Human evolution may have been influenced by dietary practices.  Big brains may have advantages, but they also make outsized demands for energy.  Several factors, including increasing the amount of meat in the diet or adopting the practice of cooking foods, may have made larger brains feasible evolutionarily.  As hominid brains grew over time, gut sizes decreased, perhaps reflecting dietary alterations and improved efficiency of nutrient extraction yielded by cooking (1).  The cooking-big brain hypothesis is intriguing, but tentative because it is unclear how far back in time our ancestors began to control fire or use it for food preparation (1).

Ideas as to how evolutionary change is driven are undergoing radical revision.  A growing body of evidence suggests the evolution of multicellular plants and animals is influenced strongly by their microbiomes, the populations of microorganisms found on or within them (2).  The collective activities of the microbiome are essential to our health and well being and while these vital partnerships are stable, they are not necessarily eternal.  Humans have been actively altering the microbial ecology of their internal and external environments by changing dietary practices, animal husbandry, agriculture and medical procedures (3).  The net effect has been a clear decrease in the species diversity of our microbiomes (3, 4).  This correlation has led to the hypothesis that the consequences of losing some of our microbial partners may be surges in chronic diseases, allergies and obesity. 

Our intestinal microbiomes react quickly to environmental factors such as dietary inputs (4, 5).  While it has long been appreciated some gut bacterial populations are healthful, evidence is also building that perturbing conditions may allow destructive communities to become established.  For example, antibiotic treatments sometimes produce catastrophic bowel disease due to the abnormal growth of the bacterium Clostridium difficile.  It is now being recognized that the impacts of gut microbe metabolic activities reach all the way to the brain (6).  Epidemiologists have noted diets high in saturated fats and sugar are linked to higher risk of Alzheimer’s disease development (7).  Several studies support the idea that gastrointestinal bacteria play a (still mysterious) role in promoting Parkinson’s disease pathology (8).  Perhaps certain neurological diseases reflect the fact that we are not properly caring for our microbial partners.      

What our ancestors ate long ago may have been a key factor promoting the emergence of big brains.  What we choose to eat today influences how those magnificent brains function.   

‘Would you like fries with that?’

The next time you hear that question take a moment to ponder its true significance. It might be the sound of evolution in progress. 


(1) J. Adler. 2013.  Why Fire Makes Us Human.  Smithsonian Magazine, June 2013.

(2) E. Rosenberg and I. Zilber-Rosenberg. 2016.  Microbes Drive Evolution of Animals and Plants: The Hologenome Concept.  mBio 7(2):e01395-15.

(3) A. Coghlan.   Microbial Mass Extinctions Were Kicked Off by Human Evolution.  New Scientist Daily News, 20 June 2016.

(4) A. Gibbons. 2016.  Microbes In Our Guts Have Been With Us for Millions of Years.  Science, 21 July 2016.

(5) N. Voreades, A. Kozil and T. L. Weir. 2014.  Diet and the Development of the Human Intestinal Microbiome.  Frontiers in Microbiology, 22 September 2014.

(6) D. Kohn. 2015.  When Gut Bacteria Change Brain Function.  The Atlantic, 24 June 2015.

(7) The Alzheimer’s Association. Prevention and Risk of Alzheimer’s and Dementia.

(8) D. Erny and M. Prinz. 2017.  Microbiology:  Gut Microbes Augment Neurodegeneration.  Nature 544:304-305, 20 April 2017.