Introduction PART 1: BUILDING HEALTH Chapter 1: Becoming Unbreakable Chapter 2: The Real Reason You Die Chapter 3: Challenge. Adapt. Thrive. Chapter 4: "Stress" for Strength PART 2: ANCESTRAL CHALLENGES Chapter 5: Muscular Challenges (Adaptations of Strength) Chapter 6: Endurance Challenges (Adaptations of Endurance) Chapter 7: Movement Challenges (Adaptations of Mobility, Agility, and Balance) Chapter 8: Thermal Challenges (Adaptations of Heat Tolerance) Chapter 9: Cold Challenges (Adaptations of Cold Tolerance) Chapter 10: Cognitive Challenges and Psychological Stress (Adaptations of Cognitive Reserve Capacity, Knowledge/Expertise, and Psychological Resilience) Chapter 11: Famine and Food Shortage Challenges (Adaptation of Metabolic Flexibility Chapter 12: Hypoxia/Hypercapnia Challenges (Adaptations of Tolerance to Low Oxygen States) Chapter 13: Ultraviolet (UV) Light, Blue Light, and Red/Near-Infrared Light Challenges (Adaptation of Cellular Detoxification and Antioxidant Defense Systems) Chapter 14: Phytochemical Challenges (Adaptation of Inborn Antioxidant Defense and Detox Systems) Chapter 15: Microbes, Infections, and other Immune System Challenges (Adaptation of Immune Systems and Immune Memory) Chapter 16: Fear Challenges (Adaptation of Courage) Chapter 17: Stress, Discomfort, and Doing Difficult Things (Adaptations of Willpower, and Mental and Physical Toughness) Conclusion APPENDIX: Unbreakable Reserve Capacity Metrics.