Tissues are expensive
Aiello and Wheeler propose that in order to compensate for increased encephalization primates and hominids shortened their gut. This allowed for the evolution of larger metabolically expensive brain tissue without a corresponding increase in BMR. Henneberg criticizes this hypothesis by pointing out that the concurrent change in these two structures does not necessarily indicate interdependence. Aiello and Wheeler argue that the adoption of a high-quality diet allows for a small gut and liberates more energy to be devoted to the development of a large brain. If high-quality food requires greater skill/intelligence to locate, I don’t see how the proposed coevolution between gut and brain size could have been initiated. It makes sense that a relaxation of metabolic constraints could allow for the development of larger brains, but what caused the change in diet? Aiello and Wheeler tried to address these complicated interactions by avoiding telling a “just so” story. Unfortunately, in the end, I think they are still storytelling. Their story just happens to involve a physiological mechanism.
Finally, Aiello and Wheeler refer to overall brain size in this paper. However, they fail to address changes in individual sections of the brain. Do all brain sections increase isometrically with an increase in size? Is it possible that there may be differential selection for some brain regions but not others depending on foraging strategy and dietary niche of an organism? For example, an increase in the size of the hippocampus could have allowed earlier hominids to adopt a more complex foraging strategy by increasing the ability for spatial memory. In this example, an increase in the hippocampus may not drastically increase overall brain size, but it might change foraging strategy and, therefore, the size of the gut.
Finally, Aiello and Wheeler refer to overall brain size in this paper. However, they fail to address changes in individual sections of the brain. Do all brain sections increase isometrically with an increase in size? Is it possible that there may be differential selection for some brain regions but not others depending on foraging strategy and dietary niche of an organism? For example, an increase in the size of the hippocampus could have allowed earlier hominids to adopt a more complex foraging strategy by increasing the ability for spatial memory. In this example, an increase in the hippocampus may not drastically increase overall brain size, but it might change foraging strategy and, therefore, the size of the gut.
posted by Ondi at 3:45 PM
5 Comments:
Aiello and Wheeler. 1995.
This paper bring a new perspective on morphological and energy changes through human and primate evolution. In overall they show how the human body, although have a large brain size, have a similar BMR compare to other mammals and primates when body mass is taken into account. Moreover, they show that humans decrease the gut size and no other organs that also have high energy expenditure (Liver, Kidney and Heart), they argue that decreasing the size of these organs may have large implications in the normal body function. They related the decrease in gut size as a consequence of the consumption of a high quality diet, they also mention that cooking may play an important role as it serve as a predigesting phase which decrease the energy invested in breaking secondary compounds. As one of the reviewers point out, this paper don’t explain why brain is larger in humans as it just give correlative evidence. I think that is hard to explain brain evolution using just one variable (gut size), although more are explore, as several change may occurred at the same time of shifting from poor to high quality diet, like farming and increasing social groups.
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Aiello and Wheeler investigate how the highly encephalized, energetically expensive primate brain, relative to other mammals, could be accommodated without any corresponding increase in basal metabolic rate. They show that a reduction in the size and metabolic expenses of other tissues, particularly the gastro-intestinal tract, could “release” energetic constraints that may have previously limited further encephalization of the primate brain. A reduction in gut size would be much more feasible than any significant changes in less flexible tissues (e.g., heart, kidneys) and could arise by a shift from a low-quality to high-quality diet (with highly digestible foods). This explanation of an association of corresponding morphological changes drew multiple comments and suggestions from other authors (and grad students):
1) What do patterns in other taxa indicate about shifts in morphological features to accommodate larger brains (e.g., birds, bats, cetaceans)?
2) How do the metabolic costs of brain regions differ, and can we better quantify the energetic needs of the highly encephalized brains by reviewing which parts of the brain expanded in primates?
3) How do we go beyond showing evidence for the association presented by Aiello and Wheeler to a falsifiable test of causation? Are we limited to piecing together a puzzle of the suite of coincident changes in form and function of primate ancestors, without knowing which were the causes and which were the effects? This article convinces me that gut size and brain size could have undergone morphological changes linked in an evolutionary energetic trade-off, but the cause is unresolved (and wasn’t meant to be in the article). I guess I agree with Katherine Milton, that, whenever possible, the ‘how’ and ‘why’ questions should be approached together.
A note on the Armstrong paper:
Going back to our discussion from last week about McNab’s 1989 figure in chapter 4 of the text, whether brain size ‘correlates’ with BMR when ‘the effects of body mass are removed’…well, does Armstrong (in Figure 2 of her Science paper) do anything to resolve the collinearity problem by putting the relationship in another context (i.e., a relationship between total energy reserves (cm^3 O2 per minute) and brain size)?
From Armstrong’s Figure 2, any deviation from this best fit line should reflect greater or lesser energetic investment in brain development, but in the form of a trade off for reducing/increasing energetic investment in other tissues? Is this a more accurate way or a potentially less misleading way to view the brain-body size relationship?
Another note…In class on Tuesday, it might be useful to go over the overall body mass-BMR relationship (when body mass scales to the 0.75-ish power) versus the mass-specific body mass-BMR relationship (when body mass scales to the -0.25-ish power), perhaps just to make some of the equations easier to decipher.
Certain tissues of the body are more metabolically “expensive” than others. The Brain, heart, kidneys, liver, and gastrointestinal tracts account for 60-70% of the body’s basal metabolic rate, yet comprise less than 7% of its total mass. The BMR of different primates is very similar despite the differences in body size and size of these metabolically expensive tissues. There is a relative budget of energy to go around and supply these expensive tissues, and this budget can be divided amongst the tissues in different ways. When one tissue requires a larger supply of energy than normal, it will be cutting into the budgets of the other tissues and visa versa. This is how Aiello and Wheeler explain the increased brain size in humans. They claim that as the humans started to diet on higher quality, cooked foods they no longer needed guts that were as large. The shrinking of the gut meant that there was more energy available to supply the brain, enabling it to enlarge. Although Aiello and Wheeler’s theory is very interesting, it does raise some questions. They state that gut size was able to decrease as food quality increased, but how could higher quality foods be obtained without the brain already making an increase in size? Their brains must have evolved somewhat before the gut decreased in size. It seems to me that there is more than just one variable explaining the increase in brain size. The expensive tissue hypothesis is as explanation in part along with many other factors.
I think Ondi makes a very good point in regards to which sections of the brain have increased in response to the metabolic liberation from an energy-expensive gut. To reiterate, brain size increase has been shown to have increased after a decline in gut size, rather than an increase in Basal Metabolic Rate. Since this is most likely the case, I can only look forward to the day when we are not shackled by our other metabolically expensive tissues, such as muscles, heart, and kidneys. (See: Krang [1])
What is still unclear to me is if brain mitochnodrial concentration has increased since our earliest anscetors [2]. This would seem to be a less drastic change than a decrease in gut size. And what about other changes that have led to better learning and memory, such as NMDA-receptor up-regulation in the Doogie Mouse? [3]
Finally, will we continue to see an increase in brain size, as high quality energy sources become more readily availible? Can exogenous mtDNA expression allow us to increase brain size? There is much to look forward to. From last week: "In an evolutionary sense, there is nothing wrong with being 'stupid' as long as you can get away with it." [4]
[1] A mutant brain who is the overlord of Dimension X. He moves in an android body and dwells in his base and ultimate weapon: A titanic, mobile fortress called the Technodrome; http://en.wikipedia.org/wiki/Krang
[2] http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9671775&dopt=Citation
[3] http://www.bioteach.ubc.ca/TeachingResources/Genetics/DoogieMouse.html
[4] More Data: http://www.cnn.com/2006/TECH/science/01/24/bat.brains.ap/
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