The digestive abilities of anatomically modern humans, however, are different from those of Paleolithic humans, which undermines the diet's core premise. During the 2.6 million year-long Paleolithic era, the highly variable climate and worldwide spread of human populations meant that humans were, by necessity, nutritionally adaptable. Supporters of the diet mistakenly presuppose that human digestion has remained essentially unchanged over time.
Fresh fruits, fresh vegetables, fresh meat—the paleo diet is all about eating foods straight from the Earth just as our ancestors did. Those ancestors didn't have livestock or crops to call their own, so Cordain advises to go with grass-fed and organic varieties whenever possible to limit exposure to pesticides, antibiotics, and other chemicals that didn't exist back then. Research from Emory University suggests that Paleolithic people obtained about 35% of their calories from fats, 35% from carbohydrates, and 30% from protein.
CrossFit is promoted as both a physical exercise philosophy and a competitive fitness sport, incorporating elements from high-intensity interval training, Olympic weightlifting, plyometrics, powerlifting, gymnastics, girevoy sport, calisthenics, strongman, and other exercises. It is practiced by members of over 13,000 affiliated gyms, roughly half of which are located in the United States, and by individuals who complete daily workouts (otherwise known as "WODs" or "workouts of the day"). CrossFit has been criticized for allegedly causing people to suffer from unnecessary injuries and exertional rhabdomyolysis.