Monday, November 26, 2007

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)



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

1 comment:

akull said...

So true and funny post. :)