Did Starch Double Early Human Brain Size?

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Evolution Changes From Diet

An evolutionary shift that has puzzled archaeologists for quite some time is the fact that early humans acquired a gene to produce a significant amount of starch-digesting enzymes conspicuously around the same time that our brains doubled in size, birth rates rose, the digestive tract size shrunk, tooth structure changed, and we grew taller and heavier. How did this happen?

All of these changes would have required a huge boost in nutrient availability, and a shift away from a high-quantity, low-nutrient-dense diet to a lower-quantity, higher-nutrient-dense diet. It has been argued that these evolutionary changes were the resultant of a meat-eating diet, but there are questions as to whether a meat-eating diet could have provided the energy demands of a bigger body and brain. The brain requires glucose, and while there is stored glycogen in meat, there are doubts that the supply of meat was regular enough to supply the brain with the fuel to double in size.

Many of these evolutionary changes started around 2 million years ago, with the brain fully doubling in size around 500 thousand years ago. Early humans were not skilled hunters, and the quantity of meat needed to consistently deliver the higher energy demands of the brain is questionable. Meat started being hunted and cooked around the same time, 500,000 to 1,800,000 years ago. (1)

There is no doubt that the availability of hunted, cooked meat had an influence on the brain size increase, but was it enough to fuel the bigger brain?

These evolutionary changes, 2 million years ago, occurred around the time that Africa became predominantly open space grasslands – making seeds, grains and tubers more readily available to gather. (1,2)

In fact, in one study, ancient humans were thought to be able to gather enough wheat berries or barley seeds in just 2 hours to feed them for an entire day. (2)

The onset of cooking is by no means set in stone with evidence dating back to 1.8 million to 500,000 years ago. (1)

Cooking may have played the most important role in supplying evolving humans with the energy they needed to support their growing brain and body size. The onset of cooking is certainly not set in stone, but there is evidence dating back from 500,000 to 1.8 million years ago. (1) Cooking the tubers, grains and meat would make the food easier to eat, require less chewing and thus less energy expenditure, and deliver a highly nutrient-dense meal that was required to support the budding modern human.

The Elephant in the Room

Around this same time that cooking became prevalent, Africa became blanketed in fields of grasses, like wheat and barley, and early humans figured out how to successfully hunt, something else happened…

Early humans acquired a NEW GENE that allowed them to make their own amylase enzyme! This enzyme breaks down starch. Today, a deficiency of this enzyme is linked to a wheat allergy called “Baker’s Asthma.” (1)

Cooking tubers, seeds and other starchy, plant-based foods release stored amylose (starch sugar) that would otherwise stay bound up in the fibrous cellulose of the plant. Cooking may have released the starchy sugars the brain was waiting for. So, instead of chewing raw roots and gobbling excessive amounts of fibrous veggies, cooking delivered the starchy sugar in levels the early human brain never experienced before. Around the same time, some 1 million years ago (although this date is still unknown), early humans acquired this new gene to make their own amylase–clearly to digest the new high-starch diet. (1)

Such structural evolutionary changes could have only been made with a new, high energy fuel supply that likely combined: The starchy sugars from cooked and more easily digested grains, tubers and meat, AND the development of skills like fishing and hunting, which delivered the omega-3 fatty acids (along with seeds). These high-quality meat proteins and glucose are just what early humans needed to double the size of their brains, shrink the digestive tract, grow taller, heavier and develop new tooth structures.

References

  1. http://www.jstor.org/stable/10.1086/682587?seq=1#page_scan_tab_contents
  2. http://archive.unews.utah.edu/news_releases/a-grassy-trend-in-human-ancestors-diets/

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