Was meat the key to humans’ evolutionary success? A new report suggests that meat was critical in allowing the increase in brain size that is the central feature of our humanity.
Similarly, Caleb Finch and others have suggested that meat eating was important in the evolution of human longevity. It is thought that the common ancestor of humans, gorillas, and chimps had a largely vegetarian diet. Finch has argued that the transition to meat eating prompted the evolution of long life span in humans (we are the longest lived mammal, as well has the animal with the biggest brain in relation to body size). We will explore this idea further on this blog post.
The hypothesis that meat eating co-evolved with large brain size and long life span in humans raises a few questions. Eating meat, at least in the modern context, has been linked epidemiologically with chronic inflammatory diseases, including obesity, hypertension, and atherosclerosis. Epidemiologist Dariush Mozaffarian convincingly showed that meat eating is associated with weight gain. Processed meat appears to be particularly dangerous. Spam (the edible kind) was associated with diabetes among Native Americans. Perhaps because of these relationships, meat eating is associated with a shortened life span among modern humans. Along these lines, a recent publication linking red meat to early death received much recent press, such as this article in the New York Times.
So, is meat good for us or bad for us? Another evolutionary concept, namely co-evolution of host and microbiota, should be taken into account when considering these questions. Evidence suggests that diets high in saturated fat, common in meat, alter the composition of the gut microbiota in ways that are associated with obesity and the metabolic syndrome. Hardly a recipe for health! In addition, saturated fat directly induces pro-inflammatory changes in immune cells, muscle cells, fat cells, and intestinal cells. These changes may explain why consumption of saturated fat in meat has been linked with premature mortality, cardiovascular disease, malignancy, and diabetes. Why do commonly encountered nutrients in meat activate costly and injurious signaling pathways? Good question, one that my co-authors Chris Kuzawa and Melissa Franklin explore in an upcoming article in the Quarterly Review of Biology.
The bottom line is this: the immune system treats some nutrients and micronutrients as danger signals, akin to danger-associated molecular patterns (DAMPS). These nutrient DAMPS anticipate adverse changes in the gut microbiota and trigger defensive changes, including a mobilization of immune resources.
Whether of not this viewpoint is correct – that nutrient signaling can anticipate dangerous changes in gut microbes – the assertion that meat eating was a key innovation during human evolution (a beneficial one) is challenged by the molecular evidence suggesting that nutrients in red meat cause damage. Can these ideas be reconciled? Meanwhile, it is clear to this writer that a diet rich in red meat is an unhealthy diet. Follow up studies are needed to flesh out (sorry!) what these epidemiological studies mean to humans and our evolution. Perhaps time will tell.
And in case you were wondering, I am not a vegetarian! For students of the UNM course in Evolutionary Medicine, stay tuned: We will discuss the evolutionary context for the body’s responses to diet at length this Fall.