Sometimes quantitative assessments lead to important ideas. I have been enjoying later chapters in my new book, The Encultured Brain: an introduction to neuroanthropology, edited by Daniel Lende and Greg Downey. Their early chapters giving synopses of later chapters to introduce the rest of the book seemed more a marketing ploy for yet another new academic discipline (worthy, yes, but do we really need a new term for every time we do cross discipline thinking?) Their chapter on “Evolution and the Brain” was, however, magnificent, and later chapters by others have so far been very interesting. So consider this list of findings from their brain evolution chapter.
- The biomass of humans is 8 times that of all the wild terrestrial vertebrates, i.e., we are successful replicants. (Also I remember von Neumann’s estimate that each human body has about a tablespoonful of genetic material in all its cells that control the soma).
- The genus Homo appeared around 2,000,000 years ago with a sudden increase in brain volume that then slowly increased until 500,000 years ago when another surge in brain size appeared.
- Human encephalization (the concentration of nervous tissue in a brain, i.e., head) is 5-7 times what would be predicted based on a mammal of our size.
- As the neocortex evolved to dominate lower brain structures, specialized cortical fields developed that facilitated complex processing and inter-connectedness throughout the brain. Early mammals have 15-20 cortical fields; humans have maybe 150.
- Larger areas both evolved later and mature more slowly.
- Our brains have continued a mammalian and primate trend in lateralizing so much that some scientist refers to us as the “lop-sided ape”. (In addition, remember that males and females have relatively different patterns in our connectome with males showing more connections within hemispheres and females more connections between hemispheres).
- Birds, fish and reptile brains grow throughout their life spans (neurogenesis or generating new neurons) but mammalian brains finish up neurogenesis relatively early.
- Our brains triple in volume after birth while other primate brains only double.
- Finally our post partum brain growth comes despite pervasive neural pruning in the first years of life; the estimates are that the adult brain has only 20-80% (quite a range, I know, but you get the idea) the number of neurons present at the peak early in life. Neurons survive because they become integrated into functional circuits; if they stay isolated, they die off.
All of these are pretty amazing and all support the idea that our brains are shaped, as Gerald Edelman maintained, first by genetic information and then in very large and important ways by experience.
Our connectome with many systems lit
Now Lende and Downey quote two well known neuroscientists (Cosmides and Tooby) that “our modern skulls house a Stone Age mind,” and I have no qualms about that because I think a Stone Age mind was actually a pretty good one (some politicians today cannot manage even that level of intelligence). They also point to the remarkable and wonderful development of our social capabilities and find that we are drawn to social interaction for “intrinsic emotional rewards” and not just self-interest for our genes’ continuation. And they recognize that the true power of human intellect, while derived from our rather different brains, is really due to the “synergy of many brains.”
Neuroanthropologists, as best I can see with my limited exposure, treat culture as part of our extended phenotype, i.e., culture is not an acquired overlay but is rather an integral component of the human Umvelt. It is a direct outgrowth of our biological roots of empathy and symbolization (though I do not see anything here about art). More to say later but I need to get to my farm work. Oh, one more recommendation for this book—the lists of references yield a lot of gems. Travel on.
The Biological Roots of Humanity 