Writing in The Quarterly Review of Biology, an international team of researchers bring together archaeological, anthropological, genetic, physiological and anatomical data to argue that carbohydrate consumption, particularly in the form of starch, was critical for the accelerated expansion of the human brain over the last million years, and co-evolved both with copy number variation of the salivary amylase genes and controlled fire use for cooking.
“We propose that plant foods containing high quantities of starch were essential for the evolution of the human phenotype during the Pleistocene,” said the team – led by Dr. Karen Hardy from the Catalan Institution for Research and Advanced Studies at the Autonomous University of Barcelona. “Although previous studies have highlighted a stone tool-mediated shift from primarily plant-based to primarily meat-based diets as critical in the development of the brain and other human traits, we argue that digestible carbohydrates were also necessary to accommodate the increased metabolic demands of a growing brain.”
Hardy and her team build a case for dietary carbohydrate being essential for the evolution of modern big-brained humans with a series of assertions, which demonstrate the importance of carbohydrates.
Firstly, the team noted that the human brain uses up to 25% of the body's energy budget and up to 60% of blood glucose. They noted that while synthesis of glucose from other sources is possible, it is not the most efficient way, and these high glucose demands are unlikely to have been met on a low carbohydrate diet.
They also suggested that human pregnancy and lactation place additional demands on the body's glucose budget and low maternal blood glucose levels compromise the health of both the mother and her offspring.
“Cooked starch, a source of preformed glucose, greatly increased energy availability to human tissues with high glucose demands, such as the brain, red blood cells, and the developing foetus,” argue the team
“Without cooking, the consumption of starch-rich plant foods is unlikely to have met the high demands for preformed glucose noted in modern humans,” they said.
Hardy proposes that after cooking became widespread, the co-evolution of cooking and higher copy number of the salivary amylase (and possibly pancreatic amylase) genes increased the availability of pre-formed dietary glucose to the brain and foetus - which in turn, permitted the acceleration in brain size increase which occurred from around 800,000 years ago onwards.
They suggest that while eating meat may have kick-started the evolution of bigger brains, cooked starchy foods together with more salivary amylase genes made us smarter still.
The team also noted that since the global increase in obesity and diet-related metabolic diseases, has increased interest in ancestral or 'Palaeolithic' diets – the importance of carbohydrate, particular in form of starch-rich plant foods, has been largely overlooked.
Source: The Quarterly Review of Biology
Published online ahead of print, doi: 10.1086/682587
“The Importance of Dietary Carbohydrate in Human Evolution”
Authors: Karen Hardy, et al