Compared with other species, we human beings have extraordinary cognitive ability - even able to think about the structure of the human brain itself. What is unique about our brains that makes us stand out?
What gives humanity extraordinary cognitive ability? If brain capacity determines cognitive ability, then why do elephants underperform?
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Brain capacity is an obvious candidate: If the brain is the site of conscious cognition, larger brain volumes mean more cognitive ability. But obviously, elephants have a greater brain capacity than humans, but do not have the same agility and complexity as we do. In addition, if larger brain volumes are assumed to equate to higher cognitive abilities, it means that the structures of the brain in all species are assumed to be identical and the relationship between brain volume and neuron numbers is also similar. However, my colleagues and I have given a negative answer in the study. Primates have a clear advantage over other mammals.
In addition, the absolute number of neurons is an option, regardless of brain size. Since neurons can produce cognition, more neurons should mean higher cognitive ability. We used to think that many cognitive abilities are unique to humans, but now people are aware that cognitive differences between humans and other animals are only a matter of degree. In other words, this is a quantitative, not a qualitative question.
The tools used by humans are very complex and can even be designed to be used to make other tools. However, in the animal world, gorillas can dig termites using branches as tools. Monkeys can learn to use rakes to get food outside of their sight. Ravens not only get food through wires, but also save them for subsequent reuse. Alex, an African gray parrot raised by psychologist Irene Pepperberg, learned to create words that symbolize certain objects. There are also chimpanzees and gorillas that, though anatomically constructed, can not sound, but they also learn to communicate in a symbolic language. Chimpanzees can even collaborate with elephants to gain access to foods that are beyond their reach. It seems as if chimpanzees and other primates can infer the mental state of their companions as a necessary condition for showing deception. Even birds seem to know the mental states of other individuals. The magpie hides food when the spectator is present, waits until the bystander leaves, and moves it to another secret position. Chimps, gorillas, elephants, dolphins and magpies seem to recognize themselves in the mirror.
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These studies have fully demonstrated the cognitive abilities of other animals. However, if you want to know exactly what is going on in your mind, letting some species acquire cognitive ability beyond that of other species, the observation of a single species can not meet the need of cross-species comparison. Here, we're stuck in a dilemma: how to measure the cognitive abilities of a large number of species, and if that standard needs to be comparable across species?
A 2014 study tested the self-control of multiple animals, a cognitive function that relies on the prefrontal cortex of the cerebral cortex. Most of the subjects tested in this study are primates, but there are also small rodents, carnivores, Asian elephants and many species of birds. The researchers found that in the self-control test, the performance is most closely related to the absolute brain volume. However, there is an exception here - Asian elephants, although the largest brain capacity, but it failed miserably in the mission. Researchers have considered many reasons, from "it does not care about this task" to "it likes to annoy caregivers through noncooperation."
However, the most interesting possibility for me is that in the prefrontal areas of African elephants, it may not have all the neurons needed to fulfill such self-control decision-making tasks.
When we realized that the brain structure of primates and rodents is different and the number of neurons per unit volume is different, we predicted that if the African elephants had rodent brain structures, their brain size would be larger Much more, but there are only 3 billion neurons in the cerebral cortex and only 21 billion neurons in the cerebellum. In contrast, the number of neurons in our cortex and cerebellum was 16 billion and 69 billion respectively. If African elephants are brain-built like primates, they could have as many as 62 billion cerebral cortex neurons and 159 billion cerebellar neurons in their brains.
So, for African elephants whose brain weights are more than three times human, do they really have more neurons than humans? If the answer is in the affirmative, then the previous hypothesis that species cognition is related to the absolute number of neurons is wrong. On the contrary, this will support my hypothesis that the simplest explanation for the extraordinary cognitive abilities of humans is the unrivaled number of brain neurons, regardless of brain size.
Six months counting test
In order to find out the answer, we decided to study the brain of the target. After we planned to dissolve the brain of an African elephant into "soup," we counted the nuclei in it. This process can handle no more than three to five grams of tissue at a time, whereas African elephants weigh more than 2.5 kilograms of the hemisphere, meaning that the brain must be cut into hundreds of small pieces to handle and count. I hope the cutting is systematic, not random. We used a cooked slicer to cut one's brain hemisphere into a succession of slices. That slicer is ideally suited for separating cortical brain back, but it has one downside: a lot of material stuck to the blade hinders the estimation of the total number of neurons in the hemisphere. If we want to know the total number of neurons in the hemisphere like the brain, we have to cut the thicker sections manually, to the point where the loss of the neurons is negligible.
Count neurons: Suzana Herculano-Houzel and her students cross a brain like this to determine the number of neurons in the brain and compare it with neurons in the human brain.
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As a result, my daughter, who started my vacation with me, bought an L-bracket and a long knife for one-handed use at a hardware store. I put my head like a hemisphere flat on the workbench, framed by two L-shaped brackets. While holding the elephant's brain in my left hand and cutting the knife back and forth smoothly and gently in the right hand, a student holds the brace and holds them in place. After several cuts, we got brain-sliced "bread" lying flat on the workbench: 16 slices of cortical area, 8 slices of cerebellum, an entire brain stem, and a giant olfactory bulb of 20 grams In a rat brain mass 10 times).
Next, we need to separate the internal structures such as the striatum, the thalamus and the hippocampus from the cortex, and then cut the cortex into small pieces and classify them by gray and white matter. In the end, we got a total of 381 organizations, but most of them were over 5 grams and we could not handle it at once. With the team's efforts, we completed all the treatment of the hemispheres of Africa in half a year.
Who has more neurons
The end result makes us happy. There are far more neurons in the African elephants than in the human brain. They have 257 billion neurons, three times as many as humans. However, up to 98% of these neurons are located in the cerebellum. Although in most other mammals studied before, most brain neurons also accumulate in the cerebellum, but never exceed 80%. Despite the large size of the cerebral cortex of African elephants, there are only 5.6 billion neurons in the entire cerebral cortex. That figure dwarfs that of the 16 billion neurons in its much smaller human cortex.
For the first question, our experiment gives the answer: There are no more neurons in the human brain than in the brain. African elephants have twice as many cerebral cortices as humans, with only one-third of human neurons. Elephant cerebellar neurons are three times as many as humans, but their cognitive ability is significantly weaker than that of humans, which can overthrow the hypothesis that "the number of neurons in the cerebellum limits or determines cognitive ability."
Therefore, the key issue may lie in the cerebral cortex. Many of the neurons in the cerebral cortex and cerebellum are naturally separated and nature has done the experiment we need. Human brain is better than brain cognitive ability can only be attributed to the huge number of neurons in the human brain.
Although we did not test the cognitive ability of all mammals, or at least the number of neurons we know of cortical neurons, we can already make verifiable predictions based on these data. If the absolute number of neurons in the cerebral cortex is a major limiting factor in the cognitive ability of a species, then my prediction of the species's cognitive ranking is based on the number of neurons in the cerebral cortex:
This is clearly more logical than the current brain-based rankings, which prioritize animals like giraffes in many primates:
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Why only the human brain, similar to other species, is so unique that it gives us the ability to think about philosophical problems? Here's a simple explanation for this: First, we are primates, which gives humans the advantage of being able to fit large numbers of neurons into smaller cortices. Secondly, thanks to the technological innovations brought by our ancestors, we are able to get out of the energy constraints. Energy consumption of the human brain can be described as extravagant: less than 2% of body weight, but accounted for at least 20% of the metabolic rate. For other animals, the amount of energy harvested in the wild from raw meat is not enough to support such a wealth of cerebral cortical neuronal activity.
In my opinion, what sets us apart is the extraordinary number of neurons in the cerebral cortex, which is the highest among all species. In the evolutionary process, what is the unique reason that we have accumulated so many neurons? I think it is cooking. The rest is well known - the numerous neurons in our cortex make all the innovations possible, followed by productive forces and cultural transmission, driving history to move forward.
References for Text and Images:
- https://askananthropologist.asu.edu/stories/brains-over-brawn
- http://nautil.us/issue/35/boundaries/the-paradox-of-the-elephant-brain
- http://www.pnas.org/content/109/Supplement_1/10661
https://twitter.com/ricard_sole/status/899354672703655938
- ricard_sole
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