The Brain-Body Paradox

As we continue on our journey through Part I of “Your Brain on Meat” in the Brain Food Series, we run into a paradox – The Brain-Body Paradox.

In the last section we saw how the brain expanded from the modest beginnings of our hominid ancestors to its enormous size which (as we’ll see in Section 3) represents the single greatest distinguishing factor of humans from all other species.

We saw how selective pressures coupled with a persistent feedback loop between climate, food sources, and the advantages of a large brain underpinned our stunning divergence from our primate ancestors and the transition from tree-dwelling herbivore to bipedal meat-eater. (r, r, r)

And while a big brain came with advantages such as fashioning ever-complex hunting tools and coordinating larger groups, it came at a cost.

The brain was our greatest weapon and our biggest liability.

In fact, it created a seemingly insurmountable paradox – The Brain-Body Paradox.

Part I: Your Brain on Meat

• Section 1: Building the Brain
• Section 2: The Brain-Body Paradox (this post – below)
• Section 3: Meat the Brain that Became Sapiens
• Section 4: Evidence for a Meat-Based Diet

Part II: Your Brain on Plants (coming soon)

Part III: Brain – Body Connection (coming soon)

The Brain-Body Paradox

The ever-expanding human brain came at a high cost and with high stakes.

The brain is extremely hungry and selfish when it comes to energy consumption. (r)

So as the climate cooled early in human evolution, we were faced with longer foraging ranges and escalating daily caloric requirements.

Selective pressures from the environment as well as the energy demands of the brain forced the body to adapt.

And one such adaptation – a shrinking gut – created a seeming paradox.

Humans had higher energy demands from longer foraging ranges and an ever-expanding brain. Yet, we had a shrinking gut, which left less surface area to absorb nutrition. (r, r)

Energy Tradeoffs

The brain required so much energy that other organs had to shrink, especially metabolically expensive ones, like the gut.

So humans found themselves in a situation needing more energy to feed the brain, but with less gut area to absorb it.

To solve this riddle, we’ll examine how and why the human body morphed over time.

Brain Food: Meat Heads and Morphed Bodies

Humans dependence on meat not only built our brains but it changed our entire body.

When the earliest humans left the trees for the grasslands, they were untrained and ill-equipped hunters. They were easy prey of fierce, better-trained predators.

Acidic Stomachs

This led to early humans subsisting off remains. We were scavengers, carrion feeders, eating the dead and rotting flesh, the scraps from the professional hunters like big cats.

However, due to the importance of meat for the existence of our specie, natural selection began selecting traits that facilitated meat consumption.

One such trait was an acidic stomach that could kill off pathogens residing in the rotting animals early humans scavenged. It also improved the digestion of meat.

Our very low stomach pH is a trait that was selected for early in human evolution to filter out pathogens and facilitate meat digestion in the small intestines.

This acidic stomach is a distinguishing trait among carrion feeders and carnivores. (r, r, r, r, r)

Shrinking Gut and Solving the Brain-Body Paradox

With a diet shifting to meat, our stomach pH changed but so did our entire gut. It shrank. (r)

This led to the paradox: Our big hungry brains demanded more energy, but our gut was shrinking.

The answer to the riddle lies in that humans no longer needed the large cecum to ferment plants into useable energy.

In fact, a big colon would prevent a meat-heavy diet, as too much meat would put it at risk of twisting, which would not only be painful, but potentially fatal. (r)

The solution to the paradox was consuming an energy dense diet, that was efficiently absorbed without the need for a large gut. All qualities that meat provided.

Obligate Meat-Eaters

This combination of a shrinking gut that could ferment less and less plant food into energy and an expanding brain that demanded tremendous amounts of energy – turned humans into obligate meat-eaters. (r)

Humans simply couldn’t survive without animal food. It was the only way we could meet our energy requirements, the only way we could feed our brains and our bodies. (r)

While we could still eat plants, alone they couldn’t fuel us.

Over time we continued to lose most of our ability to ferment fiber and traded it for the ability to eat larger and larger amounts of meat to fuel an increasingly expanding brain.

Our guts mirrored the guts’ of carnivores. Our cecum shrank to a useless vestigial appendage. And our small intestines absorbed the fat and protein from meat with extreme efficiency. (r)

In fact, we now share similar, well-developed gallbladders comparable to that of wolves and lions.

From Herbivore Roots to Carnivore Hunters

During this transition we started to share more features and habits with carnivores than our herbivore ancestors.

Instead of grazing and grinding on leaves all day, we became intermittent eaters.

Prey. eat. rest. digest. repeat.

Muscles and Mastication

Since we stopped grazing and grinding all day, our facial anatomy changed as well. Our jaws main use became vertical up-and-down chewing rather than the side-to-side, rotary mechanism of our herbivore ancestors.

We have teeth similar in size, shape and number to our primate heritage, yet we have ridged molars like wolves instead of the flat ones like sheep.

Since the earliest humans used tools to kill and cut meat, less mastication forces were needed and our jaws shrank. And with the advent of fire several hundred thousand years ago, even less mastication forces were needed. (r, r, r, r)

Our canines shrank as well.

Canines are predominately defense and intimidation mechanisms. The smaller canine represents a shift in social structure and mating behavior that results from cooperation, communication, and a reduction in male-to-male conflict.

All thanks to our big brain.

From Climber to Runner

As we became dependent on animals for food, natural selection favored traits that facilitated the capturing of meat.

About 3 million years ago when we climbed out of the trees, we were barely bipedal. Slowly we morphed into predator apes and skilled, persistence hunters.

Today humans are the only living primate adapted for endurance running.

We have a vestibular system and nuchal ligament for balanced running. We have running enhanced features like longer lower limbs, muscles like the glutes, and tendons like the achilles.

Our feet became shock absorbers. The big toe came into alignment with other toes as running replaced grasping limbs.

We evolved sparse and short body hair and millions of eccrine sweat glands to prevent hyperthermia.

The decoupling of muscles in the shoulder transformed us from tree climbers, to rock throwers, to spear and baseball hurlers.

We traded a plant-based diet for a meat-based diet that transformed us into premier runners and hunters. (r, r, r, r, r)

As we look at our bodies today, we can see that they morphed in accordance to the ability to obtain meat to fuel our brains.

This was the selective priority in which everything else fell in line.

Dr. Max Kleiber

“How much do I need to eat?”

This a question I’ve asked myself whether cutting fat for a bodybuilding show or bulking up to put on muscle. It’s also a question that Dr. Max Kleiber must have asked himself.

He discovered a law to predict how much an animal would eat based on its size. This law could also be used to predict how big an organ should be and how much energy it should use.

Not surprising, the larger an animal is, the more it eats.

Klieber’s Law and Humans

Kleiber’s Law works on humans. Sort of. We eat in the amount that his equation predicts for an animal our size.

But if we use Kleiber’s Law to predict how big and how much energy particular organs should use we no longer fit standard predictions.

Humans break Kleiber’s Law.

Using his equation we find our brains are over 7X bigger than Kleiber would predict for an animal our size. Not only that, it uses 30X more energy than would be predicted.

To accommodate the brain, Kleiber’s Law shows that the human gut shrank to just a third of the size as would be expected. Our cecum, the appendix, is practically non-existent and our colon is about half the size of what would be expected. (r)

While surprising, this is consistent with carnivorous animals. Since they eat little to no plants they have much smaller cecums often replaced by the vermiform – the appendix.

The Brain-Body Paradox: Proof in the Math

Kleiber was able to use mathematics to show just how important the brain is and how much other organs sacrificed to feed it.

Not only is the gut small as predicted by Klieber’s Law but so are human hearts, livers and muscles compared to other animals our size. (r)

Kleiber’s Law also helps answer our paradox.

Fueling a big brain demands a lot of energy. Humans would either need a very large gut with a large absorptive surface or a very energy dense diet or both.

Simply looking at our anatomy and the predictions from Kleiber’s Law we see humans could only keep a brain our size by shrinking metabolically active organs, like the gut, and eating energy dense food. (r)

The riddle can only be answered by humans becoming predominate meat-eaters.

If we take Kleiber’s findings we can draw only one conclusion – we have to eat an energy dense diet. And meat was the only practical source.

This law answers a question that still stumps the scientific community today –

“What are humans designed to eat?”

Fortunately, Dr. Kleiber answered this for us decades ago.

The New Brain-Body Paradox

The Brain-Body Paradox humans faced is solved when we see that humans switched to an energy dense, efficiently absorbed food – meat – which allowed for the gut to shrink, yet still meet energy requirements.

But we have a new paradox.

With the advent of agriculture, for the first time in human history it became possible to meet our energy requirements with plant-based foods (note: energy requirement is not the same thing as nutrient requirements) .

Yet, even with the surplus of energy (calories) we eat today, our brains are shrinking and getting sick. (r, r, r, r, r, r, r, r, r, r, r)

This is a paradox we are still working on solving.

I’ll be talking about it specifically in Part II – “Your Brain on Plants.”

• Could it be that the brain needs more than just energy? And that the type of energy matters?

• Might “plant nutrients” be different than “meat nutrients”?

• After millions of years where humans evolved on a meat-based diet – could we have become ill-equipped to eat plant-based foods?

Our brains distinguish us from every other species on the planet.

They were the force behind humans ability to hunt mammoths 2X the size of an elephant. We couldn’t have done this alone. We did it as a team.

It’s this teamwork, communication, and ingenuity – made possible by the human brain – that transformed unskilled scavengers into hunters that climbed to the top of the food chain.

We didn’t get to the top alone. We did it by working together, forming robust social structures, by coordinating and communicating, relying on each other and helping each other for the greater good of all.

No other animal does this like humans. And as we’ll see in the next section, no other animal has a brain like the human brain.

Solving the new Brain-Body Paradox is of paramount importance.