It is two in the morning in a police precinct, and a shaken witness is asked to describe a stranger’s nose. Most people cannot do it, and this episode explains why. For decades the facial composite depended on a skilled sketch artist, a luxury most small departments never had. The fix was the mechanical kit: Identikit arrived in the United States in 1959, and Jacques Penry’s Photofit followed in the UK in 1970. Officers and witnesses stacked transparent sheets, each printed with a single nose, mouth, or pair of eyebrows, until a face appeared.

The trouble is that the brain does not store faces as parts. People remember the whole impression and the spacing between features, a point the Thatcher Effect makes vivid. Desktop programs in the 1990s only digitized the same mistake, with endless drop-down menus of eyes and chins. The real break came in the 2000s from university researchers in the UK and South Africa, whose evolutionary systems show a witness a grid of whole faces and breed the best matches together, generation after generation.

• Archaeologists use the same principles to reconstruct the faces of ancient mummies from skeletal remains.
• In a 2012 police field trial, EvoFIT composites led to an arrest and conviction in 29 percent of cases.
• An EvoFIT image drawn from a victim’s memory led directly to the arrest of Manchester serial rapist Asim Javed.
• Television shows such as America’s Most Wanted and Crimewatch turned millions of viewers into a search party for sketched faces.
• The 1995 John Doe Number One sketch of Timothy McVeigh and the smirking 1971 portrait of D.B. Cooper became cultural icons, records of a witness’s fear as much as a face.