Charles Darwin’s 1871 book The Descent of Man described rudimentary structures like body hair, wisdom teeth, and the tailbone. In 1893 anatomist Robert Wiedersheim went much further and listed 86 human organs as wholly or partly functionless, including the pineal gland, thymus, and pituitary, whose roles were not yet understood. That outlook fed the focal infection theory, and a 1916 journal reported removing an appendix to cure facial nerve pain. This episode tours the body’s genuine leftovers and the ones that turned out to matter, starting with the appendix as a safe house for gut bacteria that has evolved independently at least 32 times in mammals.

Softer cooked food shrank human jaws while genes such as PAX9 kept the tooth count, leading to impacted wisdom teeth. Every embryo grows a tail around days 31 to 35 before cells remove it. Thin ear muscles once swiveled the ear, the neck’s platysma is a remnant of the skin muscle horses use to flick off flies, goosebumps are left from fur, and newborns can grip hard enough to hang their own weight. The episode ends with the broken vitamin C gene GULO and hiccups traced to the way tadpoles gulp water past their gills.

• Wisdom tooth agenesis is almost zero among Tasmanian Aboriginal people and nearly 100 percent in some Indigenous Mexican populations
• The embryonic tail reaches about a sixth of the embryo’s length, and rare babies are born with 12 centimeter pseudo tails
• A 1932 study found over 37 percent of infants could support their body weight hanging from a rod
• The vomeronasal organ appears in over 90 percent of adults but has no working neurons, and its TRPC2 gene is a pseudogene
• Premature infants spend about 2.5 percent of their time hiccuping