About this book
Half an Eye is a free, openly licensed book on how eyes evolved. This page explains who made it, how AI was used, how every claim is checked, and how you may reuse it.
What this book is
Half an Eye is a free online book about how eyes evolved, from light-sensitive patches of skin to camera eyes and compound eyes. It is written for readers who want the evidence and its sources in one place, and who need a page they can link to with confidence. The chapters set out the science. A separate part, Claims and responses, takes common claims that the eye could not have evolved and answers each one, linking back to the chapters that hold the evidence.
The book is published chapter by chapter. A page goes live only after it has been fact-checked, so the contents list shows some chapters as “coming”.
How it was made
The research and first drafts were done by Claude, an AI model made by Anthropic, working to written rules set by the editor. The editor, Paul Thewlis, checks every page against its sources and approves it before it is published.
AI models can misread a paper, or cite a reference that does not exist. The process below is designed to catch that. Each page passes five stages before it appears here:
- Research. Every page starts as a research dossier: a numbered list of claims, each tied to a source that has been read, not just found. The dossier records which part of the source was read, such as the abstract, the results or a figure.
- Draft. The page is written only from the approved dossier. A sentence that cannot be traced to a source is cut and listed as unsourced, not published.
- Fact-check. The editor checks the claims against their sources.
- Figures. Each figure is planned in a visual brief and checked against the source it illustrates.
- Approval. The editor approves the finished page for publication.
The rules for sources
Every factual sentence carries a numbered note. Each note points to one source, with a DOI for a paper or a printed page number for a book, and every source is listed in the bibliography.
- Primary research first. Peer-reviewed papers and reviews are the default source. Academic books, chiefly Land and Nilsson’s Animal Eyes and Schwab’s Evolution’s Witness, are used for established facts and cited by printed page.
- Journalism finds papers; it is never the source. Press releases and science journalism are used only to find the original paper. Wikipedia, blogs, videos and AI summaries are never cited.
- Numbers as published. Figures, units and sample sizes are copied exactly as the paper gives them. Where the text rounds a number, it says “about”.
- Settled or not, said plainly. Each claim is labelled as established, supported by one or two strong papers, or a hypothesis. Hypotheses are written as hypotheses, and for any claim that is not yet settled we look for papers that disagree and report the debate.
- Retractions checked. Sources are checked for retractions and corrections.
- Quotes kept short. At most one short quote from any source, with its page.
Creationist and intelligent-design writings are cited only in Claims and responses, and only to quote a claim accurately.
How figures are made
Diagrams and charts are drawn for this book as vector graphics by Claude. Each diagram is redrawn from a named figure in a cited paper or book and checked against it, and its caption names that figure. Charts use only numbers from the page’s sources, with units shown on every axis.
Photographs of animals and fossils are never made, only sourced: from Wikimedia Commons, iNaturalist, open-access papers and the Biodiversity Heritage Library, under licences that allow reuse. The species in each photo is confirmed from its source page. No AI image generator is used for any image in the book. Every image is credited on the Figure and media credits page, with its author, licence and source.
Corrections and changes
Every page shows when it was first published and when it was last revised, and every change is recorded in the changelog. When new research changes the picture, the relevant page is revised. Chapter and section numbers never change once published, so links and citations keep working. Each page has a “Cite this page” box with a ready-made citation.
Licence
The text of Half an Eye is published under the Creative Commons Attribution 4.0 International licence (CC BY 4.0). You may copy, share and adapt it, including commercially, as long as you credit Half an Eye, link to the licence and say if you made changes.
Photographs and some figures are not covered by that licence. Each carries its own licence, shown on the Figure and media credits page, and some require you to share adapted versions under the same terms.