Primary sources are the "raw materials" of scholarship. For your research project on your chosen scientific controversy, you'll likely need to identify scientific primary sources and historical (or humanities) primary sources.
Finding Primary Sources:
What you ultimately decide to use as primary source material is highly contextual and depends on both your topic and your approach. Consultation with a librarian and/or your professor is highly recommended. That said, here are some places to get started:
In the sciences, a primary source:
Primary scholarly references are the gold standard for your background research as a scientist. Secondary scholarly literature—review articles, books, encyclopedias, handbooks, etc.—are useful entry points, but shouldn't be used alone. Follow up on the citations you find in secondary sources to get to the primary scholarly references.
Scientific primary literature is peer reviewed, or refereed, before being published. This enhances the quality and validity of the work. In the peer-review process, two or three specialists in the field read and critically evaluate the work before it can be published. Peer review is a quality-control measure to ensure that the primary literature includes only high quality, valid scientific information. Primary authors may revise and resubmit articles to improve them. Secondary literature is less stringently reviewed.
In humanities disciplines, including history, a primary source: