In a Chinese farming village in 1248, the investigator Song Ci solved a sickle murder without a single interrogation. He had every villager lay a sickle in the sun and waited until flies gathered on the one blade that still carried traces of blood. That scene opens this history of forensic science, the long effort to replace forced confessions and faulty memory with a witness that cannot lie. The story moves through rice powder ordeals that exploited a dry mouth, a torn newspaper wad that convicted John Toms in 1784, and the arsenic test James Marsh published in 1836.

From there the episode tracks the race to identify people: Alphonse Bertillon’s body measurements, the rise of fingerprinting through William Herschel, Henry Faulds, and Francis Galton, Edmond Locard’s rule that every contact leaves a trace, and Alec Jeffreys’s discovery of DNA profiling in 1984. It also faces the failures. Bullet lead analysis and bite mark comparison fell apart under scrutiny, crime labs have fabricated evidence, and genealogy databases now raise hard questions about privacy.

• Marsh built his test after a jury acquitted an accused poisoner in 1832 because the yellow precipitate proving arsenic had faded before it reached the courtroom.
• In Argentina in 1892, a bloody thumbprint on a door frame identified Francisca Rojas as the killer of her two sons after she had blamed a neighbor.
• Police surgeon Thomas Bond produced the first recorded offender profile while reviewing the Jack the Ripper autopsies.
• The first criminal use of DNA cleared Richard Buckland, who had confessed. A sweep of more than 4,000 local men then led to Colin Pitchfork.
• The FBI dropped comparative bullet lead analysis in 2005, and in 2009 the Supreme Court ruled in Melendez-Diaz v. Massachusetts that lab analysts must testify and face cross-examination.