An Evolutionary Perspective on the Human Brain

This term I wrote an essay on the topic of human uniqueness from an evolutionary perspective. As I drew on research that is also relevant to this blog, (and to some extent has already been covered here). I'll post some of it here.
In this post I'll have a short look at the human brain from a neuroscientific, a comparative, and an evolutionary perspective:

Human Evolution

We are evolved primates. (As are all other primates of course. So maybe it is better to say that we, like all other primates, are evolved beings with a unique set of specializations, adaptations and features. )

In our lineage, we share a common ancestor with orangutans (about 15 million years ago (mya)), gorillas (about 10mya), and most recently, chimpanzees and bonobos (5 to 7 mya). We not only share a significant amount of DNA with our primate cousins, but also major anatomical features (Gazzaniga 2008: 51f., Lewinn 2005: 61) These include, for example, our basic skeletal anatomy, our facial muscles, or our fingernails (Lewin 2005: 218ff.).

What most distinguishes us as humans on an anatomical level are our bizarre hair distribution, our upright posture and the skeletal modifications necessary for it, including a propensity for endurance running, our opposable thumbs, fat deposits that are unusually extensive (Preuss 2004: 5), and an intestinal tract only 60% the size expected of primates our size (Gibbons 2007: 1558).

Finally, there is also a distinguishing feature that is a much more remarkable violation of expectations – a brain three times the size expected of a primate our size. This is all the more interesting as primates are already twice as encephalized as other mammals (Lewin 2005: 217). A direct comparison shows this difference in numbers: Whereas human brains have an average volume of 1251.8 cubic centimetres and weigh about 1300 gram, the brains of the other great apes only have an average volume of 316.7 cubic centimetres and weigh between 350-500 gram (Rilling 2006: 66, Preuss 2004: 8). In a human brain, there are approximately a hundred billion neurons, each of which is connected to about one thousand other neurons, comprising about one hundred trillion synaptic connections (Gazzaniga 2008: 291). If you would count all the connections in the napkin-sized cortex alone, you would be finished after 32 million years (Edelman 1992: 17).

Expensive Tissue

The human brain is also extremely “expensive tissue” (Aiello & Wheeler 1995): Although it only accounts for 2% of an adult’s body weight, it accounts for 20-25% of an adult’s resting oxygen and energy intake (Attwell & Laughlin 2001: 1143). In early life, the brain even makes up for up 60-70% of the body’s total energy requirements. A chimpanzee’s brain, in comparison, only consumes about 8-9% of its resting metabolism (Aiello & Wells 2002: 330). The human brain’s energy demands are about 8 to 10 times higher than those of skeletal muscles (Dunbar & Shultz 2007: 1344), and, in terms of energy consumption, it is equal to the rate of energy consumed by leg muscles of a marathon runner when running (Attwell & Laughlin 2001: 1143). In all, its consumption rate is only topped by the energy intake of the heart. (Dunbar & Shultz 2007: 1344).

Consequently, if we want to understand the evolutionary trajectory that led to human cognition there is the problem that “because the cost of maintaining a large brain is so great, it is intrinsically unlikely that large brains will evolve merely because they can. Large brains will evolve only when the selection factor in their favour is sufficient to overcome the steep cost gradient“ (Dunbar 1998: 179). We have to come up with a strong enough selection pressure operative in the Pleistocene environment of evolutionary adaptedness that would have allowed such “expensive tissue” to evolve (Bickerton 2009: 165f.).

What About the Brain is Uniquely Human?

If we look to the brain for possible hints, we first find that presently, there is “no good evidence that humans do, in fact, possess uniquely human cortical areas” (although the jury is still out) (Preuss 2004: 9). In addition, we find that there are functions specific to humans which are represented in areas homologous to areas of other primates. Instead, it seems that in the course of human evolution some of the areas of the brain expanded disproportionally, “especially higher-order cortical areas, including the prefrontal cortex” (Preuss 2004: 9, Deacon 1998: 435-438). This means that humans do not only think in a better way, but that they think differently (Preuss 2004: 7). The expansion and apparent specializations of only certain kinds of neuronal areas could indicate a qualitative shift in neuronal activity brought about by re-organization of existing features, leading to a wholly different style of cognition (Deacon 1998: 435-438 Rilling 2006: 75).

This scenario squares well with what we know about the way evolution works, namely that it always has to work with the raw materials that are available, and constantly co-opts and tinkers with existing structures, at times producing haphazard, cobbled-together, but functional results (Gould & Lewontin 1979, Gould & Vrba 1982). Given the relatively short time span for the evolution of the “most complex structure in the universe, we have to acknowledge how preciously little time the evolutionary process had for ‘debugging.’ It could well be that make the human mind so unique is that it is a imperfect ‘Kluge:’ a clumsy or inelegant – yet surprisingly effective – solution to a problem,” like the Apollo 13 CO2 filter or an on-the-spot invention by MacGyver (Marcus 2008: 3f.). It may thus well turn out that what we think makes us so special is a mental “oddity of our species’ way of understanding” the world around us (Povinelli & Vonk 2003: 160). It is reasonable then to assume that human cognition did not just simply get better across the board, but that instead we owe our unique style of thinking to quite specific specializations of the human mind.

With this in mind, we can now ask the question how these neurological differences must translate into psychological differences. But this is where the problem starts: Which features really distinguish us as humans and which are more derivative than others? A true candidate for what got uniquely human cognition off the ground has to pass this test and solve the problem how such “expensive tissue” could evolve in the first place.

References:

Aiello L.C. and P. Wheeler 1995. ”The expensive tissue hypothesis: the brain and the digestive system in human and primate evolution.” Current Anthropology 36:199–221

Aiello, Leslie C. and Jonathan C. K. Wells (2002): “Energetics and the Evolution of the Genus Homo.” In: Annual Review of Anthropology 31:323–38.

Attwell, David and Simon B. Laughlin. (2001.) “An Energy Budget for Signaling in the Grey Matter of the Brain.” Journal of Cerebral Blood Flow and Metabolism 21:1133–1145.

Bickerton, Derek (2009): Adams Tongue: How Humans Made Language. How Language Made Humans. New York: Hill and Wang.

Deacon, Terrence William (1997). The Symbolic Species. The Co-evolution of Language and the Brain. New York / London: W.W. Norton.

Dunbar, Robin I.M. (1998)“The Social Brain Hypothesis” Evolutionary Anthropology 6: 178-190.

Dunbar, R. I. M. and Susanne Shultz. (2007)“Evolution in the Social Brain” Science 317: 1344-1347

Edelman, Gerald Maurice (1992) Bright and Brilliant Fire: On the Matters of the Mind. New York: Basic Books

Gazzaniga, Michael S. (2008): Human: The Science of What Makes us Unique. New York: Harper-Collins.

Gibbons, Ann. (2007) “Food for Thought.” Science 316. 1558-1560.

Gould, Stephen Jay and Richard Lewontin (1979). "The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme". Proclamations of the Royal. Society of London B: Biological Sciences 205 (1161): 581–98.

Gould, Stephen Jay, and Elizabeth S. Vrba (1982), "Exaptation — a missing term in the science of form," Paleobiology 8 (1): 4–15.

Lewin, Roger (2005): Human Evolution: An Illustrated Introduction. Oxford: Blackwell.

Marcus, Gary (2008): Kluge: The Haphazard Evolution of the Human Mind. London: Faber and Faber.

Povinelli, Daniel .J. and Jennifer Vonk. (2003) Chimpanzee minds: Suspiciously human? Trends in Cognitive Sciences, 7.4, 157–160.

Preuss Todd M. (2004): What is it like to be a human? In: Gazzaniga MS, editor. The Cognitive Neurosciences III, Third Edition. Cambridge, MA: MIT Press: 5-22

Rilling, James K. (2006.) “Human and NonHuman Primate Brains: Are They Allometrically Scaled Versions of the Same Design?” In: Evolutionary Anthropology 15: 67-77

A Zombie’s Inquiry Into the Evolution of his Most Favorite Meal II

In my last post I listed some important factors in the evolution of the human brain, or better imagined what a zombie evolutionary biologist, named George, might dig up when investigating the evolutionary path of his Dinner Nr. 1. As Terrence Deacon (1997) puts it, there is no escaping the fact that human brains are unusually large. There are several factors why humans could develop large brains, but what is still at stake is the question why they actually did, and how they came into a position that allowed them to devote so much energy to a walnut-shaped pink lump of tissue with the consistency of a half-baked egg.
This question is critical, because organisms do not normally develop new traits just because they can. This of course also happens, in combination with random genetic drift and populations bottlenecks. But because evolution is a ‘miserable and greedy tinkerer’, or put more nicely, an economical process, it is highly unlikely that it produces needless and incredibly complex capacities that are extremely costly to maintain. As a consequence,

“some proportionately beneficial advantage must have driven brain evolution against the steep selection gradient created by the high costs of brain tissue.” (Dunbar & Shultz 2007)

Why, Dunbar and Shultz ask further, do primates have so much bigger brains than squirrel, with both facing about the same foraging decisions? (Dunbar & Shultz argue that ecological explanations fail to account for this differences, but their Chimpanzee-Squirrel dichotomy nevertheless is a bit hyperbolical, given that, among primates, those whose diet includes insects and fruits show higher encephalization rates than leaf-eaters, and strategic hunting and gathering of food and prey places additional demands on navigational, representational and other cognitive skills. However, their general argument is still valid. (Park et al. 2007))

To shed light on this issue, we can divide the big picture into several smaller ones. Useful questions include: What are we good at? Split into What are we (primates) good at? And What are we (humans) even better at than other primates? What could the ecological niche favoring big brains in humans have looked like? How exactly does our brain differ from that of other primates?
These questions essentially depend on comparative ethology (how do our minds work compared to how the minds of other animals work?), comparative (neuro)anatomy (On which evolutionary foundations are our modern cognitive abilities, and other phenotypic traits built upon?), and the kind of scenario we envision or infer from these observations togther with the fossil record and other lines of evidence. Of course it is also crucial what we think what the most salient and essentially aspects of our ancestors were. Do we see our ancestor as “Man the Tool Maker”, “Man the Hunter” or “Man the Social Animal”, or just as “Man with the extraordinarily big & expensive (and extremely delicious, George might add) brain”.

Well, of course Man should probably rather be seen as “Man the cooperative, competitive, tool-making, hunting, {…}, articulate social animal.” And all of these property probably contributed (co-evolutionary, we might say, without adding much in terms of explanatory adequacy) to our cognitive abilities and brain size, but in which order? And which driving forces were a little more pushy than others?
As Cheney & Seyfarth (2007) have show in baboons, interactions in primate groups are cognitively highly demanding and require sophisticated representational and predictive abilities, because of the intricate and complex networks and ‘friendships’ they inherit. Thus rising complexity in social life could be seen as a key selection pressure in the evolution of cognitive abilities in primates in general, and especially in humans. (Lewin 2005: 220f.)
Depending on which aspect one wants to stress, this correlations can be described in different terms. Scholars who wanted to stress the competitive aspect of social life dubbed it the “Machiavellian Intelligence Hypothesis.” (Byrne & Whiten 1988: who themselves, interestingly, didn’t want to stress the competitive aspect by giving the hypothesis the title). Now it is most widely called the “Social Brain Hypothesis” to emphasize the general complexity of primate groups including all arising affordances (Dunbar 1998, Dunbar & Shultz 2007).
Unfortunately, this is still rather vague. To get a clearer picture, it is important to make explicit the advantages and disadvantages of large social groups and the specific problems which need to be solved. However, group size indeed seem to contribute advantageously to genetic fitness by minimizing predation risk, but paired with greater ecological and reproductive competition and suppression, affording higher behavioral flexibility (Dunbar & Shultz 2007). Brain expansion theories stressing the importance of ’technological intelligence’ as a driving force. (without neglecting the importance of social factors, but seeing the latter as less crucial). According to these views, the ‘behavioral drive’ for cultural transmission and innovation is more frequent in species with large brains. As a consequence these species are led to exploit the environment in new way, opening up new possibilities regarding new selective pressures. ´(Reader and Laland 2002). Certainly, these tendencies were important, but where do they come from? Big-brains seem to be a prerequisite for ‘technological intelligence’, but how did these evolve in the first place? Rather it seems probable that

“Although innovation, tool use, and technological invention may have played a crucial role in the evolution of ape and human brains, these skills were probably built upon mental computations that had their origins and foundations in social interactions.” (Cheney & Seyfarth 2007: 283).

Supporting Reader and Laland’s emphasis on the importance of technological aspects on human cognitive evolution, Tomasello and his colleagues propose that human’s advanced Theory of Mind-skills were amplified not in the context of intention-reading present in great apes, but rather during learning and imitation of hierarchical planned and structured tool-making and tool-using. (Tomasello et al. 2005: 687). I’m not sure whether George would like this speculation. Probably, he would argue this to be a ‘just-so story’ and propose that all scientist coming up with these should be eaten. So thank God scientists are not really zombies, I wouldn’t miss the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany (and especially its co-director) for anything in the world (OK, except for the really important things such as love, life, family, donuts.

I haven’t addressed much of the questions stated in the beginning, especially What we as humans are especially better at than other primates. I will come to this issue in my next post (relying again on research done by scientists from the Max Planck Institute for Evolutionary Anthropology, so again, glad they haven’t been eaten.)

References:

Cheney, Dorothy L. and Robert M. Seyfarth. 2007. Baboon Metaphysics: The Evolution of a Social Mind. Chicago: University of Chicago Press.

Deacon, Terrence William 1998. The Symbolic Species. The Co-evolution ofLanguage and the Brain. New York / London: W.W. Norton

Dunbar, Robin I.M.1998.“The Social Brain Hypothesis” Evolutionary Anthropology 6: 178-190.

Dunbar, R. I. M. and Susanne Shultz. 2007.“Evolution in the Social Brain” Science 317: 1344-1347

Park, Min S., Andrew D. Nguyen, Henry E. Aryan, Hoi Sang U, Michael L. Levy, Katerina Semendeferi. 2007. “Evolution of the Human Brain: Changing Brain Size and the Fossil Record.” Neurosurgery 60:555–562.

Reader, S.M. and K.N. Laland. 2002. “Social Intelligence, innovation, and enhanced brain size in primates” PNAS 99: 4436-4441

Tomasello, Michael, Malinda Carpenter, Josep Call, Tanya Behne, and Henrike Moll. 2004. “Understanding and Sharing Intentions: The Origins of Cultural Cognition.” Behavioral and Brain Sciences 28

Zombies have Taste

In my last post I wrote about the fact that human brains are selfish energy-hungry little bastards, which makes the stuff they’re made of extremely ‘expensive tissue’ (Aiello & Wheeler 1995). This means that zombies have a quite extraordinary taste, equivalent to a caviar-gourmet (either that, or they are ‘informavores’ just like we are (Miller 1991)).
Now imagine (instead of the oft-cited martian scientist) a zombie-evolutionary biologist (Insert joke about the parasitic tendencies of the ‘mindless new atheism’ and/or Intelligent Design, the Idea of theistic evolution, greedy reductionism, Evolutionary Psychology or whatever floats you boat here) puzzling over the evolutionary emergence of his most favorite meal. Let’s take it for granted that our zombie-scientist is not easily satisfied by zombie-centric evolutionary concepts, just as Steven Pinker warns us that, if Elephants were the most culturally advanced species (well, and maybe they are, who knows), their evolutionary biologist (albeit only the bad ones) would probably search for the evolutionary path that inevitably climaxed in the highest form, the evolutionary optimum of trunkitude. Let’s also assume our zombie-scientist isn’t a friend of ‘just-so’ stories like ‘humans evolved bigger brains to run away from zombies more effectively’. Assuming, too, that human scientist like Aiello, Wheeler, Dunbar and others weren’t eaten before publishing their caveats about the expensiveness of brain evolution and maintenance, or that some other zombie-scientists could hold back their hunger long enough to test human subjects before eating them, coming to similar conclusions as Aiello and others did – or, rather would have come if they hadn’t been eaten beforehand. Assuming this, we could be sure that our zombie-scientist would not regard the evolution of a ‘general being-eaten-avoidance intelligence’ as unlikely.

What then, our zombie-scientist, call him George, would ask, was the reason humans developed such large, specialized brains. Looking for homologues or convergent evolution in other (hopefully not entirely eat… I mean extinct) species, and considering what makes the human mind special. George could come up with a lot of possible hypotheses as driving forces and triggers of brain evolution, and other facts he would have a hard time to make sense of such as:

• positively selected genes involved in regulating (Microcephalin: Evans et al. 2005) and determining brain size (ASPM: Mekel-Brobov et al. 2005), development of the human neocortex (HAR1F: Pollard et al. 2006), and playing a part in progressive changes in cognitive abilities (Neuropsin: Li et al. 2004)
  
• 
  
• cooking, paired with gastrointestinal shrinkage (our intestinal tract is only 60% the size expected of a primate with similar size) may have saved energy from digestion which in turn could be used to help fuel the brain. Together with the possible role of meat and more efficient upright walking and running, this could have expanded the human energy budget significantly (Gibbons 2007)
  
• 
  
• supporting this hypothesis, AMY1, a gene improving the digestion of food containing starch, is found in much greater numbers in humans than in chimpanzees (Perry et al. 2007)
  
• 
  
• Correlations between group size and neocortex size (Dunbar 1993, Dunbar & Shultz 2007) on the one hand, and significant positive correlation between innovation, social learning, tool use and brain size on the other, (Reader and Laland 2002, Reader 2003), making it likely that social and technological innovative intelligence (mediated by social learning) both played a crucial and inseparable role in human brain evolution (Cheney & Seyfarth 2007)
  
• 
  
• The possibility that the ability to evolve fat babies was the precursor for the evolution of the big and metabolically expensive brain (The article proposing this hypothesis is called ‘survival of the fattest’, What a great pun! Er… or maybe not) . In a resting newborn baby, the brain consumes 74% of the baby’s energy intake. In a 4-6 months old baby the rate is 64%, further dropping during ontogenetic development until reaching a rate of about 23% in adults. And whereas in chimpanzee infants, there is virtually no body fat, in human infants body fat contributes about 11-14% of the baby’s weight (as does the baby’s brain) (Cunnane & Crawford 2003)
  

So one thing is clear: a stable high-energy food supply was essentially necessary for human brain development, as were the possibility for longer ontogenetic development (as often observed, human (and generally primate) newborns are pretty much helpless compared to newborns of other species, with some even able to walk following almost immediately after birth).
Another important aspect is the general tendency in mammals to develop bigger brains compared to other species (they are about 10 times ‘brainier’ than amphibians or reptiles). Then, humans are part of the order of primates, which (along with toothed whales) have bigger brains than other mammals. And among primates, monkeys and apes have the biggest brains. But, as I said, our brains are even three times bigger than that expected of an ape of similar size. Another factor is the fact that the pre-natal rapid brain growth observed in other species whose infants are relatively helpless continues post-natally in human babies for about twelve months instead of changing into a slower pace.
As a consequence, human infants are even more helpless than that of other primates. This requires a much greater devotion of time, energy and other resources from the parent’s side. (Lewin 2005: 217f., John L. Locke and Barry Bogin (2005) make a similar argument concerning the unique human life history and ontogenetic development, but extending it not only to brain growth in general, but also to the evolution of language).
Making such a list, George would probably have a lot of trouble to distinguish preconditions, epiphenomena, co-evolutionary processes and driving forces of brain expansion. In my next post I will try to shed some light on this issue (Of course I will fail even more grotesquely than someone who is not a complete layman, but I hope that I will at least clarify some points)

References:

References:

Aiello L.C. and P. Wheeler 1995. ”The expensive tissue hypothesis: the brain and the digestive system in human and primate evolution.” Current Anthropology 36:199–221

Cheney, Dorothy L. and Robert M. Seyfarth. 2007. Baboon Metaphysics: The Evolution of a Social Mind. Chicago: University of Chicago Press.

Cunnane Stephen C. and Michael A. Crawford. 2003. “Survival of the fattest: fat babies were the key to evolution.” Comparative Biochemistry and Physiology Part A: 136.1: 17-26

Dunbar, R.I.M. 1993. “Co-evolution of Neocortex size, group size and language in humans.” Behavioral and Brain Sciences 16.4: 681-735

Dunbar, R. I. M. and Susanne Shultz. 2007.“Evolution in the Social Brain” Science 317: 1344-1347

Evans, Patrick D., Sandra L. Gilbert, Nitzan Mekel-Bobrov, Eric J. Vallender, Jeffrey R. Anderson, Leila M. Vaez-Azizi, Sarah A. Tishkoff, Richard R. Hudson, Bruce T. Lahn “Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans” Science 309: 1717-1720.

Gibbons, Ann. 2007. “Food for Thought.” Science 316. 1558-1560.

Lewin, Roger. 2005. Human Evolution: An Illustrated Introduction. Fifth Edition. Suffolk: Blackwell.

Locke, John L. and Barry Bogin. 2005. “Language and life history: A new perspective on the development and evolution of human language” Behavioral and Brain Sciences

Mekel-Bobrov, Nitzan, Sandra L. Gilbert, Patrick D. Evans, Eric J. Vallender, Jeffrey R. Anderson, Richard R. Hudson, Sarah A. Tishkoff, Bruce T. Lahn. “Ongoing Adaptive Evolution of ASPM, a Brain Size Determinant in Homo sapiens.” Science 309: 1720-1722

Li, Yi, Ya-ping Qian, Xiao-jing Yu,* Yin-qiu Wang, Ding-gui Dong’ Wei Sun, Run-mei Ma and Bing Su. 2004. “Recent Origin of a Hominoid-Specific Splice Form of Neuropsin, a Gene Involved in Learning and Memory.“ Molecular Biology and Evolution 21.11: 2111-2115.

Miller, G.A. 1991. The Science of Words. New York: W.H. Freeman

Reader, S.M. 2003. “Relative brain size and the distribution of innovation and social learning across the nonhuman primates.” The Biology of Traditions: Models and Evidence. Eds. D.M. Fragaszy and S. Perry, 56-93.

Reader, S.M. and K.N. Laland. 2002. “Social Intelligence, innovation, and enhanced brain size in primates” PNAS 99: 4436-4441.

Pollard, Katherine S., Sofie R. Salama, Nelle Lambert, Marie-Alexandra Lambot4, Sandra Coppens, Jakob S. Pedersen, Sol Katzman, Bryan King, Courtney Onodera, Adam Siepel, Andrew D. Kern, Colette Dehay, Haller Igel, Manuel Ares Jr, Pierre Vanderhaeghen & David Haussler. 2006 “An RNA gene expressed during cortical development evolved rapidly in humans.” Nature 443: 167-172.

Perry, George H, Nathaniel J Dominy, Katrina G Claw, Arthur S Lee, Heike Fiegler, Richard Redon, John Werner, Fernando A Villanea, Joanna L Mountain, Rajeev Misra, Nigel P Carter, Charles Lee, & Anne C Stone. 2007 “Diet and the evolution of human amylase gene copy number variation“ Nature Genetics Advanced Online Publication doi :10.1038/ng2123

Brains are Expensive

A while back, Larry Moran over at the Sandwalk wrote about previous expectations of scientist about the number of genes in the human genome. (See also his discussion of Pennisi 2005) The guesses ranged from 140,000 to 15,000 genes. Reading this I was reminded of a statement from Thompson (2001) which I quoted in a term paper about language evolution: 30,000 to 50,000 genes of the human DNA are present in all cells, but only activated in brain cells. This would be a quite extraordinary feat, given that there seem to be only about 20,488 genes at all in the human genome. (Pennisi 2007). But there are other statements that characterize the ‘hyperastronomical’ dimensions (Quine 1987) of the brain quite well. (Quine used the word when describing the vastness of Borges’ fictional Library of Babel, a universe-sized library which contains books with all possible combinations of characters there are, making it impossible to find even one comprehensible sentence. But as a complete layman, I sure sometimes feel the same when reading an article about neuroscience… or anything else for that matter)

The Brain consists of an incredible 10¹⁰ neurons and 10¹³ synapses (Gegenfurtner 2005) and the number of possible connectional combinations between is sometimes estimated to be greater than the number of molecules in the universe (Ramachandran) no wonder it is sometimes called the most complex structure in the know universe. Further, with its about 1251.8 cubic centimeters (cc) (compared to the 316.7 cc of greater apes) (Rilling 2006),our brain is about three times bigger than that of an ape would be, given the same body-size (Lewin 2005).
Just as remarkable is the amount of energy the brain consumes. Although the human brain weighs only 2% of an adult’s body weight, it accounts for 20-25% of its resting energy intake. As a comparison, a chimpanzee’s brain only consumes about 8-9% of its resting metabolism. Thus, the brain uses energy at the same rate consumed by leg muscles of a marathon runner when running (Attwell & Laughlin 2001. It seems like that one could make a good joke out of this fact, but sadly I can’t really imagine how.) The costs to run a human brain are, per unit mass, about 8 to 10 times higher than those for skeletal muscles. In the energy-consuming business, it is only topped by the heart. (Dunbar & Shultz 2007).

At the moment there doesn’t seem to be enough data to estimate which brain areas consume how much energy, but there are two promising candidates for the title of ‘hungriest brain area’. 50% to 60% of the human cerebral cortex are devoted to the perception and interpretation of visual stimuli and the reactions to it. Given that, perception is an incredibly complicated and complex task, this seems quite understandable: In sum, the brain receives about two gigabyte of data per second from approximately 200 million photo receptors in the eyes (Gegenfurtner 2005). Another aspirant for the title are the auditory areas, where metabolic rates seem to be 40% greater than in other parts of the brain (Attwell & Laughlin 2001).
Brains really are made out of “expensive tissue” (Aiello & Wheeler 1995). Because of this, specialized intelligence is far more likely to be created by natural selection than general intelligence, or a brain that is ‘just large’ due to the sheer cost of evolving and maintaining such a thing. (Cheney & Seyfarth 2007: 11) (Ha! Suck on this Jean Piaget! ;) ) As Robin Dunbar puts it: “Because the cost of maintaining a large brain is so great, it is intrinsically unlikely that large brains will evolve merely because they can. Large brains will evolve only when the selection factor in their favor is sufficient to overcome the steep cost gradient“ (Dunbar 1998: 179). In my next post I will consider some of the proposals why we do have such damn hungry and huge brains.

References:

Aiello L.C. and P. Wheeler 1995. ”The expensive tissue hypothesis: the brain and the digestive system in human and primate evolution.” Current Anthropology 36:199–221

Attwell, David and Simon B. Laughlin. 2001. “An Energy Budget for Signaling in the Grey Matter of the Brain.” Journal of Cerebral Blood Flow and Metabolism 21:1133–1145.

Dunbar, Robin I.M.1998.“The Social Brain Hypothesis” Evolutionary Anthropology 6: 178-190.

Dunbar, R.I.M. and Susanne Shultz. 2007. .“Evolution in the Social Brain” Science 317: 1344-1347

Lewin, Roger. 2005. Human Evolution: An Illustrated Introduction. Fifth Edition. Suffolk: Blackwell.

Quine, W. V. 1987. Quiddities: An Intermittently Philosophical Dictionary. Cambridge, MA: Belknap Press

Pennisis, Elizabeth. 2005. "Why do Humans have so Few Genes?" Science 309: 80.

Pennisis, Elizabeth. 2007. "Working the (Gene Count) Numbers: Finally, a Firm Answer?" Science 316: 1113.

Rilling, James K. 2006. Human and NonHuman Primate Brains: Are They Allometrically Scaled Versions of the Same Design? Evolutionary Anthropology 15: 67-77.

Thompson, Richard F. 2000. The Brain: A Neuroscience Primer. Third Edition. New York: Worth Publishers.