Homo sapiens' Brain Development: A Fish Story?

Kazuo Uyehara

            Humans underwent myriad evolutionary changes before developing into modern Homo sapiens. Homo sapiens underwent a dramatic increase in brain size in a short period of time when compared to Homo erectus.  This brain development is often cited as the reason why humans outlasted their other hominid relatives and are now the dominant species on Earth.  Although Homo sapiens’ sophisticated brains gave them significant advantages over other animals, it is possible that our brain evolution was based more on chance than a genetic predisposition for a high level of intelligence. One theory is that humans had to wait for a high-energy and nutrient rich shore-based diet that could support a more complex brain before an increase in intelligence could be achieved.

            The link between increased social interaction, cooperative hunting, use of tools, speech, and other brain intensive practices and brain evolution is often understood as a positive feedback loop.  However, brains are extremely energy and nutrient demanding, so brain evolution could only have taken place while humans had a stable diet with high amounts of energy dense food like meat or fish.  Since it would require a high level of sophistication and intelligence to hunt effectively, early humans must have found a source of food that did not already require a more evolved brain.  One possible solution is that Homo sapiens underwent their rapid brain development because of a diet close to the shore, which provided fish, mollusks, crustaceans, marine plants, bird’s eggs, frogs, and turtles (Cunnane, 21).  These foods are high in protein and fat, and would not have required the use of fire or a well-developed intestine to digest (Cunnane, 21).  A shore-based diet also provided easy access to readily available sources of nutrients specifically vital to brain development.

            There is fossil and DNA evidence in support of Homo sapiens originating in an environment close to fresh or salt water, such as the East African Rift Valley; this contrast to the forested environment of other hominids could be responsible for Homo sapiens’ brain expansion (Crawford, 3).  Studies of modern hunter-gatherer societies and fossil evidence show that it is unlikely that our ancestors ate many medium to large game animals, but instead adopted a more stable diet that was high in fish during times of seasonal stress (Stewart, 230).  Fossil evidence from the Late Pleistocene supports that marine food procurement was relatively easy and availability was high during the correct seasons.  Fish spawning in shallow water and fish that formed nests were easy to catch and others could be trapped or stranded by shallow waters with minimal technology or skill (Crawford, 233).  It is also possible that the nature of the easy access to marine food by all members of an early human group gave time for individuals to hone other skills such as hunting and tool making and that the very act of capturing fish as a group could have been important in the development of more complex social interactions (Cunnane, 21).

           In addition to being high in protein, fat, vitamins, and minerals, foods found on the shore are excellent sources of long chain polyunsaturated fatty acids and other critical brain nutrients including iodine, zinc, copper, iron, and selenium (Cuanne, 25).  Arachidonic acid (AA) and docosahexaneoic acid (DHA) are two especially critical fatty acids for brain development.  In addition to dietary DHA, which is predominantly found in marine foods, humans can naturally synthesize DHA and other essential fatty acids from dietary w3 and w6 fatty acids, both of which are rarely in high concentrations outside of food from a shore diet.  This has important implications because relative brain size generally decreases as body weight increases because animals cannot bio synthesize AA and DHA quickly and efficiently enough to support further development of their brains (Crawford, 3). Cunnane explains in, “Survival of the fattest: fat babies were key to evolution of the large human brain” that, “all mammals have brains with approximately the same proportion of protein, cholesterol, and fatty acids such as DHA, so what makes the human brain more sophisticated is not its composition but its size and the increased density of its wiring” (23). Thus, brains can only mature in relation to the availability of the limiting factors of DHA and AA.  This is congruous with fossil records showing that Homo sapiens have had exponential growth of relative brain size in the last 200,000 years, whereas other hominids such as the forest dwelling Australopithecus showed very small brain development over 3 Myr (Crawford, 3).  This implies that human’s dramatic increase in brain complexity was a result of a genetic predisposition for brain development combined with the necessary nutrients and resources for the supply and upkeep of brain growth.

           Humans tend to believe that our mental capacity far exceeds other primates because of specific genes.   Interestingly, “when they are 45-50 days old, embryos of both humans and non-human primates have heads accounting for similar and large proportion (approx. 40%) of their body weight (qtd. in Schultz, 1969).  This and the similar genetic make up of human and non-human primates suggest that primates, whether human or non-human, have the same early embryonic potential to have a large brain” (Cunnane, 21). That is, humans did not become a dominant species because of a genetic advantage over other primates; they fostered their brain development by utilizing a diet that offset the inefficient biosynthesis of DHA and the lack of other proper nutrients that limited their brain growth (Cuanne, 21). This unique relationship with their environment allowed humans to capitalize on their potential for brain growth while other primates and mammals remained incapable of acquiring a diet that compensated for their inability to provide the necessary resources for a growing brain.

           Cuanne hypothesizes that what sets humans apart from other primates is the unmatched amount of relative body fat that human infants have.  The more fat an infant had, the more biologically fit it was.  This is because the fat is a source of nourishment for babies in between feeding periods and a source of lipids vital to brain expansion. Infants’ fat also contains around 3 months worth of DHA for the high-energy requirements of their developing brains.  It is unclear how humans evolved to have considerable amounts of fetal body fat, but it is likely to have arisen because of a prolonged diet rich in nutrients, such as the shore-based diet, which allowed humans to store energy in the form of body fat.  It is possible that the diet prompted pre-existing genes to express traits that eventually evolved into the high fetal body fat of human infants.  This would also imply that human brain growth was not necessary for survival, but that it was the product of natural selection after the availability of a nurturing diet.

           Some current implications of our brain evolution can be found in the moderate decrease in brain size of hominids over the past 25-90,000 years (Cunnane, 25).  Our divergence from a diet high in marine foods to a diet of processed foods, cereal grains, and industrialized food may be a part of this brain size reduction.   Likewise, the decrease of dietary DHA and the overall imbalance of essential fatty acids in modern diets have been linked to increased occurrence of psychological disorders, vascular dysfunction, lower birth weights, and preterm deliveries (Crawford, 8).  Although there is a connection between these dietary factors and brain growth, the actual evolution of the human brain and the roles of various factors remain uncertain and speculative.

           In conclusion, differences in early humans’ diet may have been an essential part in the rapid growth of Homo sapiens’ brain.  A marine diet high in nutrients crucial for brain development such as DHA, AA, iodine, zinc, copper, iron, and selenium as well as high amounts of both fat and protein could have provided the necessary energy and materials for brain expansion and lead to the eventual expression of genes relating to fetal body fat and brain size.  Once the exponential growth of brains became nutritionally possible, social communication and increased interaction with the environment may have further promoted brain development.  However, population movement away from shores and the use of agriculture and animal domestication could be the cause of the decline of brain size and different diseases now common in our modern societies.  Even though it is still uncertain how Homo sapiens’ environment affected their mental development, it is possible that Homo sapiens acquired higher intelligence, not due to environmental pressures or a genetic advantage over other primates, but because of their location near an appropriate and rich source of food.

Works Cited

Crawford, MA., et al.  “Evidence for the Unique Function of Docosahexaenoic Acid (DHA) During the Evolution of the Modern Hominid Brain.”  Lipids 34 (2000): S39-S47.

Cunnane, C. Stephen and Michael A. Crawford.  “Survival of the fattest: fat babies were key to evolution of the large human brain.”  Comparative Biochemistry and Phsiology Part A 136 (2003): 17-26.

Stewart, Kathlyn M.  “Early hominid utilisation of fish resources and implications for seasonality and behaviour.” Journal of Human Evolution 27 (1994): 229-245.


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last updated 2/6/06