A "common ancestor" refers to an individual from which two or more species or groups have descended. This concept is fundamental in the study of evolution, as it helps scientists understand how different species are related through time. For example, humans and chimpanzees share a common ancestor that lived millions of years ago. The idea of a common ancestor is supported by genetic evidence, which shows similarities in DNA among related species. By tracing these genetic links, researchers can reconstruct evolutionary histories and better understand the relationships between various organisms, including birds, reptiles, and mammals.