Trace every living person’s mother’s mother’s line far enough and all eight billion threads meet in one woman who lived in East Africa during the Pleistocene, somewhere between 100,000 and 230,000 years ago, with 2013 sequencing studies pointing to around 160,000 years or a window of 99,000 to 148,000. She was not the only woman alive. The population never fell below the tens of thousands, and her line survived by chance as every other maternal line eventually ended, whether through women with no children or only sons. The episode explains how mitochondrial DNA, passed almost unchanged from mother to child, lets scientists count mutations like ticks of a molecular clock.

In 1987 Allan Wilson, Mark Stoneking and Rebecca Cann published a study in Nature of mitochondrial DNA from 145 women plus two cell lines, finding the oldest and most diverse branches were African, strong support for the out of Africa model. Wilson preferred the name lucky mother; the nickname Eve came from Roger Lewin in Science and spread to magazine covers. The episode then shows why the title can move: forward when a branch dies out, backward when an older lineage turns up, as happened to Y-chromosomal Adam with the discovery of haplogroup A00, and why the most recent common ancestor of all living humans is far more recent.

• Mitochondrial DNA mutating five to ten times faster than nuclear DNA, about one change every 3,500 years
• A 1990s study of the Romanov royal family that challenged the clock’s speed
• Young Earth creationists who first welcomed the Eve headlines, and why the data says otherwise
• Why Mitochondrial Eve and Y-chromosomal Adam almost certainly never met
• Models placing everyone’s shared ancestor as recently as 5,000 years ago, and why Tasmania complicates that