Do the Large Cetacean Brains Deny the "Human Advantage" in the Cerebral Cortex?

                                                                                      Orcas

I have agreed with Suzana Herculano-Houzel's argument that the (very rough) estimate of 16 billion neurons in the human cerebral cortex--out of the (very rough) estimate of 86 billion neurons in the whole human brain--gives us a "human advantage" that explains why we are the dominant animal on the Earth.  No other animal, she predicts, has more cortical neurons than the 16 billion in the human cerebral cortex.

But what about those animals that have brains larger than the human brain--elephants, whales, and dolphins?  Herculano-Houzel and her colleagues have reported that the African elephant brain is about three times larger than the human brain, and the elephant cerebral cortex is about twice the weight of the human cerebral cortex (Herculano-Houzel, et al. 2014).  Using the isotropic fractionator method for counting neurons, they counted an estimated 257 billion neurons in the elephant brain, which is three times more than the average human brain.  They also found that 97.5% of the neurons in the elephant brain (251 billion) are in the cerebellum.  So the elephant cerebral cortex has only about 5.6 billion neurons, which is only about one-third of the neurons in the human cerebral cortex.  This confirmed Herculano-Houzel's prediction that animals with brain larger than human brains would not have as many cortical neurons as in the human cerebral cortex.

As I have indicated in previous posts, these claims come with at least two kinds of reservations.  First, all of these numbers are only very rough estimates, because there is such a great range of variation in brains.  Second, the estimates here are based on a very small sample size--only the right half of one adult male elephant brain!  Counting the neurons in other elephant brains will result in widely varying numbers.  And there's always the problem of relying on "convenience samples": whether one has access to even one fresh elephant brain is happenstance.

But now if you look at Wikipedia's "List of Animals by Neuron Count," you will see (at the end of the article) the highest estimated numbers of cortical neurons.  And while humans rank near the top--with Herculano-Houzel's estimate of 16 billion--you will also see the Orca (Orcinus orca), or killer whale, has 43.1 billion cortical neurons (Ridgway et al. 2019), about two and a half times more than in the human brain.  You will also see estimates of 37.2 billion cortical neurons for the long-finned pilot whale (Mortensen et al. 2014), 12.8 billion for the common Minke whale (Eriksen and Pakkenberg 2007), and 11.8 billion for the short-finned pilot whale (Avelino-de-Souza 2018).  So here we have two cetacean species with many more cortical neurons in their brains than in the human brain, and two cetacean species whose number of cortical neurons are very close to the number in the human brain.

These data seem to falsify Herculano-Houzel's prediction in 2016 that the number of cortical neurons in the cetacean cerebral cortex would always be much lower than the 16 billion neurons in the human cerebral cortex (Herculano-Houzel 2016, 103-107).  But if you look more carefully at the various reports of neuron counts for cetacean brains, it's not so clear that her prediction has failed.

Eriksen and Pakkenberg (2007) reported using the stereological technique in counting 12.8 billion neurons in the cerebral cortex of the Northern common minke whale based on averaging the counts for five brains.  But Kamilla Avelino-de-Souza, Nina Patzke, Karl Karlsson, Paul R. Manger, and Suzanna Herculano-Houzel (2025) have recently reported using the isotropic fractionator technique in counting 3.2 billion neurons in the Northern minke whale based on a count for one adult male brain.

Why was the stereological estimate so much higher than the isotropic fractionator estimate?  Herculano-Houzel and her colleagues argue that the stereological technique suffers from undersampling.  Eriksen and Pakkenberg (2007, 87-88) report that they sampled only 12-13 sections out of over 3000 sections of the Northern minke whale cerebral cortex, counting only up to 215 neurons per brain.  Sampling from so few and minute sites creates undersampling because the neuronal densities in different areas of the brain are highly variable.  By contrast, the isotropic fractionator method eliminates undersampling by collecting samples from the brain structure of interest only after it has been homogenized into a soup in which the neuronal cell nuclei are evenly distributed (Avelino-de-Souza et al. 2025, 17; Herculano-Houzel 2016, 25-26, 28-34, 104-105).

Previously, in 2018, in her dissertation at the Federal University of Rio de Janeiro, Avelino-de-Souza had reported using the isotropic fractionator technique to calculate that the Minke whale's cerebral cortex had 3.1 billion neurons--virtually the same number as in her recent report.  But in her dissertation, she also reported 11.8 billion neurons in the cerebral cortex of the short-finned pilot whale.  That's the number given in the Wikipedia list.  And it contradicts what Herculano-Houzel had predicted in 2016--that the number of cortical neurons in the pilot whale would be around 3 billion cortical neurons (Herculano-Houzel 2016, 104).  

I asked Herculano-Houzel about this, and she responded (in an email message):

In my recollection, the pilot whale [in Avelino-de-Souza's study] was a VERY old specimen--old in the sense that it had been sitting in fixative for a very, very long time, contrary to the minke whale we reported, which was freshly caught.  The nuclei still had detectable DNA, but were very autofluorescent.  Those counts were therefore an estimate based on total numbers of nuclei, but we cannot be really certain of what fraction were neurons.  We have a better specimen of a beluga whale now that we can work on and get a proper estimate for an odontocete.

I told her that I will be happy to see the numbers for that beluga whale.

Herculano-Houzel still has to explain the 43.1 billion count for the cortical neurons of the Orca (Ridgway et al. 2019) and the 37.2 billion count for the cortical neurons of the long-finned pilot whale (Mortensen et al. 2014).  Presumably, she will argue that these calculations suffer from the undersampling problem of the stereological technique.  But if some study using the isotropic fractionator technique were to confirm these counts for these cetaceans, that would falsify her predictions.

REFERENCES

Avelino de Souza, Kamilla. 2018. "Analysis of the Cellular Composition of the Cetacean Brain."  Ph.D. Dissertation. Program in Morphological Sciences. Federal University of Rio de Janeiro.

Avelino-de-Souza, Kamilla, Nina Patzke, Karl Karlsson, Paul R. Manger, and Suzana Herculano-Houzel. 2025. "Cellular Composition of the Brain of a Northern Minke Whale." Journal of Comparative Neurology 533:e70089.

Eriksen, Nina, and Bente Pakkenberg. 2007. "Total Neocortical Cell Number in the Mysticete Brain." The Anatomical Record 290:83-95.

Herculano-Houzel, Suzana, Kamilla Avelino-de-Souza, Leber Neves, Jairo Porfirio, Debora Messeder, Larissa Mattos Feijo, Jose, Maldonado, and Paul R. Manger. 2014. "The Elephant Brain in Numbers." Frontiers in Neuroanatomy 8: Article 46.

Herculano-Houzel, Suzana. 2016. The Human Advantage: A New Understanding of How Our Brain Became Remarkable. Cambridge: MIT Press.

Mortensen, Heidi S., Bente Pakkenberg, Maria Dam, Rune Dietz, Christian Sonne, Bjami Mikkelsen, and Nina Eriksen. 2014. Frontiers in Neuroanatomy 8: article 132.

Ridgway, Sam H., Robert H. Brownson, Kaitlin R. Van Alstyne, and Robert A. Hauser. 2019. "Higher Neuron Densities in the Cerebral Cortex and Larger Cerebellums May Limit Dive Times of Delphinids Compared to Deep-Diving Toothed Whales." PLoS ONE 14 (12): e0226206.

Was Man Created in God's Image? Or in the Image of Primates, With 16 Billion Neurons in His Cerebral Cortex?

 

                                  The Hidden Neuroscience in Michelangelo's "Creation of Adam"

THE CREATION OF ADAM'S PREFRONTAL CORTEX

Michelangelo's "Creation of Adam" is his visual depiction of the teaching in Genesis 1:27--"So God created man in his own image, in the image of God created he him; male and female created he them" (KJV).  Remarkably, Adam's body is already fully formed, but God is going to transmit to him something essential to his humanity through God's extended finger.  Presumably, God is endowing Adam with a human soul.  But how exactly is that to happen?  

We should notice that Michelangelo chose not to depict another image of Adam's creation from Genesis: "And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul" (2:7).  Michelangelo decided not to show God breathing into Adam's nostrils the breath of life as the source of ensoulment.

So what is it about the image of God flying through the air and stretching out his arm towards Adam lying on the ground that conveys the emergence of a human soul in Adam?  Some neuroscientists have pointed out that one can see a hidden drawing of the human brain in the image of God.  And God's right arm is extended through the prefrontal cortex, which is the part of the cerebral cortex responsible for decision-making, planning, creativity, working memory, and language.  Previously, I have written about how the liberty or freedom to choose between alternatives is a function of the cerebral cortex, under prefrontal control, in its reciprocal interaction with the environment. 

Is This a Hidden Drawing of the Human Brain?

We know that Michelangelo studied human anatomy carefully, and that he dissected human bodies and brains.  We know this from his anatomical drawings.  (Leonardo da Vinci--a contemporary of Michelangelo's--was also a talented anatomist of the brain.)  But of the thousands of Michelangelo's drawings, he destroyed most of them, and only about 600 have survived.  Some of these show drawings of the human skull and brain, but none of them show the exact outline of the cerebral cortex that some neuroscientists have seen hidden in Michelangelo's pictures.  There is certainly no evidence that Michelangelo understood the cognitive functions of the cerebral cortex and the prefrontal cortex.  Seeing a hidden neuroscientific message in Michelangelo's picture seems fanciful.

Nevertheless, we could argue for a modern neuroscientific revision of Michelangelo's picture, in which the image of God could be replaced by the image of the human cerebral cortex.  This would not have to be seen as an atheistic revision of the picture.  Because if the theistic evolutionists (like Francis Collins, Deborah Haarsma, and the last three Catholic popes) are correct, then God works through the natural laws of evolution to execute His creative design.  Some of the theistic evolutionists (like Pope Francis) say that while God works mostly through natural evolution rather than miracles, the creation of the human soul did require an "ontological leap"--a supernatural miracle--that transcended natural evolution.  But as I have argued, there are good reasons to believe that the natural evolution of the primate brain can explain the emergence of the soul in the human brain, so that no miracle was required.  And yet we still have to wonder what properties of the evolved human brain explain the amazing intellectual and emotional capacities of the human mind.

COUNTING NEURONS

Over the past fifteen years, the research of Suzana Herculano-Houzel, a Brazilian neuroscientist now at Vanderbilt University, has shown how this natural evolution of the human brain can be understood as based on the remarkable number of neurons in the human cerebral cortex, as it has emerged from the evolution of the primate brain, and thus we can see that we were created in the image of other primates, just as Charles Darwin suggested in 1871 in The Descent of Man (Gabi et al. 2016; Herculano-Houzel 2016, 2021; Herculano-Houzel and Lent 2005).  I am persuaded by most of what Herculano-Houzel says, although I will identify two points of disagreement with her.

                                                Suzana Herculano-Houzel Counts Neurons

How many neurons are in the human brain?  For many years, the answer from many scientists was 100 billion.  But, surprisingly, when Herculano-Houzel began some years ago looking for the original scientific research that provided evidence for this number, she found nothing.  She discovered that neuroscientists had repeated this number over and over again without realizing that there was no scientific verification for it.  

Moreover, she discovered that scientists had no reliable method for counting brain cells.  The most common method for attempting to do this was stereology: virtual three-dimensional probes are placed throughout thin slices of brain tissue from some part of the brain, then the number of cells within the probes are counted, and finally this is extrapolated to the total number of cells in the entire tissue volume.  The problem is that this works only for tissues with a relatively homogeneous distribution of cells.  In fact, the highly variable density of neurons across different structures of the brain, and even within a single structure, makes stereology impractical for counting the cells in whole brains.

Herculano-Houzel developed a new technique for counting neurons that starts with creating brain soup.  She dissects the brain into its anatomically distinct parts--such as the cerebral cortex, the cerebellum, and the olfactory bulbs.  She then slices and dices each part into smaller portions.  Next, she puts each small part in a tube and uses a detergent that dissolves the cell membranes but leaves the cell nuclei intact.  By sliding a piston up and down in the tube, she homogenizes this brain tissue into a soup in which the nuclei are evenly distributed.  She stains all the cell nuclei blue so that she can count them under a fluorescent microscope.  She then adds an antibody labeled red that binds specifically to a protein expressed in all neuronal cell nuclei, which distinguishes them from other cell nuclei such as glial cells.  Going back to the microscope, she can then determine what percentage of all nuclei (stained blue) belong to neurons (now stained red).  Finally, she can estimate the number of neurons for each structure of the brain.  She has done this in studying the brains of many mammalian animals.

Now she can tell us that the total number of neurons in the whole human brain is not 100 billion but 86 billion.  Of that total, 16 billion are in the cerebral cortex, which includes 1.3 billion neurons in the prefrontal cortex.  The cerebral cortex is the outer covering of the surfaces of the cerebral hemispheres.  The prefrontal cortex covers the front part of the frontal lobe of the cerebral cortex located behind the forehead.

FOUR ADVANTAGES

Comparing the numbers of neurons for the human brain with the numbers for other primate brains and other mammalian brains allows Herculano-Houzel to explain the conundrum of how the human brain can be at once so similar to and yet so different from other animal brains.  She makes four arguments about how the human brain gives human beings four kinds of advantages in their mental abilities: the primate advantage, the human advantage, the advantage of cooking, and the advantage of cultural learning.

Her claim that differences in cognitive capabilities across species can be explained by differences in the absolute numbers of neurons depends on a fundamental assumption.  If neurons are the basic units of brain networks for processing information, and if the networks are structured in similar patterns, then the greater the number of neurons in a network, the greater the capacity of the network for processing information.

The primate advantage.   Not surprisingly, larger brains tend to contain more neurons than smaller brains.  But different groups of animals show different scaling rules in proportioning brain size to number of neurons.  For example, a rodent cortex will have fewer neurons than a primate cortex of similar mass.   Primates always concentrate larger numbers of neurons in the brain than rodents of a similar, or even larger, brain size.

So here a capybara brain of 76 grams has 1.6 billion neurons, while a capuchin monkey brain of 52 grams has 3.7 billion neurons.  Comparison of primates with other mammals shows the same primate advantage:  in primate brains, the neurons are packed more tightly in a similar volume.  As primate brains, human brains have the same primate advantage over other mammals.  Thus, while the number of neurons in the human brain is remarkable, it is not extraordinary, because it falls on the primate scaling line: it's what one would expect for a primate with a body the size of the human body.  As Herculano-Houzel says: "The human brain is just a scaled-up primate brain: remarkable but not special."

One common objection to this is that the bodies of gorillas and orangutans can be as large as human bodies, but the brains of gorillas and orangutans are only about one-third the size of the human brain.  Gorillas and orangutans can weigh about 165 pounds, but their brains have only about 30 billion neurons, in contrast to the 86 billion neurons in the human brain.  Doesn't this show that the human brain is three times larger than what one would expect for a primate with the same size body? 

Herculano-Houzel's answer is that if one excludes the great apes (gorillas and orangutans), the scaling of the human brain in proportion to body size follows the same scaling rule as all other primates.  So the outliers here are not human beings but the great apes: it's not that human brains are too large for their bodies, but that gorillas and orangutans have brains that are too small for their bodies.

She explains this as showing that as primate evolution reaches the outer limits of the primate energy budget--the extra effort to find food--there is a tradeoff between brains and brawn.  More energy for a big body means less energy for a big brain.  The evolutionary history of the great apes shows a tradeoff  favoring big bodies and small brains.  By contrast, the evolutionary history of the human species shows a tradeoff favoring a typically primate slim body but a big brain.

The human advantage.  Herculano-Houzel's counting of neurons has allowed her to see that we human beings have one great advantage over all other animals: "we are the species that owns the largest number of neurons in the cerebral cortex--the part of the brain responsible for finding patterns, reasoning logically, expecting the worst and preparing for it, developing technology and passing it on through culture" (2016, x).  Notice that she is not saying that we have the largest number of neurons in the whole brain.  The African elephant brain has 257 billion neurons, three times our count of 86 billion.  But an amazing 98 percent of the elephant's neurons are in the cerebellum.  Although the elephant's cerebral cortex weighs over 6 pounds, it has only 5.6 billion neurons, far fewer than the 16 billion neurons in the human cerebral cortex weighing 2.6 pounds.  The sperm whale has the largest animal brain--weighing about 18 pounds--but Herculano-Houzel predicts that the number of neurons in the sperm whale's cerebral cortex is probably far fewer than 16 billion.

What Herculano-Houzel says about this human advantage confirms what I have argued about how the human mind or soul can arise from a natural process of emergent evolution in which differences in degree become differences in kind after passing over a critical threshold in the size and complexity of the primate brain.  As a consequence of her research, I can now identify that critical threshold as the remarkably large number of neurons in the human cerebral cortex and particularly in the prefrontal cortex.  Unfortunately, she herself does not see how her research supports the idea of emergent evolution.  That's my first disagreement with her.

In her book chapter on Darwin's Descent of Man--particularly, Darwin's chapter on "Comparison of the Mental Powers of Man and the Lower Animals"--she supports Darwin's claim that human mental powers differ from the mental powers of other animals only in degree and not in kind, and that if this difference were a difference in kind, that would deny his theory of human evolution from lower animals (Herculano-Houzel 2021, 48-49, 51, 61).

Herculano-Houzel does not see that Darwin in The Descent of Man is forced to contradict himself by both affirming and denying that there are differences in kind between human beings and other animals in their mental capacities.  Despite Darwin's explicit statement that humans differ only in degree, not in kind, from other animals, he implicitly recognized differences in kind.  That is to say, Darwin recognized that human beings have some mental traits that other animals do not have at all.

Darwin noted that self-consciousness is uniquely human: "It may be freely admitted that no animal is self-conscious, if by this term it is implied, that he reflects on such points, as whence he comes or whither he will go, or what is life and death, and so forth" (Descent, Penguin edition, 105).  Morality is also uniquely human: "A moral being is one who is capable of comparing his past and future actions or motives, and of approving or disapproving of them.  We have no reason to suppose that any of the lower animals have this capacity. . . . man . . . alone can with certainty be ranked as a moral being" (135).  And language is uniquely human: "The habitual use of articulate language is . . . peculiar to man" (107).

Darwin could implicitly affirm such emergent differences in kind without affirming any radical differences in kind.  Emergent differences in kind can be explained by natural science as differences in kind that naturally evolve from differences in degree that pass over a critical threshold of complexity.  So, for example, we can see the uniquely human capacities for self-consciousness, morality, and language as emerging from the evolutionary increase in the neurons of the primate brain, so that at some critical point in the evolution of our ancestors, the size and complexity of the brain (perhaps particularly in the frontal cortex) reached a point where distinctively human cognitive capacities emerged at higher levels of brain evolution that are not found in other primates.  With such emergent differences in kind, there is an underlying unbroken continuity between human beings and their hominid ancestors, so there is no need to posit some supernatural intervention in nature that would create a radical difference in kind in which there is a gap with no underlying continuity.  

Like Darwin, Herculano-Houzel fails to see how evolutionary science can recognize emergent differences in kind that arise from natural evolution while denying radical differences in kind that would require supernatural miracles.

The advantage of cooking.  If human beings are created in the image of primates, and yet they are unique among the primates because of those 16 billion neurons in the human cerebral cortex, why are human beings the only primate species to evolve such an increase in those neurons?  Herculano-Houzel's answer is that the evolutionary expansion of the human brain was made possible by the human invention of cooking.  Here she adopts the cooking hypothesis of Richard Wrangham in his book Catching Fire: How Cooking Made Us Human (2009).

The energy cost of the human brain is huge.  The brain is only 2 percent of the total body mass, but it consumes on average about 25% of the daily total energy budget for the human body.  On average 3.3 billion molecules of glucose are consumed per human neuron per minute.  6 kilocalories are consumed per billion neurons per day.  And so the more neurons in a brain, the more energy that a brain costs.

The caloric intake from the typical raw food diet of primates is not enough to supply the energy needs of the human brain.  The human controlled use of fire and the invention of cooking about 1.5 million years ago expanded the daily supply of energy to support the evolutionary expansion of the human brain, which can be seen in the fossil record of the huge expansion of the brain from Homo erectus to Homo sapiens.

The slicing and dicing of food and cooking with heat start the process of digesting food before it enters the mouth.  Cooking with heat breaks down the collagen fibers of meat and softens the hard walls of plant cells.  Cooked foods yield 100 percent of their caloric content to the digestive system.  Without cooking, our ancient human ancestors could never have fed their hungry human brains.

In a previous post, I have suggested that this cooking hypothesis for human evolution confirms what Lucretius said about the importance of fire and cooking for the evolutionary emergence of a fully human species.

The advantage of cultural learning.  The controlled use of fire and cooking are technologies that are invented and passed down across the generations by cultural learning.  As Herculano-Houzel observes, this shows us that "plenty of neurons aren't enough," because while having lots of neurons endows a brain with the capacities for complex cognition, turning those capacities into abilities requires cultural learning across an individual lifetime and across many human generations.

As I have indicated in some previous posts, cultural learning is not uniquely human because some other animals have cultural traditions of behavior.  But human beings are unique in their capacity for symbolic culture.

The behavioral inheritance system is the transmission of information among animals through social learning. For example, among some animals (including human beings) mothers transmit food preferences to their offspring, because information about what mother is eating is transmitted either in the womb or through suckling, so that the offspring inherits a preference for that food. More complex forms of social learning come through animal culture. For example, some chimpanzees can discover how to open nuts with a stone, and then pass on this practice within their group so that it becomes a social tradition. Different communities of chimps in Africa have different cultures based on distinctive profiles of traditional practices transmitted by social learning. As opposed to genetic evolution, cultural evolution is not blind but targeted to functional change.

The symbolic inheritance system is uniquely human because it shows the qualitative leap that defines our humanity as based on our capacity for symbolic thought and communication. Other animals can communicate through signs. But only human beings can communicate through symbols. The evolution of human language was probably crucial for the evolution of symbolism. Symbolic systems allow us to think about abstractions that have little to do with concrete, immediate experiences. Symbolic systems allow human beings to construct a shared imagined reality. These symbolic constructions are often fictional and future-oriented. Art, religion, science, and philosophy are all manifestations of human symbolic evolution. 

Herculano-Houzel gives a good account of human cultural evolution as driven by six technological revolutions (2016, 206-213).  But she doesn't explain why the pace of that technological progress has become so rapid over the past 300 years.  I have argued that the best explanation for this accelerated human progress is the symbolic niche construction of Lockean liberalism, which has sustained the freedom for innovation that has made it possible for the Earth today to support a population of over 8 billion people who are living longer and healthier lives on average than has ever been possible.  That is a stunning enhancement of Darwinian survival and reproductive fitness through cultural evolution.

THE SYMBOLIC EVOLUTION OF RELIGIOUS BELIEF IN "METAPHYSICAL ORIGINS"

The evolved cognitive capacity of the human brain for symbolic cultural learning includes a propensity to religious belief, which can include the belief that human beings were created in the image of God, and that this endowed human existence with a supernatural purpose.  Herculano-Houzel fails to take that religious belief seriously as a part of human cultural evolution, and that's my second point of disagreement with her.  She does say that the human brain gives human beings "the ability to ponder our own material and metaphysical origins" (2016, 215).  But she says nothing more about how the religious belief in "metaphysical origins" might be rooted in the evolutionary psychology of the human brain.  In The Descent of Man, in his chapter on the "mental powers of man and the lower animals," Darwin comments on a long list of mental powers, concluding with "belief in God, spiritual agencies, superstitions."  In her commentary on this chapter, Herculano-Houzel is silent about this.

If she had examined that last part of Darwin's chapter on the evolution of the mental powers as leading to religious belief, she would have seen that Darwin anticipates the modern evolutionary psychology of religion as an expression of the human brain's evolved theory of mind or the "hyperactive agency detection device."  Darwin conveys this idea when he says that the earliest manifestation of religious belief is "when anything which manifests power or movement is thought to be endowed with some form of life, and with mental faculties analogous to our own" (117).  Through symbolic cultural evolution this "belief in spiritual agencies" or animism could eventually be expressed in the belief in the existence of an Omnipotent God who is the "Creator and Ruler of the universe" (116, 118).  This is the God depicted in Michelangelo's "Creation of Adam"--the God who created human beings in His image.

The evolutionary science of the brain can explain how this ability for believing in the existence of the Creator God evolved through the genetic evolution of the human cerebral cortex and prefrontal cortex and the symbolic cultural evolution of religious belief in monotheism.  But while some Darwinian psychologists (such as Justin Barrett) see this as showing that Darwinian science is compatible with the truth of believing in God, others (such as Jesse Bering) see this as exposing belief in God as a fictional construction of the evolved human mind. For those like Barrett, religious belief is an adaptive truth. For those like Bering, religious belief is an adaptive illusion.

Thus, we see that Darwinian science cannot resolve the Reason/Revelation debate.  But Darwinian liberalism can support the freedom of thought that promotes that debate.

Part of that Reason/Revelation debate is whether human beings have a natural need to see the purpose for their existence that can only come from believing in the revelation of a divine purpose for human life, or whether natural reason alone can give human life some purpose or meaning.  Some theistic scientists (like Deborah Haarsma) say that in answering our questions about the Universe, science can tell us when and how, but only religion can tell us who and why.

Friedrich Nietzsche thought that the cultural history of the "teachers of the purpose of existence" over thousands of years had changed human nature so that human beings had a culturally acquired need to see some cosmic purpose to their lives.  "Gradually, man has become a fantasizing [phantastischen] animal that has to fulfill one more condition of existence than any other animal: man has to believe, to know, from time to time why he exists; his race cannot flourish without a periodic trust in life--without faith in reason in life" (The Gay Science, sec. 1).  Consequently, Nietzsche observed, "I fear that the animals consider man as a being like themselves that has lost in a most dangerous way its sound animal common sense; they consider him the insane animal, the laughing animal, the weeping animal, the miserable animal" (sec. 224). 

Is this what happens when you have a brain with 16 billion neurons in its cerebral cortex fed by some delicious, cooked meals?

REFERENCES

Darwin, Charles. 2004. The Descent of Man. 2nd edition.  New York: Penguin Books

Gabi, Mariana, et al. 2016. "No Relative Expansion of the Number of Prefrontal Neurons in Primate and Human Evolution." Proceedings of the National Academy of Sciences 113 (no. 34): 9617-9622.

Herculano-Houzel, Suzana. 2016. The Human Advantage: A New Understanding of How Our Brain Became Remarkable.  Cambridge, MA: MIT Press.

Herculano-Houzel, Suzana. 2021. "Remarkable But Not Extraordinary: The Evolution of the Human Brain." In Jeremy M. Desilva, ed., A Most Interesting Problem: What Darwin's 'Descent of Man' Got Right and Wrong About Human Evolution, 46-62.  Princeton: Princeton University Press.

Herculano-Houzel, Suzana, and Richard Lent. 2005. "Isotropic Fractionator: A Simple, Rapid Method for the Quantification of Total Cell and Neuron Numbers in the Brain." Journal of Neuroscience 25: 2518-2521.

Wrangham, Richard. 2009. Catching Fire: How Cooking Made Us Human. New York: Basic Books.

The Very Rough Estimate of 86 Billion Neurons in the Human Brain--Or At Least in the Brains of Four Men--Explains Our Human Nature

As I said a few years ago, I have been persuaded by Suzana Herculano-Houzel's argument that the number and distribution of neurons in our brain largely explain why we have become the dominant animal on planet Earth (Herculano-Houzel 2016).  The brain is the organ that organizes our thinking and our behavior.  Of the three main components of the brain--neurons, glial cells, and vasculature--neurons are the functional units that integrate synaptic activity and then pass it on.  We can infer, therefore, she argues, that the ultimate causes for the flexibility and complexity of cognition and behavior arise from the numbers of neurons in well-defined circuits in the brain.  

But now, after reading Alain Goriely's recent article in Brain (2025) criticizing Herculano-Houzel's claim that she has correctly counted the number of neurons in the human brain, I have been reconsidering her whole argument.

Let's begin by reexamining the techniques for counting neurons in the brain.

THE ANSWER IS IN THE BRAIN SOUP

How many neurons are in the human brain?  For many years, the answer from many scientists was 100 billion.  But, surprisingly, when Herculano-Houzel began some years ago looking for the original scientific research that provided evidence for this number, she found nothing.  She discovered that neuroscientists had repeated this number over and over again without realizing that there was no scientific verification for it (Bartheld, Bahney, and Herculano-Houzel 2016). 

Moreover, she discovered that scientists had no reliable method for counting brain cells.  The most common method for attempting to do this was stereology: virtual three-dimensional probes are placed throughout thin slices of brain tissue from some part of the brain, then the number of cells within the probes are counted, and finally this is extrapolated to the total number of cells in the entire tissue volume.  The problem is that this works only for tissues with a relatively homogeneous distribution of cells.  In fact, the highly variable density of neurons across different structures of the brain, and even within a single structure, makes stereology impractical for counting the cells in whole brains.

Herculano-Houzel developed a new technique for counting neurons that starts with creating brain soup.  She dissects the brain into its anatomically distinct parts--such as the cerebral cortex, the cerebellum, and the olfactory bulbs.  She then slices and dices each part into smaller portions.  Next, she puts each small part in a tube and uses a detergent that dissolves the cell membranes but leaves the cell nuclei intact.  By sliding a piston up and down in the tube, she homogenizes this brain tissue into a soup in which the nuclei are evenly distributed.  She stains all the cell nuclei blue so that she can count them under a fluorescent microscope.  She then adds an antibody labeled red that binds specifically to a protein expressed in all neuronal cell nuclei, which distinguishes them from other cell nuclei such as glial cells.  Going back to the microscope, she can then determine what percentage of all nuclei (stained blue) belong to neurons (now stained red).  Finally, she can estimate the number of neurons for each structure of the brain.  She has done this in studying the brains of many mammalian animals.

Now she can tell us that the total number of neurons in the whole human brain is not 100 billion but about 86 billion.  Of that total, about 16 billion are in the cerebral cortex, which includes about 1.3 billion neurons in the prefrontal cortex.  The cerebral cortex is the outer covering of the surfaces of the cerebral hemispheres.  The prefrontal cortex covers the front part of the frontal lobe of the cerebral cortex located behind the forehead.  

Other animals with larger brains--like elephants, whales, and dolphins--have larger brains with more neurons.  But what makes the human brain unique is the large number of neurons in the cerebral cortex, which is the part of the brain responsible for the highest levels of cognition--such as self-consciousness, abstract thinking, social engagement, language, memory, and emotion.

8 BILLION HUMAN BRAINS AND NO TWO ARE THE SAME

Notice that in stating the numbers of neurons, the numbers are qualified by the word "about."  That points to the problem identified by Goriely, a mathematician at Oxford University who has noted that if you look at the research of Herculano-Houzel and her colleagues, you will see that the average numbers they give are only rough estimates with a wide range of variation based on a small sample of human brains.  

There are two main papers--Von Bartheld et al. (2009) and Andrade-Moraes et al. (2013).  The 2009 paper reports an analysis of the brains from four deceased males aged 50, 51, 54, and 71, who had no cognitive impairment when they died.  The authors declare that the adult male brain contains an average of 86 billion neurons.  But if you look at the numbers for the four brains, you will see that 86 billion is the average of 78.82, 79.72, 90.30, and 95.40 billion.  This is a wide range--from 79 billion to 95 billion--and it's based on only four data points.

The 2013 paper reports a counting of the neurons in the brains of five elderly females--between the ages of 71 and 84--who died of non-neurological causes.  The numbers for these five brains were 62.1, 63.3, 67.3, 72, and 72.06 billion.  This is a wide range, and it's well below the range for the 2009 paper.  The average for these women was 67 billion, as opposed to 86 billion for the men.

Goriely suggests that the only statements we can make about these two studies are rather weak:

(i) Experiments have shown variations between 62 and 95 bn neurons in the human brain (n = 9).

(ii) An experimental study on the number of neurons suggests an average between 79 and 95 bn neurons in the healthy male brain (n = 4).

(iii) An experimental study on the number of neurons suggests an average between 62 and 72 bn neurons in the healthy female human brain (n = 5).

Clearly, none of these statements is satisfactory or as catchy as "the human brain has 86 billion neurons" (Goriely 2025, 691).

Notice that depending upon how you aggregate these nine data points, you could have three different averages.  If you average the four data points for the males, the average is 86 billion.  If you average the five data points for the females, the average is 67 billion.  If you average all nine data points, the average is 76 billion. 

In a response to Goriely, Christopher von Bartheld (2025) has conceded that, of course, it would be absurd to say that all human brains contain exactly 86 billion neurons.  About 8 billion human brains are currently operating in the world today, and because of the biological variability of those individuals, no two brains are exactly alike.  But it is still justifiable to look for rough estimates and approximate ranges for the number of neurons in the human brain.

We can expect that the number of neurons in different brains will vary according to the effects of gender, age, brain mass, and variation in life history.  For example, people with Down syndrome are thought to have perhaps 40% fewer neurons than a normal brain.  People with neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, and Huntington's disease have fewer neurons in some part of the brain.  Malnutrition both in utero and after birth can impede the growth of neurons.  Some of the women in the 2013 study were born in World War II or shortly thereafter, and it's possible that they suffered from the malnutrition caused by the war.  That or other factors related to their life history might explain why the five women had a lower average number of neurons than the four men.

Actually, the brains of the five women in the 2013 study were used as "controls" in comparison with the brains of five other women with dementia from Alzheimer's disease, and with four women with the neural plaques and neurofibrillary tangles associated with Alzheimer's but without dementia.  The study concluded that the five women with dementia had suffered a loss of neurons, while the other nine women had not shown any dementia from loss of neurons.

For a long time it was assumed that dementia and severe mental decline were the inevitable consequence of a normal process of aging with the loss of neurons.  What this and other studies have shown is that loss of neurons is not part of the normal aging process, and therefore the neurogenerative diseases of the elderly come from an abnormal loss of neurons.

Hey, this is great news for us old folks--the rotting of our brains is not an unavoidable consequence of our aging!

So what does all of this mean for assessing Herculano-Houzel's argument?  This does not refute her claim that she can estimate the number of neurons in human brains.  But this does suggest that she needs to qualify her claim by stressing that these numbers are only very rough estimates within wide ranges created by the biological variability in individual brains--with a range between 62 and 95 billion for normal brains, and an even wider range for abnormal brains.  She should also admit that the average of 86 billion is only for the four adult male brains in the 2009 paper.  Perhaps she should put into italics this sentence from her 2009 paper: "Exact numbers are probably highly variable among humans, particularly given the variation of over 50% in the number of cortical neurons among individuals of the same sex described recently in the literature."  But she should drop the word "probably"--exact numbers are highly variable among humans!

She also needs to make a second qualification that has not been brought up by critics like Goriely:  although the sheer number of neurons in the human brain is surely a critical factor in explaining the power of the human brain, we also need to recognize the importance of the structural organization of those neurons into complex circuits.  Actually, Herculano-Houzel and her colleagues point to this when they stress the importance of how those 86 billion neurons are organized in the human brain, and particularly having 16 billion neurons organized into the circuitry of the neocortex, of which 1.3 billion are in the prefrontal cortex.

Although Herculano-Houzel's main argument is that large numbers of neurons in the neocortex are a key component of complex cognition, she recognizes that, of course, many other factors are also important--such as "cellular morphology, connectivity patterns, neurochemical properties, and cognition-related regulatory genetic sequences" (Strockens et al. 2021, 1602).  Neurons in the brain are connected by synapses that enable the firing of one neuron to enhance or suppress the firing of other neurons.  To fully explain the functioning of the brain we would need to understand this circuitry. Those roughly 86 billion neurons in the human brain probably have over 100 trillion connections.  It will be difficult to map such a complex circuitry.  We have complete maps--"connectomes"--only for some small brains: the roundworm, Caenorhabditis elegans, and the fruit-fly, Drosophila melanogaster (Devineni 2024).

At this point, many of you are thinking of questions that I haven't answered.  What about those huge cetacean brains?  And what about those really smart bird brains that don't even have a cerebral cortex?  I'll take up those questions in my next posts.

REFERENCES

Andrade-Moraes, Carlos Humberto, et al. 2013. "Cell Number Changes in Alzheimer's Disease Relate to Dementia, Not to Plaques and Tangles." Brain 136: 3738-3752.

Azevedo, Frederico A. C., et al. 2009. "Equal Numbers of Neuronal and Nonneural Cells Make the Human Brain an Isometrically Scaled-Up Primate Brain." The Journal of Comparative Neurology 513: 532-541.

Devineni, Anita. 2024. "A Complete Map of the Fruit-Fly Brain." Nature 634:35-36.

Goriely, Alain. 2025. "Eighty-Six Billion and Counting: Do We Know the Number of Neurons in the Human Brain?" Brain 148: 689-691.

Herculano-Houzel, Suzana. 2016. The Human Advantage: A New Understanding of How Our Brain Became Remarkable. Cambridge: MIT Press.

Strockens, Felix, et al. 2022. "High Associative Neuron Numbers Could Drive Cognitive Performance in Corvid Species." The Journal of Comparative Neurology 530:1588-1605.

von Bartheld, Christopher S. 2025. "Understanding and Misunderstanding Cell Counts of the Human Brain: The Crux of Biological Variation." Brain 148: e72-e74.

von Bartheld, Christopher, Jami Bahney, and Suzana Herculano-Houzel. 2016. "The Search for True Numbers of Neurons and Glial Cells in the Human Brain: A Review of 150 Years of Cell Counting." The Journal of Comparative Neurology 524: 3865-3895.

How Bird Brains Endow Corvids and Parrots with Primate-Like Intelligence

 

                                               Comparing the Brains of a Monkey and a Raven

                                                          A Diagram of the Avian Brain

"Bird brain" is a derogatory term.  But it shouldn't be.

Yes, bird brains are smaller than primate brains.  And bird brains don't seem to have a mammalian cerebral cortex or prefrontal cortex, which are largely responsible for complex cognition among mammals.  So we might not expect much from a walnut-sized bird brain.

But many of the studies of animal intelligence have shown that some birds--particularly, corvids (crows, jays, ravens, and jackdaws) and large parrots--have a complex cognitive psychology comparable to that of primates (Emery and Clayton 2004).  They use and manufacture tools.  They engage in causal reasoning.  They show flexible learning strategies.  They have some social intelligence.  They can use their imagination to form cognitive maps of their world and run simulations of future situations.

This complex cognitive functioning in corvids is controlled by the avian pallium, which corresponds to the mammalian cerebral cortex.  The pallium is the layers of grey and white matter that cover the upper surface of the cerebrum in vertebrates.  In mammals, the cortical part of the pallium forms the cerebreal cortex.  Within the avian pallium, the highest cognitive functions are controlled by the high numbers of associative neurons in the mesopallium and nidopallium, which correspond to the prefrontal region of primates (Herculano-Houzel 2020).

Using the isotropic fractionator method, Herculano-Houzel and her colleagues have shown that birds have large numbers of neurons in their whole brain, in their pallium, and in their mesopallium and nidopallium.  Among the songbirds studied, with brain mass ranging from 0.36 to 14.13 g, the total number of neurons in the brain ranges from 136 million to 2.17 billion.  In the parrots studied, brain mass ranged from 1.15 to 20.73 g, while the numbers of neurons ranged from 227 million to 3.14 billion. By comparison, a rhesus monkey brain weighing 70 g has about 1.7 billion neurons. This illustrates how neurons are more densely packed in the brains of birds than in primate brains--just as neurons are more densely packed in primate brains than in the brains of other mammals, which Herculano-Houzel calls the "primate advantage."  So here we see the "avian advantage" (Olkowicz et al. 2016).

                                                              A Blue-and-Yellow Macaw

                                                                A Bonnet Macaque Monkey

The avian pallial neurons of corvids and parrots significantly outnumber the pallial neurons of primate brains of the same mass.  For example, a raven pallium weighs about 10.20 g, and it has about 1.2 billion pallial neurons, while a capuchin monkey pallium weighing about 39.18 g has about 1.1 billion neurons.  A blue-and-yellow macaw with a pallium weighing about 14.38 g has about 1.9 neurons, while a bonnet macaque pallium weighing about 70 g has about 1.7 billion neurons.

The associative pallial neurons in the avian mesopallium and nidopallium are thought to be the functional equivalent to the mammalian prefrontal cortex, which drives flexible and complex cognitive performance.  So if we find similar or even higher numbers of neurons in associative pallial areas in corvids and parrots as in some primate species, that would help explain why these birds have primate-like intelligence.  And, indeed, some research has shown that corvid mesopallium and nidopallium combined can have between 200 and 300 million neurons per hemisphere, which is more than the estimated 68 million neurons in the prefrontal cortical region of the rhesus monkey.

But even though these large numbers of neurons in the bird brain are a major factor in explaining the complex intelligence of corvids and parrot, we should keep in mind that "complex cognitive functions obviously also depend on many other variables like cellular morphology, connectivity patterns, neurochemical properties, and cognition-related regulatory genetic sequences" (Strockens, et al. 2022, 1602).

What we see here is one of the major themes of my writing--the emergent evolution of animal minds in the brain:  differences in degree in the mental capacity of the brain can produce differences in kind when they pass over a critical threshold in the size and complexity of the brain.  That's why the approximately 16 billion neurons in the human cerebral cortex helps to explain why human beings have some mental capacities--such as language and symbolic abstraction--that other animals do not have at all.

Or should we say that the uniqueness of the human mind in its capacity for intellectually understanding the Universe can only be explained by God's miraculous creation of man in His own image by creating the human mind as the image of the Divine Mind?

One of the best arguments for theism is that this theistic doctrine of the human mind as created by God in His image provides the necessary support for the validity of human thought, including the validity of modern science. If we embrace Naturalism--the view that nothing exists except Nature, and so there is no God or nothing like God--we are caught in self-contradiction: if human thought originated not from a divine Mind but from the irrational causes of Nature, then we cannot trust our minds as reliable, and thus we cannot trust our belief in Naturalism. Naturalism destroys itself by destroying the rationality of believing in Naturalism, or anything else. Insofar as science--including evolutionary science--depends on the validity of human thought, and insofar as theism is the indispensable support for trusting in the validity of human thought, science is not only compatible with theism, science depends upon theism.

I have written a series of posts in response to this evolutionary argument against naturalism.

REFERENCES

Emery, Nathan J., and Nicola S. Clayton. 2004. "The Mentality of Crows: Convergent Evolution of Intelligence in Corvids and Apes." Science 306:1903-1907.

Herculano-Houzel, Suzana. 2020. "Birds Do Have a Brain Cortex--And Think." Science 369:1567-1568.

Olkowicz, Seweryn, et al. 2016. "Birds Have Primate-Like Numbers of Neurons in the Forebrain." Proceedings of the National Academy of Sciences 113:7255-7260.

Strockens, Felix, et al. 2022. "High Associative Neuron Numbers Could Drive Cognitive Performance in Corvid Species." The Journal of Comparative Neurology 530:1588-1605.

Darwinian Conservatism Supports Emergent Complexity and Immanent Teleology: A Response to Robert Koons

The Darwinian conservatism of the permanent things requires a biological teleology of human nature.  Because one needs to claim that evolved human nature is directed to certain ends or goals that constitute the natural norms (the permanent things) of human existence, so that the fulfillment of those norms can be understood as the human good.

By contrast, the metaphysical conservatives argue that modern science--and particularly Darwinian biology--denies teleology in denying that human beings can have natural ends.  The metaphysical conservatives claim that for modern science, all reality is understood to be nothing but the blind motion of particles in space governed by material and mechanistic laws of motion; and in such a world, there can be no final causation.  At the Kirk Center conference, that claim was made by Robert Koons (University of Texas at Austin) on a panel on "Conservatism and Religion."

Koons said this as part of his general argument that modern science has been "mostly a very bad thing."  On this point, he saw himself as agreeing with C. S. Lewis in The Abolition of Man that "modern science was born at an inauspicious hour and in an unhealthy neighborhood."

I saw two problems in Koons' presentation.  First, he identified modern science with a reductionistic physics, and he was silent about the emergent complexity of biology.  Second, in arguing that modern science rejects teleology, he ignored the distinction between cosmic teleology and immanent teleology.

EMERGENT COMPLEXITY IN DARWINIAN BIOLOGY

In my chapter on "Emergence" in Darwinian Conservatism, I argue for the "emergent evolution of the soul in the brain," and I defend the idea of emergence as an alternative to both reductionism and dualism. As I have argued in previous posts, the simplest expression of the idea of emergence is that the whole can be greater than the sum of its parts. Emergent phenomena are those complex wholes with properties that we could not explain or predict from our knowledge of the parts. For example, when hydrogen and oxygen combine chemically to form water, we see emergent properties in water that we could not have predicted from the properties of hydrogen and oxygen. This emergence of novelty is manifested throughout the evolution of the universe. As we pass through levels of complexity, we find new properties at higher levels that are not fully reducible to the lower levels. This idea of emergence denies strong reductionism, because it denies that the higher levels of organization can be completely reduced to the lower levels. But the idea of emergence also denies dualism, because it denies any radical separation of matter and mind.

In his Descent of Man, Charles Darwin recognized--if only implicitly--that human beings show an emergent difference in kind from other animals.  Darwin noted that self-consciousness is uniquely human: "It may be freely admitted that no animal is self-conscious, if by this term it is implied, that he reflects on such points, as whence he comes or whither he will go, or what is life and death, and so forth" (105).  Morality is also uniquely human: "A moral being is one who is capable of comparing his past and future actions or motives, and of approving or disapproving of them.  We have no reason to suppose that any of the lower animals have this capacity. . . . man . . . alone can with certainty be ranked as a moral being" (135).  And language is uniquely human: "The habitual use of articulate language is . . . peculiar to man" (107).

Darwin could implicitly affirm such emergent differences in kind without affirming any radical differences in kind.  Emergent differences in kind can be explained by natural science as differences in kind that naturally evolve from differences in degree that pass over a critical threshold of complexity.  So, for example, we can see the uniquely human capacities for self-consciousness, morality, and language as emerging from the evolutionary expansion of the primate brain, so that at some critical point in the evolution of our ancestors, the size and complexity of the brain (perhaps particularly in the frontal cortex) reached a point where distinctively human cognitive capacities emerged at higher levels of brain evolution that are not found in other primates.  With such emergent differences in kind, there is an underlying unbroken continuity between human beings and their hominid ancestors, so there is no need to posit some supernatural intervention in nature that would create a radical difference in kind in which there is a gap with no underlying continuity.

Now we know from the work of neuroscientist Suzana Herculano-Houzel that human beings are unique as compared with other primates in that human beings have 16 billion neurons in the cerebral cortex, which includes 1.3 billion in the prefrontal cortex.  The human mind or soul can arise from a natural process of emergent evolution in which differences in degree become differences in kind after passing over a critical threshold in the size and complexity of the primate brain.  As a consequence of her research, we can now identify that critical threshold as the remarkably large number of neurons in the human cerebral cortex and particularly in the prefrontal cortex.  That probably explains the three points of human uniqueness that Darwin saw--self-conscious awareness, morality, and language.

Koons is silent about this emergent complexity of human biology in asserting that all of modern science must reduce reality to the motion of particles in space.

IMMANENT TELEOLOGY IN DARWINIAN BIOLOGY

Koons is also silent about how Darwinian biologists recognize the immanent teleology of living beings even though they deny the cosmic teleology of the universe.  In insisting that modern science denies teleology, Koons has to assume that cosmic teleology is the only kind of teleology.  And thus he ignores the fact that for Aristotle natural teleology was more clearly biological than cosmological.

As I have argued previously, Darwin recognized the teleological character of his evolutionary science.  In an article in Nature, Asa Gray wrote: "let us recognize Darwin's great service to Natural Science in bringing back to it Teleology; so that instead of Morphology versus Teleology, we shall have Morphology wedded to Teleology."  In response to this, Darwin wrote to Gray (June 5, 1874): "What you say about Teleology pleases me especially and I do not think anyone else has ever noted that."  So, we can infer, that when Darwin read Aristotle's explanation of biological teleology in the Parts of Animals and expressed his admiration for Aristotle's biological science, Darwin saw that he really was bringing back into science a teleological conception of living nature that was originally formulated by Aristotle.

We need to see that while Darwin denied cosmic teleology, he affirmed immanent teleology. The natural evolution of living beings does not conform to any cosmic design. But that natural evolution does produce species that show an internal teleology in being directed to ends or goals. For example, a mammalian species is naturally adapted for parental care, so that mothers caring for their offspring can be explained teleologically as goal-directed behavior. Such species-specific, immanent teleology is the only kind of teleology that Aristotle saw in the living world, and modern biology confirms such Aristotelian teleology.

Insofar as Darwinian biology allows for emergent complexity and immanent teleology, this is enough for a Darwinian conservatism.