While I am myself wildly enthusiastic about the need to understand the complexity of the human brain, I fear that the stampede towards emphasis on "the daunting quantitative magnitude of the brain" has dangerously tipped our canoe to one side. Biologists like Eric Kandel have shown that memories can be produced and learning can occur at the level of individual synapses. While it is true that most learning and memory involves groups of neurons, we need not be consumed by a "bigger is automatically better" attitude with respect to brains. For example, many people fret over brain size issues like
the fact that Neanderthals had larger brains than modern humans
and
the relative sizes of male and female brains.
Men, monkeys, and even mice have enough neurons to produce intelligent behavior. There are other issues that are more important than shear size of brains. One of these issues that Edelman touches on (all too briefly) is WHEN brain growth occurs. Modern humans are most remarkable in terms of the amount and duration of POST-natal brain growth. Edelman mentions this aspect of human neotenic development, but does not make a big deal out of it.
In his book, The Origins of Order, Stuart Kauffman has provided an analysis of the immune system as containing a "universal tool kit" for recognition of molecular structures. We produce and carry around inside us enough different types of antibodies and T-cell receptors that we are able to respond to any possible foreign invader that might show up, such as the AIDS virus.
We can in a similar way ask how many neurons and synapses an animal brain needs in order to be able to have a "universal neural net" capable of dealing with everything a brain might encounter in the course of a human life. My GUESS is that a modern human brain is probably much larger than what is required. The evolutionary biologist will then demand of me, "Oh, ya, then why would our large brains have been evolutionarily selected for in the first place?"
I suspect that most of large brain size has been generated starting during early non-human primate evolution and continuing until about 100,000 years ago within the direct human-specific lineage as an unavoidable side effect of another evolutionary process. I think that what WAS actively selected for (and what gave primates in the human lineage selective advantage) was neotenic brain development and forms of brain plasticity that were extended into post-natal life. If you made a type of primate with a brain the same size as a modern human but a with a developmental program that caused the brain to develop to full size mostly before birth (as is the case in all non-human primates) then you would not end up with a primate that could be human. The flip side of this proposal is that if we genetically altered a chimp so that more of its brain growth was post natal, then you would end up with a much more human-like chimp, for example, you would get a much more linguistically capable chimp. Size is not the key difference between chimp brains and human brains. Four million years ago there were bipedal human ancestors with chimp sized brains. There were tool-using human-lineage primates living about 3.5 million years ago with brains no larger than the brain of a chimp.
Unfortunately, post-natal brain growth does not leave fossils, but I suspect that delay in brain maturation has usually gone hand-in-hand with increases in primate brain size. If we look at living primates (see Deacon's The Symbolic Species for some of these data), we see correlations between increasing brain size, delays in maturation, and even lengthening of life span. If you are going to shift a primate survival strategy from instincts to social learning, you need post-natal brain plasticity and long periods of time to learn and use what you have learned. My guess is that in terms of developmental mechanisms, the easiest way to increase brain plasticity in a post-natal animal is to allow a brain to continue to grow after birth.
This correspondence between learning and brain growth has been found in birds that learn to sing mating songs. The brain region responsible for learning the songs has active growth and production of neurons during mating season and can quickly double in size. I suspect that most of the bulk of human brains has been produced as a side effect of selection for post-natal human brain plasticity. The easiest way to get plasticity and an increased capacity for learning was to let the brain continue to grow after birth. Recent evidence has been found that shows that new neurons and synapses continue to be produced throughout human adult life in brain regions that are important for learning and momory such as the hippocampus.
During the past 100,000 years there has probably been selection to prune down the size of the human brain while retaining postnatal plasticity. The large size of the modern human brain is probably an energetically expensive extravagance that is actively being selected against. As long as we can have post-natal brain plasticity, we do not need so many neurons. Neanderthals may have disappeared because they could not compete with SMALLER brained, more efficient humans.
So my proposal is that the human brain's development was shifted in time with respect to birth. For humans, most brain growth happens after birth. This allows for greater learning from one's social group, such as learning language.
Some ideas on how to integrate the study of language into the study of the mind.
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