Australopithecus sediba, our vegetarian past? What does it take to grow a brain?

Very detailed and scientifically correct human skullcutaway, with all brain details, mid-sagittal side view, on white background. Anatomy image.

I’m watching a PBS special about some new fossils of our ancestors, specifically Australopithecus sediba, discovered in a cave in South Africa. I noted that the reconstruction of the ribcage appeared to be wide and consistent with an individual having a large abdomen, similar to a gorilla, providing room for a long enough digestive tract to digest a mostly vegetable diet. Just as horses and cattle need a relatively large abdomen to provide a “fermentation vessel”, so too the gorilla (and A. sediba) have a skeletal structure to accommodate this structure. Later in the show, it was revealed that dental calculi (plaque) provided evidence of a vegetarian high-fiber diet. This validates my observation of the ribcage dimension and clinches, in my mind, that this ancestor was a veggie eater. The braincase suggests that this ancestor had not yet evolved the larger brain, particularly frontal lobes, characteristic of modern humans.

Scientists have identified two requirements for the evolution of large brains (termed “encephalizaton”). The first requirement is sociality, as much of the brain enlargement is frontal lobe and this part of the brain is where much of our social dynamic originates from. Executive functions, self control, altruism are frontal lobe functions for the most part. Species with large frontal lobes have advanced social societies (dolphins, whales, elephants are prime examples). The second requirement is DHA, an essential fatty acid found mostly in seafood but also in fats of prey animals.

It is thought by many research authors (see Ben-Dor and Jandhayala below in references) that mankind’s encephalization involved a shortening of the digestive tract to support eating more energy-dense foods like meats and fats. The consumption of plant material for a diet requires that most of the waking time be spent foraging and eating, and the digestive process depends on a large gut. The development of tools and weapons was a necessary step to becoming an omnivore, as was the development of social structure necessary to successfully hunt larger prey animals.

It is for these reasons that a diverse and unique gut bacteria were a critical component of our development. The gut bacteria help produce BDNF (brain derived neurotrophic factor), required for the development of new brain cells and is essential for plasticity and learning. High levels of DHA in the diet also help with BDNF production. The human brain requires essential fats in the diet to support both its energy requirements and its health. There is evidence that during the phases where early man was developing the large brain characteristic of our species, diet shifted from plant based to include meat and fat. Those near the ocean likely obtained all the DHA fats needed from seafood, while those dwelling inland may have obtained it from…brains and marrow fat. Fossil remains show that prey animals consumed by early man have been found with cranial vault damage suggesting that the brain was removed for consumption. Obviously, those living near the ocean had a much easier time of things, as clams do not run fast nor pose much of a threat! Ancient settlements near water are often characterized by large piles of seashells.

What this boils down to is that we developed our big and (usually) useful brains due to a dietary shift away from vegetarian to eating meat and animal fats. Accompanying this shift, and codependent on it, was a behavioral shift toward toolmaking, weapon use, and increased social interaction. It should also be noted that as I explained in The Symbiont Factor, a higher functioning brain is codependent on a diverse microbiome. More brain activity helps the gut become healthy and encourages a healthy microbiome, and vice versa. The modern Western diet has high levels of omega-6 fats, which displace DHA and increase inflammation, leading to more disease and less brain development! That type of diet is also typically very unheathy for the microbiome, further hindering brain function. A vegetarian or vegan diet can still be brain-healthy, but apparently not without additional supplementation of DHA as even the presence of healthy fats in vegetable sources does not provide enough DHA due to a low conversion rate. That subject is discussed at length in some of the references provided, and may be fodder for many who have strong opinions one way or the other.


The Symbiont Factor:

Energetic and nutritional constraints on infant brain development: implications for brain expansion during human evolution:

Docosahexaenoic acid, the aquatic diet, and hominin encephalization: difficulties in establishing evolutionary links.

Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality.

Evidence for the unique function of docosahexaenoic acid during the evolution of
the modern hominid brain.

Man the Fat Hunter: The Demise of Homo erectus and the Emergence of a New Hominin Lineage in the Middle Pleistocene (ca.400 kyr) Levant

New Insights into the Evolution of the Human Diet from Faecal Biomarker Analysis in Wild Chimpanzee and Gorilla Faeces

Docosahexaenoic Acid (DHA): An Ancient Nutrient for the Modern Human Brain

Role of the normal gut microbiota:


2 thoughts on “Australopithecus sediba, our vegetarian past? What does it take to grow a brain?

  1. Andy

    “Those near the ocean likely obtained all the DHA fats needed from seafood, while those dwelling inland may have obtained it from…brains and marrow fat.”

    Good article, the interesting thing is that all the jumps hominids made in brain size all seem to have occurred, when we had access to large quantities of, i.e in the rift valley or along the coast. Meaning we survived in land, but we never thrived.

    1. Dr. Richard Matthews Post author

      Andy that is a really good point. Fats, combined with evolutionary pressure for a more functional brain, are a powerful combination. A healthy microbiome facilitates adaptation and evolution too, as it allows intragenerational adaptation as well as intergenerational. Interestingly, all of the animals that have brains as big or bigger than humans also have complex social structures: elephants and whales/dolphins. I’m not sure what elephants do for fats, but all the others are aquatic. Life at the shoreline may have provided more opportunity and benefit for socialization compared to being a hunter inland.


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