The Number of Neurons in the Brain

There's a nice TED talk circulating on the Internet by a fellow Brazilian scientist about the number of neurons in the brain. Here it is.

The talk is quite interesting, but if you have ever read anything here on my blog (which I confess I update once every 6 months) you know that I do not like unjustified hypes and what I've been reading on the Internet seems to be characterizable as such.

I can understand well why the talk has been so popular. She starts by saying that there is a number in science, the number of neurons in the brain, which she looked everywhere to know where it comes from and she couldn't find. That's popular. People love when someone says that. It's like 'those old scientists were so full of them that they didn't bother to check if that was true or not!' She says that nobody could tell her the origin of the number. The number, by the way, is that that the brain has about 100 billions neurons.

Then she proceeds to talk about brain size in different animals and how the size of the brain is not a determinant in the level of intelligence. After a while, she describes her work on an experimental method to count the neurons with a higher precision than before. She finds that the previous number was wrong. It's not 100 billions, it's 86 billions.

Finally, comes the twist. She first says that the neuron density in the primate brain is higher than in other classes, but is almost constant among the primates. If it's the same, what differs us from the other primates? She talks about the amount of energy that the brains consumes and finally concludes, for the surprise of everyone, that cooking is what makes humans different! That's because by cooking we can digest the food better and we liberate time for our brains to develop other activities! We are humans because we cook! 

I'm not being sarcastic, I did enjoy the lecture. However, I must make some harsh observations.

First of all, let me talk about the number of neurons in the brain. 100 billion neurons is not an exact number. Never was. It was an ESTIMATE. If you are not a scientist, than you are excused of not knowing what we mean by an estimate. When we scientists estimate a number, we are mainly worried with something which is called order of magnitude. This is roughly to estimate how close to a power of 10 is the true result. If you pay attention, 100 billion IS the same order of magnitude of 86 billion because 100 is very close to 86. In fact, it's amazingly close! It could be any number, but it's almost the correct one. This cannot be a coincidence. A number so close to the real one MUST have some explanation, otherwise it would be extremely lucky to get it!

I suggest you to read the following book:

How many licks? Or how to estimate damn near everything
by Aaron Santos

This will help you to understand that an estimate is different from a precise measurement. They serve different objectives.

There's one thing that I dislike in the talk. The speaker says that she looked everywhere in the literature to find where that number came from, but she couldn't find and no one knew. Well, I decided to try my own literature review. I went to Wikipedia and looked for neuron. If you click the link and go to that page, you will see that reference number 27 is the article:

The Control of Neuron Number, by Williams and Herrup

By clicking the above link, you can read the HTML version of the paper for free. There, under the section 'Total Neuron Number in Different Species', you will find a list of references to experimental work that estimate the number of neurons in the brain to be, guess what, around 100 billion. In fact, there is the following reference from 1975

Lange, W. 1975 Cell number and cell density in the cerebellar cortex of man and other mammals. Cell Tiss. Res. 157: 115–24

This reference (have I said it was from 1975?) estimates the number of neurons to be... around 85 billion. I am not sure what she meant exactly when she said that she couldn't find in the literature where the number came from, but the above search took me around 10 minutes including the time to browse the papers.

Once again, don't get me wrong. I'm not criticizing her work. I think that measuring things with higher precision is extremely important and I'm sure her work must be extremely good, but facts are facts.

Finally, although the talk does not say that explicitly, it makes you think that what makes humans different from other species is cooking. That is definitely not true. If it was, why the other primates have not copied us during all this time? Cooking requires the use of tools. Requires the use of fire. It seems reasonable to say that cooking brought us an evolutive advantage. Other things did that as well. Still, there is more to the human brain than cooking and it seems clear that there was something different even before cooking. Cooking might have helped to increase this difference. Writing did that as well. Don't even talk about science.

I feel obliged to repeat one thing here. Question everything. Always. Especially things that are nice to hear. Those are the ones which will probably be too good to be true. No matter who says that. Especially me.