Suzana Herculano-Houzel’s
Discoveries
It all started with brain soup
It seemed like such a basic question: how many neurons was the human brain made of, and how did that compare to other animals? The popular answer was “100 billion neurons and 10 times as many glial cells”, but nobody could point out the references to those numbers. Her suspicion: nobody actually knew - and using standard microscopy methods would not give an answer. So, in 2004, she decided to turn brains into soup to figure it out. Twenty years later, she has that answer - and SO much more!
The human brain is remarkable, but not special
We have exactly as many cortical neurons as predicted for a generic primate of our brain size
Yes, our 16 billion cortical neurons is the MOST neurons in ANY cerebral cortex - but that is still just as many as expected. Our brains grew as humans evolved - but we are still like other primates in how our brains are made and scale.
We live exactly as long as predicted for a warm-blooded vertebrate with as many cortical neurons
No, humans are NOT “extraordinarily long-lived” - not once we realize that longevity doesn’t scale with body mass; it scales with the number of cortical neurons. In that case, we are exactly as long-lived as a generic warm-blooded vertebrate with our number of cortical neurons.
Our childhood is exactly as long as predicted for our number of cortical neurons
We did not “evolve an extended childhood”; just like longevity, sexual maturity depends on the number of cortical neurons - and we, again, meet the mark on the age at which we become sexually mature.
A brain is not a brain is not a brain
Bird brain, me? Why, thank you!
They have tiny brains for their body size, but they pack a TON of neurons. They are birds - and songbirds, parrots and owls have as many neurons as primates in their tiny brains!
Their brains may be as small as a rat’s or as large as a giraffe’s, but they hide SO many more neurons inside!
The hidden power of the primate brain
T. rex and its cousins were the primates of their time
If they were warm-blooded, then their numbers of brain neurons can be estimated using modern chickens and ostriches… and they were baboon-like, which is terrifying!
The cortex folds because… physics
Like a crumpled paper ball
Why is the cerebral cortex folded? Because it follows the laws of physics as it expands - no exceptions made.
The cerebellum, too!
The cerebellar cortex is much more folded than the cerebral cortex. Why? Because it is thinner - and physics still applies.
Why so much white matter?
It’s not “optimization” when it can’t be any other way, and that includes the ratio of white/gray matter of the cortex.
It’s the absolute numbers that matter
Forget relative size, count the neurons
Think of neurons as the LEGOs that build brains. The more you have in structures that make behavior flexible, the more intelligent the species.
The largest bird brain is the ostrich’s, but crows seem so much smarter. Why? Tons of neurons in associative areas, despite their size.
Crows have SO many more
Whales have primate-like numbers of neurons
The minke whale brain is twice the size of ours but has only 1/5 the cortical neurons… which is still as many as a baboon - or T. rex!