For most of hominid evolution, newer meant bigger. Newer species had bigger brains than older ones, and later members of a species had bigger brains than earlier members (Rightmire, 2001). And for generations researchers have puzzled over the Neanderthal’s quick demise (Hrdlicka, 1927), especially puzzling in light of apparently developed communicative abilities (Arsenburg, Tilier, Vandermeersch, Duday, Schepartz, & Rak, 1989) and the fact that some Neanderthals may be more closely related to humans than other members of their own species (Paablo, 2003). Yet fifteen thousand years ago the human brain began shrinking (Ridley, 2003). Though perhaps the decline is older than that – Neanderthals may have had larger brains than we do (Klein, 2003).
I do not know what this means. We know that “within primates the relative size of the neocortex is significantly correlated with group size” (Bjorklund & Pellegrini, 2002, 102). We like to think that our brains make us special, though apparently the seemingly-simpleminded purposes are large-brained as well (92). Additionally, considering that “brain size is correlated (negatively) with litter size” and that larger-brained “animals tend to have smaller litters and to give birth to infants at longer intervals” (97), this implies that modern humans are more expendable and less precious than our ancestors of fifteen thousand years ago, or even the ancient Neanderthals! Clearly humans are evolving, but how and why?
Bjorklund & Pellegrini give hints of an answer. They write that “brain growth continues into adolescence” (100) and (quoting Bjorklund & Green, 1992) “lessons learned as a young child will not interfere with the qualitatively different tasks required of an adult” (108). These facts must be synthesized with a view of evolution that leads to us, an agricultural species, to having smaller brains and the Neanderthals, another recent non-agricultural species, to have larger ones. The most likely explanation to me is that human agriculture allowed even young children to become productive workers, as there are a myriad of tasks on a farm requiring little muscluar or intellectual strength (such as feeding chickens, etc.), and that human society allowed the formation of an “anatomically distinct worker caste” (Wilson & Holldobler, 2005, 13368). In other words, children are something like worker bees, who learn lessons appropriate for worker bees, but upon adolescence are able to be reprogrammed to be functioning adults. Thus the claim that “childhood and adolescence, are not observed in any other species” (Bjorklund & Pellegrini, 2002, 99) misses the mark – asexual workers exist in many species, and adolescence is a form of functional cocooning. And this is why (quoting Mason, 1968) “Developmental stages are less sharply delineated in humans than in other primates. Sensitive periods in development are more difficult to establish…” (Bjorklund & Pellegrini, 2002, 106): humans develop twice, once into a worker, and then into an adult.
Clearly, a view of children as “worker humans” should not be taken to extremes. Deprived environments will hamper children through the rest of their lives (Bjorklund & Pellegrini, 2002, 105) and children are safer when cared for by biological parents (Buller, 2005). Yet many children are surprisingly resilient to early traumas (Caspi, et al., 2003) and the traits that predict criminality may be largely heritable (Pinker, 2002, 315) so most children may be all but assured a good life. Other policy implications of resilient cihldren – everything from social services to educational styles – are too many to list.
Yet this gets me away somewhat from my primary question, about brain size. Clearly it would be possible for humanity to develop children as a worker caste without limiting the skull size of adults. Even if skull size and less reproductively valuable children correlate, unless these effects are caused by the same alleles there still has to be a reason for our smaller brains. My guess is that this is also from socialization, and that there is less need for us to think now that we have evolved to live in agricultural communities. If a caveman is largely on his own, with only his band to protect him, he must be a jack-of-all-trades. Everything from possible ritual cannibalism (White, et al., 1991) to warfare (Zollikofer, Ponce de Leone, Vandermeersch, & Leveque, 2002) would have to be done with the same band, meaning a successful live with a cognitively flexible life. However, humans in a modern economy rely on others for most of their needs, and they only need to learn a few things well. Thus the human brain may be evolved to be a specialist – an extraordinary mind (Gardner, 1998) — in only one domain, and a naïve generalist in others. Anyway – that’s my guess.
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Bjorklund, D.F. & Green, D.L. (1992). The adaptive nature of cognitive immaturity. American Psychologist 47: 46-54.
Bjorklund, D. F., & Pellegrini, A. D. (2002). The origins of human nature: Evolutionary developmental psychology. Washington, DC: American Psychological Association.
Buller, D.J. (2005). Adapting Minds. MIT Press: Cambridge, MA.
Capsi, A., et al. (2003). Influence of Life Stress on Depression: Moderation by a Polymorphism in the 5-HTT Gene. Science. Vol. 301 No. 5631 pp. 386-289.
Gardner, H. (1998). Extraordinary Minds. Basic Books: New York, NY.
Hrdlicka, A. (1927). The Neanderthal phase of man. The Journal of the Royal Anthropological Institute of Great Britain and Ireland 57: 249-274.
Klein, R.G. (2003). Whither the Neanderthals? Science 299(5612): 1525-1527.
Mason, W.W. (1968). Early social deprivation in the nonhuman primates: Implications for human behavior. In D.C. Glass (Ed.), Environmental influence (pp. 90-101). New York: Rockefeller University Press.
Paabo, S. (2003). The mosaic that is our genome. Nature 421: 409-412.
Pinker, S. (2002). The Blank Slate: The Modern Denial of Human Nature. Viking Adult: New York, NY.
Ridley, M. (2003). Nature via Nurture. Harper Collins: New York, NY.
Rightmire, G.P. (2001). Brain size and encephalization in early to Mid-Pleistocene Homo. American Journal of Physical Anthropology 124(2): 109-123.
White, T.D., et al. The question of ritual cannibalism at grotta guattari [and comments and replies]. Currently Anthropology 32(2): 118-138.
Wilson, E. O., & Holldobler, B. (2005). Eusociality: Origin and Consequences. PNAS 102(38)-13367-13371.
Zollifoker, C.P., Ponce De Leone, M.S., Vandermeersch, B., & Leveque, F. (2002). Evidence for interpersonal violence in the St. Cesaire Neanderthal. PNAS 99(9): 6444-6448.
Evolutionary Cognitivism, a tdaxp series
1. Selection and Cognition
2. Epigentics and Diversity
3. Children and Civilization
4. The Implicit and the Explicit
5. Man Among Men
6. More Than Genes