Survival of the Fattest: Fat Babies and Human Brain Evolution (research summary)

What came first: the ability to build tools, hunt animals, communicate, etc. or the unique size and cognition of the modern human brain?

Researchers Stephen Cunnane and Michael Crawford make a sound case for how and why a shore-based diet and fat babies explain the evolution of the large human brain that led to the sophistication and intelligence to hunt, develop language, and socialize.

Here I summarize their argument.

In order for the brain to expand by nearly one third in under a million years, it must have required an environment that supported the increasing metabolic and nutritional demand to allow for such. By weight, the adult brain consumes more energy than any other organ. In fact, the adult brain accounts for only 2% of total body weight, yet consumes up to 23% of our daily energy requirements. This is even more pronounced in infants whereby the brain accounts for ~11% of total body weight but consumes ~74% of the daily energy requirements!

How could we have supported this massive energy demand? What kind of environmental circumstances would allow for this?

Hunting live animals would certainly explain how we were able to meet the high energy demands of a larger brain. However, this would mean we developed the skills and intelligence to successfully hunt before the brain expansion occurred. Thus, we have a paradox. Which brings us back to what came first: the ability to hunt or the larger brain? Based on this paper, the answer is the larger brain. Cunnane and Crawford suggest that whatever advantage was offered by having a larger brain was not necessary for survival, but was naturally selected for.

But what environmental circumstances allowed us to metabolically afford a larger brain for natural selection to take hold of in the first place? And why was this unique to humans and not other primates?

More fat, more meat, less plants

A high quality diet containing more fat and meat and less plant material seemingly played a crucial role in the evolution of the human brain. This type of diet allowed for higher concentrations of nutrients and energy, since by weight, plant material supplies far less energy than animal products.

The structure of our jaws and teeth, as well as the modern gut indicate humans were not designed for constant grinding, or the digestion and fermentation of large amounts of plant material, respectively. Rather, fossil records indicate that the human diet evolved to require less grinding compared to the diets of other primates.

I think it’s fair to say it wasn’t a plant-based diet of low to moderate energy in the same environments as other primates that got us to the modern brain.

Access to consistent, reliable, and nutritious foods would have been needed for the incremental changes in brain evolution

Expansion of the brain would have required sustained and consistent changes in gene expression, random mutations are very unlikely to explain an increase in blood supply to the brain, due to its increase need for oxygen and nutrients. In addition to genes that promoted the storage of fat in fetuses, since fat is the energy and nutrient insurance as the brain develops. This would have required the mother to be receiving long-term sustainable nutrient and energy intake during pregnancy and lactation.

Plants, animal carcasses, and hunting live animals would not have been sustainable enough to create this consistent change in gene expression that were controlling the architecture of the brain, a diet like this offers too much variability in nutrients and energy. Fruits and nuts can’t explain it either since these foods are seasonal. Therefore, a diet of solely these foods does not support the molecular and genetic requirements needed for brain expansion.

So, we know the expansion of the brain required a lot of energy and nutrients and that these same high quality foods needed to be available over hundreds if not thousands of generations in order to continually express the genes needed for the process to occur. It doesn’t make sense that we would have continued to express these genes if the mother wasn’t consuming enough calories since the developing brain is highly influenced by maternal nutrition.

Now that we’ve covered the environmental circumstances that likely didn’t contribute to the expansion of the human brain, let’s get to what Cunnane and Crawford believe did.

The case for a shore-based diet and the evolution of the human brain

Abundant, reliable, and nutrient dense foods, if sustained long enough, would meet all the requirements mentioned above to allow for a larger brain. A shore-based diet checks all the boxes. Over the long-term, natural selection would have favoured this increased brain size that would lead to improved intelligence, contributing to improved hunting skills, etc.

It is proposed that early hominids took advantage of the abundant food availability on the shores of lakes, rivers, marshes, and seas of East and South Africa. The foods here would have been fresh and saltwater mollusks, crustaceans, birds’ eggs, spawning fish, frogs, turtles, and aquatic plants, all of which would have provided nutrient and energy dense foods. Additionally, these foods are easy to collect, require no cooking, or a well-developed intestine to extract their full energy and nutrient value (unlike roots and tubers). A shore-based diet would not replace hunting and foraging other foods, it would simply make those foods optional.

A shore-based diet provides some of the highest nutrient dense foods available and enough energy to support the growth of the brain. This theory, therefore, makes the argument for the shore-based diet preceding the expansion of the brain, plausible. Lastly, this diet could be accessed by anyone regardless of age, gender or size, and required no skill set.

What does this have to do with fat babies?

Although humans and other non-human primates have similar embryonic potential to have a large brain, human infants have substantially greater amounts of body fat, this is unique to humans. The hypothesis is that the fittest adults were the fattest babies, and therein lies the survival of the fattest.

Baby body fat ensures optimal brain development throughout the first 5 years of an infant’s life. Premature babies do not have adequate fat stores at birth and have an increased risk for slower neurological development and smaller brain size than term babies. It is very clear that the developing brain is very vulnerable to energy and nutrient supply, since body fat accounts for ~90% of a newborn’s weight and serves as an insurance for both energy and nutrients.

Ketones: my favourite part of the story

Body fat does not serve as a direct energy source for the brain, rather ketone bodies are an integral piece of the human-brain-evolution puzzle. See, the brain cannot be fuelled by fatty acids, but it can be fuelled by ketones.

Glucose is the primary fuel for the adult brain, but when food is restricted, within 24hr your glycogen stores are depleted, and ketone production is initiated as an alternative fuel to glucose for the brain. More body fat equals more ketone potential, it’s just a matter of being able to access them (but that’s a story for another day). We can make ketones from stored body fat, or from the fat we eat.

Infants are always in a state of mild ketosis, regardless of their feeding — this is not the case for adults. During pregnancy, ketones are an essential fuel source to the baby’s brain, supplying up to 30% of its energy requirements.

Ketones don’t only supply a source of fuel, they are also a preferred carbon source for the synthesis of major lipid building blocks for developing brain cells (cholesterol and fatty acids, essential to the membranes of the developing nerve connections).

The human brain has the ability to make almost all the saturated fatty acids and cholesterol it needs and excludes most fatty acids (with the exception of the EFAs) and cholesterol present in circulation. In other words, the human brain requires cholesterol and saturated fatty acids in its membranes but does not take them up from the blood. Therefore, it needs access to water-soluble carbon sources to assemble these components, such as ketones.

In infants, the cholesterol and fatty acid synthesis in the brain is indirectly tied to the mobilization and breakdown of fatty acids from stored body fat.

So to wrap it up, ketones play two important roles in the developing brain:

1. Reliable fuel source between feeds
2. Precursors to essential lipid building blocks for proper brain cell development

Humans have the fattest babies (therefore largest ketone reserve) of all mammalian infants, in a proportion that matches the energy and structural demands of the developing brain.

But fat isn’t just simply a fuel source. Besides being a reserve for ketones, it serves other crucial purposes.

Every cell membrane incorporates fatty acids, cholesterol, carbohydrates, and proteins, and the composition of such is very highly regulated. Our cell membranes control our very being — digestion, respiration, heart beat, etc. The membranes in mammalian brains have a lot of electrical activity, and also contain higher proportions of DHA; humans have an increased density of connections between neurons to account for.

At birth, the amount of DHA in our body fat is the highest than at any other time in our life. Born at term, the supply of DHA in baby’s fat is enough to support the brain’s DHA requirements for the first ~3 months of life, regardless of how much DHA is in the mother’s diet or breast milk.

Therefore, body fat not only supplies ketones to the brain for fuel and lipid synthesis, but it also provides specialized fatty acids such as DHA for healthy brain cell membranes. This is unique to humans; no other mammal has this amount of body fat and thus lacks this fuel and DHA insurance for their young. It should all be making sense how fat babies could have led to the expansion of the human brain, since other primates lack this metabolic prerequisite for the brain to expand.

Land plants contain no DHA, and with the exception of the brain, animal tissue provides very little if any. However, aquatic foods such as shellfish and fish are rich sources of DHA. A shore-based diet would have provided DHA and other nutrients to support the developing brain, available to the infant brain from both body fat and in mother’s milk.

Today, storing body fat is an inconvenience but it may be something to be thankful for

In order to store body fat, we require the appropriate enzymes to take glucose and fatty acids from circulation and build fat molecules (triglycerides) to deposit in fat cells. We also need excess energy (calories). The storage of excess carbohydrates and protein is not sufficient for survival in face of food scarcity. Being able to store body fat was crucial to our survival. In fact, infants (born at term) have enough fat to sustain them for 3 weeks if necessary.

Acquiring the ability to store body fat during pregnancy would have required access to a diet of both high energy and high nutrients for a sufficient amount of time to allow for consistent gene expression that would promote fat storage. A shore-based diet supports these requirements, since as noted earlier, they are found in abundance, easy to access, and have high energy and nutrient value. They also require far less energy expenditure, which in combination with being more energy-dense, would have facilitated the accumulation of fat.

Early in evolution, storing body fat would not have offered any immediate benefit. However, in time, the babies that were fattest would have had the prerequisites essential to the human brain expansion, improved cognition, and eventually naturally selected for.

The proof is in the pudding… or the fossil records.

Fossil records also link hominid evolution to lake- and seashores. The exact types of foods available would have been shellfish, crustaceans, turtles, frogs, fledglings, molting waterfowl, and eggs. Marsh plants, too, however there are no fossil records to confirm this. Humans exploited these rich and mostly immobile foods, and as their skills improved, fishing, hunting, and cooking less nutrient dense foods (e.g. tubers and roots), would have supplemented the diet. Fishing would have required skill and tools, therefore it was likely optional. However, as with hunting, was something we would learn to do with the expansion of our brain and improved cognition.

Hunting, seasonal fruits, nuts, roots, and even insects were not eliminated from the shore-based diet, rather reliance on these foods exclusively would have been insufficient to have permitted the expansion of the brain. If this were true, similar evolution would have taken place in other non-human primates.

In addition…

DHA was not the only nutrient supplied by a shore-based diet. Other important nutrients such as iodine, zinc, copper, iron, and selenium, in greater quantities than found in most inland foods and likely were required for successful brain expansion.

Iodine itself plays a very important role in energy expenditure and metabolism and its deficiency can cause mental retardation and infertility. Iodine-rich foods therefore appear to have been a logical requirement for our evolution and population growth. It is very difficult to assimilate iodine from plant foods given their goitrogenic properties (prevent iodine uptake in the thyroid gland). It is very possible that as early humans started to move inland, we started developing iodine deficiencies and this could have contributed to the unexplained decline in human brain size over the past 25,000–90,000 years.

A shore-based diet supports the evolution of fat babies and large brains — traits that are unique to humans — by:

• Providing a food supply that is reliable, abundant, and accessible
• Offering little competition by other primates
• Offering little predation by other carnivores
• Requiring little energy expenditure to gather food
• Requiring less time gathering food which allowed for more time to develop tools, language, and improve social interaction
• Offering a better opportunity for the expression of genes promoting fat storage in the human fetus
• Being accessible to anyone regardless of age, sex, or size
• Providing a source of key nutrients needed for proper brain development

In this regard, it was the shore-based diet that allowed the fattest babies to become the mentally fittest adults. The larger brain therefore preceded the intelligence to hunt, fish, develop language and social skills, the list goes on.

The short story

Humans have uniquely larger brains and give birth to the fattest babies compared to any other primate. Cunnane and Crawford propose that fat babies were key to the evolution of the modern human brain. The survival of the fattest explains how the fittest adults with the greatest mental acuity were once the fattest babies, and that natural selection selected for the advantages offered by fat babies (ie. increased brain size and intelligence). Baby body fat serves as a reserve for fatty acids, which in addition to being a fuel source, provided the precursors for ketone bodies, which in turn provided the precursors for brain lipid synthesis. Body fat also provided a source of DHA required for proper brain development. The unique circumstances that were needed for the expansion of the human brain can be explained by a shore-based diet which could provide energy and nutrients in abundance to evolve the ability to store body fat and support the growth and development of the brain. The human brain has a high energy demand, requires synthesis of its own cholesterol and saturated fatty acids (since it can’t import these from the blood), and depends on the specialized fatty acid DHA. Living next to lake-, river-, and seashores provided the environmental circumstances needed to evolve the fat babies needed for human brain expansion and evolution.