This is an odd sort of book, with on the face of it quite a narrow audience. To properly enjoy it you have to be a) the sort of person who enjoys tales of human endurance, and b) interested in the detail of evolutionary biology and sports physiology. If you DO tick both of those boxes, then this is definitely for you. The author tells tales of his own endurance adventures, including ultra marathons in the Sahara and treks to the Poles, and uses the extreme impacts they had on the bodies of he and his companions to illustrate the theories of evolutionary adaptation that make up the other third of the book. Stroud tells excellent stories, and his own adventures are colourful and vivid. While a little one-sided in places, the science of what kind of animal the human being is and why is also very clearly mapped. While there are small details here and there that are debatable, Stroud is balanced enough to point most of these out as he goes, and at least references the counterpoints even if he doesn't delve into them in the same detail as the perspectives he himself is most interested in. Overall, the book stands well as a fascinating and demonstrated narrative of what the human body is for, and why it so often goes wrong in the modern age.