The Brain versus the Gut: Experience versus Strength
Living organisms take up and metabolise substances that are rich in energy and their development and activities are limited by the corresponding balance between the uptake and the consumption of energy. In mammals, the brain and the gut both consume a large amount of energy. In humans, the brain consumes about 20% of the total energy ingested. From an evolutionary viewpoint, an organism needed to choose between an energy consuming alimentary system requiring little mental capacity – i.e. one where large amounts of difficult-to-digest food were consumed – or an energetically more favourable alimentary system, which could be sustained by more easily digested food (animal proteins), but which would also require a higher mental capacity and thus a larger brain. It is for this reason that mammals have either large guts or large brains; they cannot afford to have both (1). This difference in specialisation goes along with a difference in the sources of nutrition: a large gut was needed by herbivores because the vegetation they eat is difficult to digest, while carnivores required a large brain in order to hunt successfully.
In human development, hunting and gathering have played an important role over long time periods. During this time, the development of the current human species and, above all, the development of the brain, took place. It is most likely that hunting and gathering played a major role in human brain development.
Hunting – with a bow and arrow, for example – is one of the human activities that requires intense strength and experience in order to finally obtain food. In light of this, investigations into the determinants of hunting success in human societies still living in conditions similar to those of the Stone Age are of great importance for the understanding of human development. Such studies could also shed some light on the importance of physical strength vs mental capacity in coping with daily life.
A series of investigations has shown that the best hunters in such a tribe are not simply those who are in the best physical condition: successful hunting also requires substantial experience, which only older members have. Like playing music or manufacturing, hunting requires endless practice to achieve perfection. Thus Lee remarked ((4), p.47) that tracking an animal is a “skill cultivated over a lifetime, that builds on literally tens of thousands of observations”.
It is assumed from these investigations that with a change in diet (from plants only to a diet including animal proteins) an increased human lifespan co-evolved with a lengthening of the juvenile period, a correspondingly extended learning period and the development of a larger brain with increased capacities for information processing and storage (3). As noted above, hunting, unlike gathering – which can be quickly learned – is a skill that requires an extraordinary amount of experience. Thus it is not astonishing that the possession of this skill is correlated with having more offspring and a higher probability of their survival 2 and is consequently an extremely important skill. It is therefore quite useful to find out upon which skills and activities hunting rely, and it is of special interest to find out how physical strength, on the one hand, and experience, on the other, influence success in hunting.
Starting from this point, Walker and colleagues (7) investigated the Ache tribe of Eastern Paraguay. The people of this tribe reach the peak of physical strength at the age of 24, i.e. a bit later than individuals in modern societies, who reach theirs around the age of 20. The Ache tribe are hunters and gatherers with no tradition of agriculture. In the 1970s they came into peaceful contact with the Western civilization, but retained their hunting traditions. Thus the Ache still leave their settlements, for anywhere from few days to a month, to hunt in groups in the woods, using only their hands, machetes, bows and arrows: no guns or other modern weapons.
Starting in the early 1980s, notes were taken regarding which members of the tribe had killed what prey. The daily average amount of prey was 4 kg and there were considerable differences between the individual hunters, differences that were consistent over time, with a good hunter bringing home more than 10 times more meat than a bad hunter. In addition, a clear link between age and success in hunting was found: the men who brought home the most prey were about 40 years old.
As well as monitoring the tribe’s hunting activities, the authors organised arrow shooting contests, which required hitting a relatively small target located at great distance. An analysis of more than 2000 shots showed that none of the adolescent hunters had hit the goal, while the hit rate of grown men was 4.2%. Moreover, successful hits showed an age dependency: the number of hits increased until around the age of 40 and then stayed constant for the following two decades. The same result was obtained in respect to tracking animals.
The authors also gave a six week crash course in archery to the members of the tribe that no longer hunted, but without the slightest success. Overall it became clear that with hunting, as with skills such as playing soccer, violin or chess, performance is best after practicing for about two decades 6.
What does this mean for the way we understand ourselves? First, this example shows that youth and strength are not the only variables determining our success in life. Experience, which comes with age, also plays a role; and one which is at least as important as other factors. It is only at the age of 60 that a decline in hunting ability begins, – though this is an approximation of circumstances in the Stone Age. Even at that time, when human behaviour was formed more by nature and less through culture, learning and experience were already of great importance for success in life. But since then, the influence of nature and culture has shifted: like it or not our environment is, more than ever, moulded by cultural influences rather than by nature. As a consequence, youth and strength has played less of a role in successful living, while the influence of age and experience has increased.
The human brain is an energy-consuming repository that absorbs and analyses life experiences, using them to modify behaviour in order to achieve success. In doing so, the human brain is better than any other brain. Possessing a large brain (in order to learn well), having a long youth (in order to learn for a longer period) and reaching an old age (in order to use what has been learned) in a society with young people (so that they may profit from the older people’s knowledge (5)) are eventually only different aspects of the same developmental process. The phenomenon of “aging” has as less to do with disease as the phenomenon of a “long youth” or the phenomenon of a “large brain”. We often overlook this, especially in medicine. Certainly there are diseases that appear more often with age (as there are childhood diseases). Age alone however, is no more a disease than learning; at least for those organisms that decided to invest in large brains instead of large guts.
(1) Allman, J. (2000), Evolving Brains, Scientific American Library, WH Freeman & Co, New York.
(2) Hill K. and M. Hurtado (1996), Ache Life History: The Ecology and Demography of a Foraging People, Aldine de Gruyter, New York.
(3) Kaplan H., et al. (2000), “A Theory of Human Life History Evolution: Diet, Intelligence and Longevity”, Evolutionary Anthropology, Vol. 9, No., pp. 156-184.
(4) Lee, R. B (1979), The Dobe !Kung, Rinehart & Winston, New York.
(5) Spitzer, M. (2001), “Die Weisheit des Alters”, Nervenheilkunde, Vol. 20, pp. 302-305.
(6) Spitzer, M. (2002), “Lernen: Gehirnforschung und die Schule des Lebens”, Spektrum Akademischer Verlag, Heidelberg.
(7) Walker, R., et al. (2002), “Age-Dependency in Hunting Ability among the Ache of Eastern Paraguay”, Journal of Human Evolution, Vol. 42, No. 6, June, pp. 639-657.
(Translation of Spitzer, Manfred (2003), "Gehirn versus Darm, Erfahung versus Kraft" in Verdacht auf Psyche: Grundlagen, Grundfragen, und Grundprobleme der Nervenheilkunde, Schattauer GmbH, Stuttgart, pp. 90-93)