Last Tuesday I faced a horrible dilemma as two books I'd been eagerly awaiting, Too Like the Lightning and The Age of the Em, both came out the same day. Luckily I was nearly done with the book I'd been reading, The Human Advantage by Suzana Herculano-Houzel, so I could quickly finish that before moving on and I wasn't tortured by a third option of whether to just put it aside.
In some ways this book was a good complement to The Secret of Our Success since it's another book about humanity's place in the world. But this one is much more about where we fit in with other species in terms of our brains and less about the story of how we got there. The real meat of the book is how the author figured out a good way to measure the number of neurons in a brain and was able to do the first real comparisons across species as to the number of neurons in their heads.
The method was surprisingly simple as brilliant ideas sometimes are. You can't just count the number of neurons in a small part of the brain and multiply because neuron density can vary quite a bit depending on where in someone's head you look. The simple solution is just to stick a brain in a blender first. Then you can sample and multiply to get a good count. There's really more to it than that but that's the basic idea.
The interesting thing the author discovered is the way that the number of neurons in a brain varies with brain volume works very differently in primates than it does in pretty much all the other animals she looked at. In most neurons are bigger in bigger brains and the number of neurons goes up as the 3/4 power. But with primates neurons stay the same size and the brain volume and number of neurons are directly proportional. The difference between 1 and 3/4 might not sound like much but different animals can vary in size but a factor of 10,000 easily which would give you a brain 10 times larger for one animal than another.
Another interesting discovery she was able to make was that the energy consumed by a brain is almost exactly proportional to the number of neurons in the brain, even though some brains have neurons that are much bigger than those in other brains. She does on to describe the things about humans that let us get enough calories to support our huge brains in a chapter I would have found fascinating if I hadn't read The Secret of Our Success recently.
I did find a few annoyances. The author made a big deal about the fact that the conventional wisdom was that human brains had 100 billion neurons but that she had discovered that humans really only had 86 billion. Except that all her samples were from men between the ages of 50 and 70 and we know that humans lose neurons as they age to some extent. And the largest of her 4 samples weighed in at 91 billion. So we can't really be totally sure from her observations that the average number of human neurons isn't exactly 100 billion even before you consider reasonable amounts of rounding for a variable number.
The book was maybe longer than it needed to be and trying too hard to impress in places but I enjoyed it overall and I'm glad I read it. There are a bunch of interesting facts about brains I haven't included here and another bunch of interesting anecdotes about how she got her hands on the different brains she examined. So I'd recommend reading it but don't be afraid to skim.
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