View Full Version : Brains and guts in human evolution.

Darryl Shaw
02-09-2009, 05:45 AM

The brain is a very expensive organ in metabolic terms. Each unit of brain tissue requires over 22 times the amount of metabolic energy as an equivalent unit of muscle tissue. There is no correlation across mammals, however, between the relative size of the brain and the relative basal metabolic rate. The Expensive Tissue Hypothesis explains this apparent paradox by looking at the metabolic cost of the brain in the context of the costs of other metabolically expensive organs in the body. The results show that the increase in brain size in humans is balanced by an equivalent reduction in the size of the gastro-intestinal tract. In other words, the increased energetic demands of a relatively large brain are balanced by the reduced energy demands of a relatively small gastro-intestinal tract. This relationship also seems to be true in non-human primates.
The size of the gastro-intestinal tract is dependent on both body size and the quality of the diet. It is argued that humans (and other primates) could not have developed a relatively large brain without also adopting a high quality diet that would have permitted a reduction in the relative size of the gastro-intestinal tract. Dietary change is therefore viewed as a prime releaser in brain evolution. It is argued that a high quality diet is necessary for the evolution of a relatively large brain. However, the change to such a high quality diet, which involved an increased proportion of animal based products, need not have been one of the prime movers in brain evolution. In this context, and based on the archaeological and palaeoanthropological record, the factors most probably surrounding the evolution of the human brain are discussed.

Dietary quality has played a prominent role in theories of human evolution in general and the evolution of the human brain in particular. One of the most memorable of these theories is the Man the Hunter (Ardrey, 1961; Washburn and Lancaster, 1968). This theory argued that increasing amounts of meat in the hominid diet lead to increasing levels of cooperation among the males in the hunt, which lead to brain expansion and the associated development of cognition, language and symbolic culture. This hypothesis was fuelled by the realisation that an increase in the apparent consumption of meat correlated with the increase in brain size seen in Homo habilis and Homo erectus. It was also supported by the recognition in the archaeological record of the basic elements of a hunter-gatherer life-style (home bases and food sharing) (Isaac, 1971). Although the rather simplistic reasoning underlying the Man the Hunter hypothesis has lost favour in more recent years (eg. Tanner, 1981; Power and Aiello, in press) the importance of a high quality diet, and meat eating in particular, has been a common theme (eg. Foley and Lee, 1991; Leonard and Robertson, 1992, 1994).

Ideas of brain evolution centring on dietary quality have not been confined to humans and human evolution. Parker and Gibson, 1979 and Gibson, 1986 coined the Extractive Foraging Hypothesis to explain the relationship in primates. They argued that a relatively large brain correlates with omnivorous feeding in primates, which requires relatively complicated strategies for extracting high-quality foodstuffs. Alternatively, and in the context of primate frugivores, Milton (1979, 1981) and Clutton-Brock and Harvey (1980) suggested that relatively large brain size is associated with the need for a more sophisticated mental map for the location and exploitation of widely spread high quality food resources.

There is no doubt that there is an association between dietary quality and brain size across primates, including humans (Leonard and Robertson, 1994). However, how do we account for this association? There are two logical possibilities. The first is that there is a direct causal relationship between diet (and/or the associated ecological circumstances and foraging strategy) and brain size. Simplistically, hunting or extractive foraging or mental mapping would be directly selecting for large brain size. If this were true, then diet would be seen as a prime mover, or major selective agent, in the evolution of the brain (Falk, 1995). The alternative would be that diet is a prime releaser in brain evolution. In this sense, diet would release constraints on brain expansion and, given other selective agents or a combination of agents, permit brain expansion to take place.

The purpose of this contribution is to argue that diet must be viewed in the first instance as a prime releaser in brain evolution. Arguments will be presented to show that a relatively large brain requires a high quality, easy to digest diet. This conclusion does not necessary exclude diet as a prime mover in brain evolution, but it does not require it. Rather, it opens up the possibility of considering the importance of other selective agents (prime movers) in brain evolution without discounting the clear association between dietary quality and relative brain size (Leonard and Robertson, 1994). These other selective agents might include socio-ecological factors such as group size (Dunbar, 1992, 1993, 1994; Aiello and Dunbar, 1993) and social (or Machiavellian) intelligence (Byrne and Whiten, 1988) or a combination of these factors, as well as others which might include a clambering type of locomotion (Povinelli and Cant, 1995), or fully committed terrestrial bipedalism (Aiello, 1996a,b).

Brains and guts in human evolution: The Expensive Tissue Hypothesis. - Leslie C. Aiello. (http://www.scielo.br/scielo.php?pid=S0100-84551997000100023&script=sci_arttext)

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With the emergence of species of our own genus (Homo habilis) at least 2.3
million years ago [1], a rapid increase in hominid brain mass relative to body
mass (encephalization) occurred [2, 3]. Figure 1 shows that the range of cranial
capacities for Homo habilis significantly exceeded that of earlier Australopithecus
species, whose brain volumes remained constant for at least 2 million years.
Slightly prior to the emergence of Homo habilis in the fossil record was the
appearance of primitive stone tools [4] whose function was to butcher and disarticulate either scavenged or hunted carcasses of African prey animals [5, 6]. The advent of stone tools as well as the appearance of stone-tool cut marks on the fossilized bones of prey animals suggests that early members of our genus were increasingly exploiting animal foods as a source of sustenance. This dietary shift from a predominantly plant based diet to one in which animal foods became
increasingly important allowed for the relaxation of the selection pressures that
had formerly constrained encephalization in Australopithecus species [7, 8].

Fatty Acid Composition and Energy Density of Foods Available to African Hominids. - Loren Cordain. (http://www.thepaleodiet.com/articles/Encephalization%20Final%20PDF.pdf)

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First is the expensive tissue point about body RMR, brain size and metabolic activity of tissues. The computation never seemed logical to me (I'm a computer professional) and in this version of your description the words "and metabolic activity" are the key point (and IMHO should need a little more emphasis. For example: Given the fact that brains take 25% of RMR in humans and 8% in other primates. And the brain has a high metabolic rate (h) compared to other tissues (l). So the rmr would be:
RMRh = 0.25*h+0.75*l for humans and
RMRp = 0.08*h+0.98*l for other primates

If we accept that RMRc and RMRp are the same (Klieber), then it's obvious that simply reducing the size of the "other tissues" with lower RMR, actually the RMR per weight would *increase*. But as you told it's constant. If the gut is of the type "low RMR" and you decreased it's size, this would actually result in an *increase* of the summary RMR, because the brain would be relative bigger.
That's a contradiction. It only makes sense if the gut was also a tissue with elevated metabolic resting activity. Like reducing some of the highly active gut while increasing also highly active brain and leaving the rest at the same. "Expensive Tissue" only makes sense if the gut also was highly active, at rest. Maybe this is the case. I think it could be a little more active than other organs at rest because it has to move it's contents.

Personally I think a improved dietary quality alone can explain the gut size reduction. We should think of the purpose of each gut part. What is reduced in humans is the last part where cellulose fermentation is going on in other primates. They need it because they eat very much cellulose material (like leaves) and need to derivate their energy from it.

If a human get's more fruit (untrue) or more starch (true if you think of Wrangham/tubers) cellusole processing is less necessary. The fermenting part gut can be thrown overboard.

Increased dietary quality from animals can hardly explain how additional energy can come in. Game meat is a energetically a low density food, unless it's fat. As you often pointed out. A little added tuber starch can easily explain the gut reduction, because it delivers glucose energy (needed for the 25% of the brain) After all glucose is exactely the part of the energy demand which is increased
It's easily digested with a small, non-fermenting gut.

Comment on "Evolutionary Implications for Human Brain development" by Prof.Loren Cordain et al. - Amadeus Schmidt-Philipp. (http://www.geocities.com/paleolix/DHAtheories.htm)


See also -

Roots and Tubers in Diet of Early Human Ancestors. (http://www.performancemenu.com/forum/showpost.php?p=44100&postcount=1)

The Critical Role Played by Animal Source Foods in Human (Homo) Evolution. (http://jn.nutrition.org/cgi/content/full/133/11/3886S)

Garrett Smith
02-09-2009, 08:55 AM
I've been having more tubers (sweet potatoes and parsnips) lately and feeling & recovering better. Thanks for the post.