Common descent is the scientific theory that all living organisms share a common ancestor. This means that different species have evolved over time from a single or a few ancestral forms. The concept is a fundamental principle of evolutionary biology and helps explain the diversity of life on Earth. The idea of common descent is supported by various lines of evidence, including fossil records, genetic similarities, and comparative anatomy. These elements show how species are related and how they have changed through natural selection and other evolutionary processes over millions of years.