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The hominid brain grew at an accelerating pace until it reached its present size of 1500cc about 200,000 years ago. Yet uniquely human abilities such the invention of highly sophisticated "standardized" multi- part tools, tailored clothes, art, religious belief and perhaps even language are thought to have emerged quite rapidly around 40,000 years ago — a sudden explosion of human mental abilities and culture that is sometimes called the "big bang." If the brain reached its full human potential — or at least size — 200,000 years ago why did it remain idle for 150,000 years? Most scholars are convinced that the big bang occurred because of some unknown genetic change in brain structure. For instance, the archeologist Steve Mithen has just written a book in which he claims that before the big bang there were three different brain modules in the human brain that were specialized for "social or machiavellian intelligence", for "mechanical intelligence" or tool use, and for "natural history" (a propensity to classify). These three modules remained isolated from each other but around 50,000 years ago some genetic change in the brain suddenly allowed them to communicate with each other, resulting in the enormous flexibility and versatility of human consciousness.

I disagree with Mithen ingenious suggestion and offer a very different solution to the problem. (This is not incompatible with Mithen's view but its a different idea). I suggest that the so-called big bang occurred because certain critical environmental triggers acted on a brain that had already become big for some other reason and was therefore "pre-adapted" for those cultural innovations that make us uniquely human. (One of the key pre adaptations being mirror neurons.) Inventions like tool use, art, math and even aspects of language may have been invented "accidentally" in one place and then spread very quickly given the human brain's amazing capacity for imitation learning and mind reading using mirror neurons. Perhaps ANY major "innovation" happens because of a fortuitous coincidence of environmental circumstances — usually at a single place and time. But given our species' remarkable propensity for miming, such an invention would tend to spread very quickly through the population — once it emerged.

Mirror neurons obviously cannot be the only answer to all these riddles of evolution. After all rhesus monkeys and apes have them, yet they lack the cultural sophistication of humans (although it has recently been shown that chimps at least DO have the rudiments of culture, even in the wild). I would argue, though, that mirror neurons are Necessary but not sufficient: their emergence and further development in hominids was a decisive step. The reason is that once you have a certain minimum amount of "imitation learning" and "culture" in place, this culture can, in turn, exert the selection pressure for developing those additional mental traits that make us human . And once this starts happening you have set in motion the auto-catalytic process that culminated in modern human consciousness.

A second problem with my suggestion is that it doesn't explain why the many human innovations that constitute the big bang occurred during a relatively short period. If its simply a matter of chance discoveries spreading rapidly,why would all of them have occurred at the same time? There are three answers to this objection. First,the evidence that it all took place at the same time is tenuous. The invention of music, shelters,hafted tools, tailored clothing, writing, speech, etc. may have been spread out between 100K and 5k and the so-called great leap may be a sampling artifact of archeological excavation. Second, any given innovation (e.g. speech or writing or tools) may have served as a catalyst for the others and may have therefore accelerated the pace of culture as a whole. And third, there may indeed have been a genetic change,b ut it may not have been an increase in the ability to innovate ( nor a breakdown of barriers between modules as suggested by Mithen) but an increase in the sophistication of the mirror neuron system and therefore in "learnability." The resulting increase in ability to imitate and learn (and teach) would then explain the explosion of cultural change that we call the "great leap forward" or the "big bang" in human evolution. This argument implies that the whole "nature-nurture debate" is largely meaningless as far as human are concerned. Without the genetically specified learnability that characterizes the human brain Homo sapiens wouldn't deserve the title "sapiens" (wise) but without being immersed in a culture that can take advantage of this learnability, the title would be equally inappropriate. In this sense human culture and human brain have co-evolved into obligatory mutual parasites — without either the result would not be a human being. (No more than you can have a cell without its parasitic mitochondria).


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