Encephalisation And Diet Explained

Debunking A Popular Meat Eating Theory
by J S Coleman

Introduction

Many anthropologists* have promoted the idea that disproportionate brain growth in humans has been promoted by the introduction of meat, in the diet of early hominids. It is commonly supposed that only this rich source of calories, protein and fat can support human brain growth. Popular authors such as Richard Leakey (1), use the claims of Robert Martin from the Institute of Anthropology in Zurich, saying, "this increase in brain size could have occurred only with an enhanced energy supply" (p.69, op. cit.). Leaky states that this additional nutritional requirement could have been provided by meat, adding that, "only by adding a significant proportion of meat to its diet could early Homo have 'afforded' to build a brain beyond australopithecine size." (p.70, op. cit.) However, nuts and seeds also provided a concentrated source of calories fats and proteins in early Homo nutrition, as well is in modern gatherer-hunter diets. But does a scientific analysis of the available evidence support the supposition that brain growth is or was nutrient accelerated and meat dependant, in humans?

* see also: T. Perper, C. Schrire, J. Robinson and R. Ardrey

Encephalisation

When reading popular anthropology, we often find that presumption and scientic fact are mixed without any clear boundaries being made. In the realm of quality scientific work pertaining to amounts of things, we must find numbers, graphs or other measurements and statistics. This is necessary both to support the factual basis of the claim, and its precision and significance. A study of reputable evolutionary literature does not support the meat eating to brain growth causal link, promoted by popular anthropologists. In fact, the evidence can only be used to support a very different conclusion on the importance of nutrition in brain growth, during human evolution.

An encephalisation quotient (EQ) is a number reflecting the increase in brain size relative to body size, such as seen in human evolution. It is obtained by comparing the estimated brain weight of, for example, a human, with that of a hypothetical 'average mammal of equal body weight (The Cambridge Encylopedia of Human Evolution, p.462). An EQ of 1 indicates that the brain is at a size in proportion to the rest of the body. The human EQ is equal to 5, when scaled to the trend for placental mammals (Robert Martin, ibid., p.42), or about 3 times larger than predicted for an 'average' monkey or ape with our body size. This means that if the rest of the human body were to scale, in primate terms, we should weigh about 1000 pounds and be 10 feet tall (The Cambridge Encylopedia of Human Evolution, p.118).

But how does the greater ratio of brain to body size occur in humans? There are four possible postulations for the ways that such a divergance can occur. Postulate number one, is that body growth does not keep up with brain growth, in other words a form of 'dwarfism'. The opposite explanation, postulate two, as implied by anthropologists such as Leakey, is that the brain grows excessively for the body, that is 'hypertrophy' is occurring, with the assumption that this is fueled by increased nutrient density of foods. Postulate three is for an embryonic reduction in body growth, but not brain growth, as seen in primates.

The fourth postulate is that neither dwarfism or hypertrophy are occuring, but instead a simple dichotomy in growth patterns is being observed, as if a large headed species had been merged with a smaller bodied species - thus, two growth trajectories in one animal. Neither an abnormal reduction in body growth rate, or an accelerated brain growth are necessary.

Growing Apart

"brain growth prolongation occurs in humans without an extension of body growth."
(Terrence Deacon, p.174, op.cit.)

So which of the four encephalisation postulates is supported by available data? The answer to this question is explained at length in chapter six of Terrence Deacon's book, The Symbolic Species, in a section entitled 'The Chihuahua Fallacy' (2).

There is no mention in Deacon's analysis of accelerated brain growth due to meat eating, as suggested by popular anthropologists, since this simply does not occur. We know that people who have no history of meat eating, develop brains of the same size at the same rate, as the general population. It seems fairly clear that there is no necessity for meat eating in the neurological development of humans.

Of the three remaining postulates Deacon comments decisively. According to Deacon, "[P]rimates do not grow their brains faster than other mammals, but they grow their bodies slower." (p.170, op.cit.) This then, eliminates hypertrophy as a factor in brain growth as the "increase of encephalization in primates is then, more accurately, a decrease in somatization." (p.170)

Deacon adds that the above facts seriously challenge the traditional view of primate evolution characterised by selection for increased intelligence - "why should this produce a change in body growth but not brain growth?" (p.170-171) However, humans do not follow the characteristic growth patterns of dwarfism, since "[D]warfed animals exhibit slowed body growth in the later phases (late fetal and postnatal)" (p.172) phases of development. But primates "follow a shifted, parallel gestational growth curve compared to that of other animal mammals from the start." (p.172) Within the primate foetus the brains and body are growing at nearly identical rates, yet the primate body remains smaller at all corresponding stages in gestation, despite the fact that primate brain growth keeps pace with other mammals. As Deacon says, "primate brains grow at essentially the same rate as other mammal brains." The key difference between humans and other primates is that our brain growth period is longer than expected, though no faster.

Conclusion

The hypertrophic theory of encephalisation is proven false, by the available growth trajectory data. A more parsimonious explanation is the opposite one, because we know that a restriction in nutrient availability prolongs developmental phases.

Key Facts And Figures

The graphs below demonstrate conclusively that human brain growth is not exceptional in any fundamental way. We have the same growth rate for brain tissues as other mammals, and the growth curves follow the same general trends. The additional brain growth in humans is achieved because the maximum growth rate continues for longer, after birth, than for the other primates.

Brain/Body Growth Patterns In 3 Primates

Comments

The straight line indicates the average adult brain and body size allometry trend for primates. Diamonds (blue) indicate final body size for adults. Asterists (*) indicate post-natal development (green) in humans 'H' and chimpanzees 'C'. The triangle 'X' (black) shows the hypothetical growth pattern for humans, if body growh were to match brain growth so as to fit the general pattern of primate development. Humans are unique amongst primates in that the body growth curve is truncated - the 'H' to 'X' gap (grey) is absent.

sources: adapted from The Symbolic Species, T. Deacon (p.173), and The Cambridge Encyclopedia of Human Evolution, S. Jones, R. Martin (p.118)

Brain Growth Rate In 3 Species

Comments

There is a belief that human brain growth is more rapid than for other species. In fact human brain growth rate follows the same general trend as for other mammals, as this graph indicates. "According to the traditional view of our place among other mammals, primates evolved bigger brains than other mammals as a response to a more cognitively demanding niche . . . [B]ut now it apears that primates did not evolve bigger brains, just smaller bodies . . ." The Symbolic Species (p.172) As we can see from the graph, the rate of brain growth is approximately 0.6 grams per day, the same rate of growth as for pigs and macaques.

source: The Symbolic Species, T. Deacon (p.171)

Finally, we should like to know what kind of nutrient requirements brain growth has - does it justify a need for a high protein and fat diet? Without specific figures it is difficult to answer this question precisely. However, we might reach a reasonable lower estimate by just looking at how much brain material is actually built each day. The nutrient profile for mammal brain (bovine brain) is shown in the table below. If we take our 0.6 grams of daily brain growth, we can see that in terms of nutrient makeup, this would consist of 0.47 grams of water, 0.059 grams of protein, 0.056 grams of fat, and 0.0084 grams of ash. This trivial nutrient requirement is, of course, easily met by human breast milk during the period of maximum growth rate. The turnover rate of molecules in the structural components of brain tissues is low, because they are long lived.

Water	Protein	Fat	Ash
78.3%	9.8%	9.3%	1.4%

At rest, the brain is the most metabolically expensive organ, consuming up to ten times more glucose and oxygen for its mass than any other organ. But, during physical activity, the relative amount of energy used by the brain is probably much lower (Deacon, p.157, ibid.). Therefore the amount of protein and fat required by the growing brain, is small when compared to glucose requirements. This difference would be much greater for an adult brain.

"I am afraid that our predisposition to see brain size evolution where there is none mostly grows out of extra-scientific prejudices having to do with the comparatively large size of our own brains."
T. Deacon, The Symbolic Species, p.168

References

(1) Leakey R, The Origin of Humankind, Phoenix, 1994
(2) Deacon T, The Symbolic Species, Penguin Books, 1997, pp. 165-174

Last Updated on 19/07/99
By John Coleman