The data for Cordain's book only came from six contemporary hunter-gatherer groups, mainly living in marginal habitats. One of the studies was on the !Kung, whose diet was recorded for a single month, and one was on the Inuit. Due to these limitations, the book has been criticized as painting an incomplete picture of the diets of Paleolithic humans. It has been noted that the rationale for the diet does not adequately account for the fact that, due to the pressures of artificial selection, most modern domesticated plants and animals differ drastically from their Paleolithic ancestors; likewise, their nutritional profiles are very different from their ancient counterparts. For example, wild almonds produce potentially fatal levels of cyanide, but this trait has been bred out of domesticated varieties using artificial selection. Many vegetables, such as broccoli, did not exist in the Paleolithic period; broccoli, cabbage, cauliflower, and kale are modern cultivars of the ancient species Brassica oleracea.
Greg Glassman and Lauren Jenai founded CrossFit, Inc. in 2000. The company was conceived a few years earlier, in 1996, as Cross-Fit. The original CrossFit gym is in Santa Cruz, California, and the first affiliated gym was CrossFit North in Seattle, Washington; there were 13 by 2005, and today there are more than 13,000. Coaches associated with CrossFit include Louie Simmons, John Welbourn, Bob Harper, and Mike Burgener.