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A recent study proposes that ancient hominids cooked and ate starchy tubers,
like the manioc pictured above being harvested with a digging stick by homo erectus.
The scientists' theory is based on the glucose needed by a large brain and available from starches.
Image Credit: MouseWorks

About that Paleo diet, eat some carbs!

Understanding how and why we evolved our large brains is one of the most puzzling issues in human evolution. Widely accepted is that brain size increased with changes in diet including meat and the introduction of cooking.

However, in a new study published in The Quarterly Review of Biology, Dr. Karen Hardy and her team combine data from archaeology, anthropology, anatomy, physiology and genetics to argue that eating carbohydrates, particularly starchy vegetables, along with cooking over fire, were critical to the expansion of the human brain. The entire article can be read online.

With a global increase in obesity and diet-related metabolic diseases, interest has intensified in replicating our ancient or 'Palaeolithic' diet, for better health. It is assumed that our physiology is the result of the nutrition we ate during our evolution. But up until now, it was thought cooked animal meat contributed to our brain increase through increased amounts of easily digested protein. The importance of carbohydrates had largely been ignored, particularly starchy plants.

Hardy's team however, builds a case for a carbohydrate rich diet as the true essential nutrient for evolution of our big-brains — based on the following:

(1) The human brain uses up to 25% of the body's energy and up to 60% of blood glucose. High glucose demands were not likely to have been met on a low carb diet of plants and fruits.

(2) Human pregnancy and lactation make added demands on human glucose. Low maternal glucose levels negatively affect mother and baby.

(3) Starches were easily available to ancient hominids in tubers (plants with enlarged stems or roots that store nutrients), as well as in some seeds, some fruits and nuts.

(4) Raw starches are poorly digested in humans. But cooked, they lose their crystalline structure and become much more easily digested.

(5) Salivary amylase genes typically average about 6 copies in humans, but in other primates only 2. These genes produce enzymes to change starch into sugars. Geneticists suggest our salivary amylase genes multiplied at least 1 million years ago.

Hardy proposes that after cooking became widespread, our higher number of salivary amylase genes (and possibly pancreatic amylase genes as well) increased glucose being sent to fetal brains — and eventually perpetuated throughout hominid communities. These diet changes permitted our brain to grow larger at least 800,000 years ago, and possibly earlier.

So, eating cooked meat may have kick-started the evolution to bigger brains, but cooked starchy foods — together with our increase in salivary amylase genes — made us smarter still.

Abstract
We propose that plant foods containing high quantities of starch were essential for the evolution of the human phenotype during the Pleistocene. 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. Furthermore, we acknowledge the adaptive role cooking played in improving the digestibility and palatability of key carbohydrates. We provide evidence that 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 fetus. We also highlight the auxiliary role copy number variation in the salivary amylase genes may have played in increasing the importance of starch in human evolution following the origins of cooking. Salivary amylases are largely ineffective on raw crystalline starch, but cooking substantially increases both their energy-yielding potential and glycemia. Although uncertainties remain regarding the antiquity of cooking and the origins of salivary amylase gene copy number variation, the hypothesis we present makes a testable prediction that these events are correlated.

Karen Hardy, Jennie Brand Miller, Katherine D. Brown, Mark G. Thomas, and Les Copeland. "The Importance of Dietary Carbohydrate in Human Evolution." The Quarterly Review of Biology: September 2015.

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