Neurons all the way down

My views on intelligence have shifted recently after listening to an episode of the Rationally Speaking podcast where Dr Herculano-Houzel talked about her book The Human Advantage. She made a number of fascinating points based on her pioneering research on measuring the number of neurons in various species' brains:
  1. The brains of primates have higher neuron density than the brains of other mammals, particularly in the cerebral cortex, which is largely responsible for higher-level abstract thought.
  2. Amongst primates, number of neurons is roughly proportional to body mass.
  3. Apes are the main exception to the latter rule; their diets aren't calorie-rich enough for them to support brains as large as the trend would suggest.
  4. Humans, by contrast, fit right on the trend line: our innovations such as using tools and fire to cook food allowed us to obtain more calories and therefore grow bigger brains than other apes (bipedalism also helped, reducing the energy cost of walking by a factor of 4).
  5. This means that humans have many more cerebral cortex neurons than any other species, even those which have much larger brains than ours like elephants and whales (edit: except for the long-finned pilot whale, which apparently has over twice as many cerebral cortex neurons as we do).
I take these claims to be significant evidence that there are few 'hard' steps between animal-level and human-level intelligence. Firstly because the simplest explanation (that human brains grew fairly normally, without major cognitive innovations) is now more likely to me. Secondly because, if modifying mammalian brain architectures to support human-level intelligence were hard, then we should probably expect to observe other mammals with similar numbers of cerebral cortex neurons as we have, but which don't have the right brain architectures to use that capacity and so aren't as intelligent as us. As noted in point 5, though, that's not the case. Thirdly, because other primates - even apes - don't seem to be much smarter than elephants or dolphins, and so the main effect of the "primate advantage" described in point 1 is probably more about making neuron-dense brains physically viable than making fundamental changes to the mammalian cognitive architecture.

The podcast also reminded me of the well-known observation that AI researchers used to expect tasks involving abstract thought to be hard, and tasks involving primitive brain functions to be easy, and it turned out to be the other way around. I really should have taken that idea more seriously before now in considering the difficulty of implementing general intelligence.

In conclusion, I expect building an animal-level AI to take longer than I thought; that AI to go from animal level to human level much more quickly than I thought; and a greater likelihood that we'll do the above using neural networks than I thought.


  1. Wikipedia has a List of animals by number of neurons which lists the long-finned pilot whale as having 37.2 billion cortical neurons, versus 21 billion for humans.

  2. Moravec's paradox may be of interest

    Symbolic species argues that it isnt just more neurons though. It is especially more neurons in the PFC and more recently we are realizing also in the cerebellum.


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