Originally Posted by Scott Hanson
Why would anyone consider comparing diets of different species more relevant than comparing diets within different populations of a species (ie, "civilized" humans to modern hunter-gatherers)?
To make an analogous comparison of a generally omnivorous family, look at the bears (different species within a taxonomic family, similar to humans and chimpanzees). The diets of bears range from purely herbivorous (pandas), to largely herbivorous (inland grizzlies, black bears) to purely carnivorous (polar bears). Does the diet of any one of these species imply that the diet is optimal for another?
Species are adapted to their environment and its food resources. To conclude that humans (successfully adapted to virtually every ecosystem on earth) should model our diets on apes (a group of animals inhabiting exclusively tropical forest habitats) defies logic and evolutionary adaptation.
Your referenced author (Milton) acknowledges the role of animal food sources in human evolution here:
You're correct in saying that "species are adapted to their environment and its food resources" and that's why it's important when discussing the optimal diet for our species that we take into account the fact that our basic anatomy and physiology was established in Africa ~200,000 years ago. We evolved as a species that was strongly herbivorous with some opportunistic consumption of meat and only became efficient hunters (as opposed to scavengers) long after our basic physiology and nutritional requirements were established and to quote Katherine Milton "although humans can thrive on a diversity of diets, we know of few specific genetic adaptations to diet in our species.
* Therefore when attempting to determine the optimal diet for our species it seems reasonable that we study the diets of both contemporary African hunter-gatherers and our closest genetic relative the chimpanzee.
*Source: Hunter-gatherer diets—a different perspective.