Bohannon details Captain Griest’s grueling training at the Army Ranger School in Dahlonega, Georgia, in 2015. She explains that male individuals are supposedly much stronger, durable, and faster than female individuals and that most of the greatest Olympic athletes and military soldiers have been men. Because Captain Griest is a woman, Bohannon asks why she was doing this extremely treacherous training program before answering that while their ancestors used to dwell in trees, to survive the changing world, they needed to adapt by becoming bipedal.

She traces bipedalism back to Ardipithecus ramidus, or “Ardi,” whose skeleton was found in Ethiopia and who is estimated to have lived three to four million years ago. She states that Ardi is the reason that female individuals commonly have foot and knee problems. One major reason for this is the corpus luteum’s release of relaxin before a person’s period, which is meant to make the ligaments in the body more flexible. Ardi also had an S-shaped curve in her spine, which is why so many female individuals are prone to lower back pain, especially during pregnancy.

Bohannon explains that female individuals tend to have less upper-body muscle mass, which made Captain Griest’s training even more challenging.