A primary source in biological research refers to original, firsthand evidence or data produced during an investigation. Examples include original research articles, dissertations, and raw data sets, which present the methodologies and results directly from the researchers. Primary sources are crucial for advancing scientific knowledge, allowing other scientists to verify findings, replicate studies, or build upon prior research. In contrast, secondary sources, such as reviews and meta-analyses, summarize or interpret primary data and do not provide original evidence.