of Species Y. This inference is based on the principle that morphological (physical), biochemical, and genetic traits that are shared across species, known as homologous traits, can be used to reconstruct evolutionary histories.
By comparing the anatomy, DNA, and other features of fossils and living species, scientists can assess relatedness and common ancestry. The fossil record supports the idea of evolution and shows that over millions of years, species have evolved and adapted to their environments. Therefore, the shared characteristics between the fossils of Species X and the living Species Y suggest an evolutionary relationship rather than interspecies mating or one species outcompeting the other.