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HonoreDB comments on Brain shrinkage in humans over past ~20 000 years - what did we lose? - Less Wrong
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The optimist's explanation is that we're shedding hardwired macros.
What happened 20,000 years ago to make a large chunk of macros useless?
Our environment and lifestyle started changing, rendering some instincts obsolete. Also possibly we got better at conscious reasoning, so our instincts became less necessary or counterproductive.
A few years ago a friend dropped a soda can, and it developed a small hole, causing soda to hiss out and the can to start writhing around in the grass, propelled by the escaping stream. The two of us leaped back in terror, my heart started pounding, and I had trouble speaking for a moment. It took a bit for me to figure out why we had that reaction...it was much more than being startled. I'm pretty sure we have a hardwired macro that says "hissing cylindrical thing writhing around in the grass" => "leap away from it, stare at it, get scared" but never bothers to dump the symbol "snake" into our conscious thought-stream. Presumably we used to have a lot more instincts like that, but as the ratio of real hyena encounters to false positives decreased, the hyena macros became liabilities, or not worth the brain mass.
Or we're getting dumber. I could definitely believe that.
Hmm, our cats are packed with instincts like this. The housecat's brain is 30 grams source I dunno how much weight we can lose by shedding instincts but instincts don't take up a lot of brain.
BTW, one thing that really surprises me about cats is how smart they are for the brain size. Our (my and my gf's cats) a: are loaded with various instincts and reflexes, b: can understand stuff on first try. I have a small spray bottle that I tried to use as minor negative reinforcement to get dog and cats to stop doing annoying stuff. Dog seems negatively reinforced all right.
Cat sees the bottle pointing in his direction first time, looks with curiosity, then keeps on trying to get into food on stove, i spray him, he sort of lazily runs off. From that time on, he sees bottle turning his way, he just knows what the bottle does, he runs away. He understands how this works. Dog isn't like that, she's behaving more like getting conditioned over time.