Darwinian Evolution in Four Dimensions

People often question me about why I stress the importance of reading Charles Darwin's writings rather than relying on modern textbook presentations of evolutionary biology. After all, hasn't there been great progress in biological science since the publication of Darwin's Origin of Species a 150 years ago? This is certainly true. But it is also true that in many respects Darwin's original understanding of evolution is superior to the prevailing views of today's "Neo-Darwinians." Contemporary proponents of Darwinism like Richard Dawkins tell us that biological evolution is ultimately reducible to the natural selection of random genetic mutations. More and more critics are pointing out the flaws in such a genetic reductionist view of evolution, and they are arguing that we need to explain the complex interaction of multiple levels of evolution that cannot be reduced to the gene-centered view of Neo-Darwinism. But if one studies Darwin's writings, one notices that what these critics are proposing is actually a return to Darwin's original theory.

Consider Darwin's summary of his theory in the last paragraph of the Origin:

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing a Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

Notice that Darwin states his "laws" of evolution in a very general and abstract way--the laws of reproduction, inheritance, variability, and struggle for life. If some entities can reproduce themselves, if these entities show heritable variation, and if some of this heritable variation affects their chances of surviving and reproducing in the competitive struggle for existence, then evolution by natural selection will occur. Stated in such a general way, these laws could apply as well to the evolution of human culture as to the evolution of animal anatomy. And, in fact, much of Darwin's Descent of Man is a study of the cultural evolution of human morality.

Notice also that Darwin thinks that heritable variation can arise from "the indirect and direct action of the external conditions of life, and from use and disuse." This means that Darwin embraces Lamarck's idea of the inheritance of acquired characters. So the common story that Darwin overthrew Lamarckianism is false.

In the 1930s, the "Modern Synthesis" of evolutionary biology combined a Neo-Darwinism that rejected all Lamarckianism with Mendelian genetics. It was assumed that genes were the only units of heredity, that variations in genes are random and not affected by the developmental history of the individual, and that selection favors individuals with genes that make them more adapted to their environment than others. Consequently, evolution was understood as some change in the genetic composition of some group of organisms.

In recent decades, empirical research and theoretical arguments have thrown this Modern Synthesis of Neo-Darwinism into doubt, because it seems that a gene-centered theory cannot fully account for the evolution of life. Some people are even saying that this is an intellectual revolution that will destroy Darwinism.

One of the best critiques of Neo-Darwinism is by Eva Jablonka and Marion Lamb, Evolution in Four Dimensions: Genetic, Epigenetic, Behavioral, and Symbolic Variation in the History of Life (MIT Press, 2005). But rather than overturning Darwinism, Jablonka and Lamb see their critique as a renewal of Darwin's Darwinism.

The argument of Jablonka and Lamb is that in evolution the genetic system of inheritance is only one of four dimensions of evolutionary inheritance, and that these multiple dimensions of inheritance show a Lamarkian evolution of acquired characters, just as Darwin believed.

To illuminate their general point, Jablonka and Lamb use an analogy to show how different systems of heredity can work along with the genetic system. We can think of a piece of music that is represented by a score, the notes written on paper. This score can be copied as it is passed on through the generations. Although a few mistakes in copying might occur over time, generally the score will be accurately transmitted. We might then see the relationship between the musical score and the musical performance as analogous to the relationship between a genotype and a phenotype in biology. Although mutations in the genotype will be transmitted to future generations, changes in the phenotype will not be transmitted, and so the Lamarckian inheritance of acquired characters does not occur.

But with the invention of new means of transmitting music--such as musical recordings and broadcasts--it becomes possible to transmit the musical performance (the "phenotype") by inheritance. It is also possible that popular performative interpretations of the music might bring notational changes in the score, and in this way the musical "phenotype" would change the musical "genotype."

In a similar way, Jablonka and Lamb argue, there are systems of inheritance beyond the genetic system that allow phenotypic variations to be transmitted across generations. All organisms have two systems of inheritance--the genetic system and the epigenetic system. Many animals have a third system--the behavioral system. Human beings are unique in that they have not only these three systems, but also a fourth--the symbolic system. The full complexity of evolution arises from the intricate interaction of these four dimensions of evolutionary inheritance, which correspond to various levels of complexity from the genome to cells to organisms to groups.

The genetic inheritance system is the foundation for the Neo-Darwinian theory of evolution. Jablonka and Lamb accept this as true, but they also criticize genetic reductionism and determinism for failing to see how gene action depends on the complexity of interacting causes within the genome, within cells, within organisms, within groups of organisms, and within ecological circumstances. Except for a few single-gene genetic disorders, "genetic astrology"--the idea that genes directly control specific traits--must be dismissed as foolish.

The epigenetic inheritance system is evident in the differences between specialized cells. Brain cells, liver cells, and skin cells are very different, although the nucleus of each cell has the same genome. Their differences are epigenetic, rather than genetic, because they have arisen through their developmental history in which there were different patterns of gene activation and interaction within the cell. This developmental information is passed on as these cells divide to produce more cells of the same kind. It is possible for evolution to occur through heritable epigenetic variation even without genetic variation. Just as a musical recording transmits interpretations in musical performances of a musical score, so does an epigenetic inheritance system transmit interpretations of the information in DNA, so that there is a Lamarkian inheritance of phenotypes instead of genotypes. One version of such inheritance that is now under active study is DNA methylation: strands of DNA are chemically modified during development, and these modifications can be transmitted through reproduction.

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. What Karl Jaspers called the Axial Age (600-200 B.C.) would be an example of a turning point in the symbolic evolution of humanity, in which Confucius, the Buddha, the Hebrew prophets, and Plato acted as agents of symbolic innovation that has been inherited across the generations for the last two thousand years.

Neo-Darwinian theorists can hardly deny the reality of symbolic or cultural evolution. So they have developed their own theories of how this works. Dawkins is famous for his theory of "memetics"--the idea that "memes" (units of cultural replication) can evolve as "viruses of the brain." The evolutionary psychologists (like John Tooby and Leda Cosmides) explain cultural evolution as a historical process of social learning constrained by the genetically evolved biases of the brain. Although Jablonka and Lamb see some partial truth in these approaches, they also see them as inadequate for explaining the uniqueness of symbolic evolution as shaped through the constructive activities of individual and social agents in history.

The four-leveled account of evolution could explain Aristotle's biological studies of political animals. Human beings are not the only political animals, Aristotle observed, but human beings are more political than the other political animals because the human capacity for logos--speech or conceptual reasoning--allows human beings to organize their political life around shared conceptions of the good and the just. Jablonka and Lamb might say that while human beings share with other animals a capacity for political culture based on behavioral inheritance, only human beings have a capacity for political symbolism that creates a shared symbolic meaning for political life.

I will be writing more posts on these four dimensions of evolution.

The Evolution of Heaven and Hell (5): Dante

The evolution of Heaven and Hell illustrates that one dimension of evolution that is uniquely human--symbolic evolution.

As I have indicated in a previous post, I agree with Eva Jablonka and Marion Lamb that the genetic system of inheritance is only one of four dimensions of evolutionary inheritance. All organisms have two systems of inheritance--the genetic system and the epigenetic system. Many animals have a third system--the behavioral system--by which learned traditions of behavior are passed from one generation to another. Human beings are unique in that they have not only these three forms of evolutionary inheritance, but also symbolic inheritance.

Symbolism is distinctively 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 crucial for the evolution of symbolism. Symbolic systems allow us to think about abstractions far beyond our concrete, immediate experiences. Symbolic systems allow human beings to construct a shared imagined reality. Art, religion, science, and philosophy are all manifestations of human symbolic evolution.

Dante's Divine Comedy is the supreme literary moment in the symbolic evolution of Heaven and Hell--and of Purgatory, by the way! He inherited 2,000 years of oral, visual, and written symbolism of the afterlife. He appropriated and transformed that inheritance by passing it through his individual experience and judgment and then passed it on to future generations. He thus introduced variations into the imagery of the afterlife that would compete in the evolutionary battle of symbols that constitute the spiritual history of humanity. His success depended upon his astute artistic judgment as to what kind of symbolism could survive and reproduce itself in the mental space of human intellectual and emotional responses, as constrained by the capacities and propensities of the human brain and nervous system as modulated by the behavioral and symbolic history of human culture over the past 50,000 years.

An evolutionary explanation of The Divine Comedy must move through at least three levels of evolutionary history--the natural history of the human species, the cultural history of Dante's world, and the individual history of Dante's life. His writing would be unintelligible and uninteresting if it were not responsive to our universal human nature--our natural desires and inclinations. And because of that appeal to human nature, a book like The Divine Comedy will resonate with human readers for as long as human beings have natural desires that are satisfied by the symbolism of an afterlife. But within the broad constraints of human nature, Dante's work was also constrained by the cultural history of Christendom and of Dante's Italy. And yet, natural history and cultural history constrained but did not determine exactly what would emerge from the individual history of Dante's unique mind.

NATURAL HISTORY
From the beginning of Dante's Inferno, we see what he calls the principle of contrapasso--"counter-penalty" or "retaliation" (28.142). The punishment fits the crime as appropriate retribution. Dante learned this principle from Thomas Aquinas who defined contrapassum as "equal passion repaid for a previous action; and the expression applied most properly to injurious passions and actions, whereby a man harms the person of his neighbor; for instance, if a man strike, that he be struck back" (ST, II-II, q. 61, a. 4). He also learned this from Aristotle who identified antipeponthos--"reciprocity"--as a fundamental principle of justice (NE, 1132b21-23). He also learned this from the Bible's teaching "an eye for an eye" and "whoso sheddeth man's blood, by man shall his blood be shed" (Gen. 9:6

Of course, Dante didn't need to learn this principle from reading Aquinas or Aristotle or the Bible, because he could have learned it from his own natural experience. Indeed, all human beings--even those who have never read Dante, Aquinas, Aristotle, or the Bible--understand the principle of reciprocity, because there is a natural human desire to respond in kind--returning benefit for benefit and injury for injury. That's why I have identified the desire for justice as reciprocity as one of the twenty natural desires that belong to our evolved human nature.

The symbolic world of an afterlife might seem so far beyond our natural human experience that a skillful artist like Dante would have a completely free hand in designing that world in any way he pleases. That's not true, because the evolutionary success of an artist in constructing a symbolic world depends on his satisfying the natural desires of his human audience. We naturally want people to be rewarded appropriately for their virtues and punished appropriately for their vices. The ideas of Heaven and Hell have evolved in response to those natural human expectations, and so we will judge the plausibility of those ideas by how well they fit our expectations for fair reciprocity of rewards and punishments.

Much of the power of Dante's art in describing the afterlife comes from his skill in depicting appropriate punishments and rewards. For example, he presents the second circle of Hell as reserved for those being punished for the sin of lust. Although lust is one of the seven deadly sins, Dante ranks it as one of the least of the mortal sins. The more serious sins--violence, fraud, and treachery--are punished in the deeper circles of Hell.

In the second circle, the punishment of the sin of lust conveys the passionate power that drives the lustful by having the lovers blown about incessantly by a hurricane:

"I reached a place where every light is muted,
which bellows like the sea beneath a tempest,
when it is battered by opposing winds.
"The hellish hurricane, which never rests,
drives on the spirits with its violence:
wheeling and pounding, it harasses them.
"When they come up against the ruined slope,
then there are cries and wailing and lament,
and there they curse the force of the divine.
"I learned that those who undergo this torment
are damned because they sinned within the flesh,
subjecting reason to the rule of lust.
"And as, in the cold season, starlings' wings
bear them along in broad and crowded ranks,
so does that blast bear on the guilty spirits:
now here, now there, now down, now up, it drives them.
"There is no hope that ever comforts them--
no hope for rest and none for lesser pain" (V.28-45).

Here the punishment fits the crime, because the lustful reenact the restless obsessiveness of their lust eternally. The natural punishment for a vice is to be driven forever by one's vicious activity.

Nevertheless, Dante feels compassion for these lovers, particularly, Francesca and Paolo. Francesca had been unhappily married to Giancotto, and she had fallen in love with his handsome brother, Paolo. When Giancotto discovered their adulterous love, he murdered both of them. Francesco tells Dante how she fell into sin. She and Paolo were reading a romantic story about how Lancelot fell into an illicit love for Guinevere. When they read how Lancelot and Guinevere kissed for the first time, Francesca relates, "this one, who never shall be parted from me, while all his body trembled, kissed my mouth," and "that day we read no more." As Francesca finishes telling her story, Paolo weeps, and then Dante faints with pity.

We the readers are made by Dante to feel the sweet power of romantic lust, and so we also feel Dante's pity for these lovers, which persuades us to agree with Dante's judgment that their sin is lighter than the sins punished deeper in Hell. But we also know that adultery is so disruptive to social life that it deserves some punishment.

Some scholars have argued that romantic love is not a universal natural desire but a cultural invention of the medieval courtly love tradition. If so, then those readers who have not been shaped by that cultural tradition might not share Dante's sympathy for these doomed lovers. But if romantic love is a natural desire of our evolved human nature--as I believe it is--then Dante's depiction of the temptations of this passion should evoke some agreement from most readers universally.

Even readers who don't believe in the reality of an afterlife with Heaven and Hell can be moved by, and learn from, Dante's Divine Comedy because its poetic symbolism illuminates natural human experience.

CULTURAL HISTORY
But within the limits set by our universal human nature, there is room for cultural variation. And so conceptions of the afterlife will express both the natural universality and the cultural variability of our experience. Although our 20 natural desires are universal, different cultural traditions will rank and organize those natural desires in different ways, and this will be manifested in views of the afterlife.

For example, the cultural history of medieval Christendom had shaped a view of the afterlife in which the supreme good of religious understanding was to be recognized by being placed at the peak of heavenly rewards. All other goods were to be ranked as inferior to this dominant good. The goods of the body were inferior to the goods of the mind. The goods of the political life were inferior to the goods of the contemplative life. And of the goods of the mind, philosophic or scientific understanding was inferior to the religious understanding that would come from the eternal contemplation of the Christian God.

Moreover, the traditional Christian doctrines of original sin and predestination dictated that all human beings were condemned to eternal punishment, and that those few who attained eternal salvation did so only by the unmerited grace of God through the atonement of Christ. Therefore, all those outside the tradition of Christian revelation were to be condemned forever.

Dante's Divine Comedy is constrained by this Christian cultural history, which provides him the symbolic material for his poetic art. But even so, if he is crafty enough, he can exercise his individual judgment in altering and even criticizing that cultural tradition. And, certainly, the individual readers of his poetry, including many who might be skeptical of Christian culture, can engage in a critical assessment of the Christian afterlife and Dante's representation of that afterlife, which contributes to the continuing symbolic evolution of Heaven and Hell.

For instance, one might question the assumption of eternal punishments in Hell. Why do they have to be eternal? Even if Francesca and Paolo deserve punishment for their adulterous lust, do they deserve it for eternity? Doesn't it seem unfair and unreasonable that God would never give them a chance to repent of their momentary yielding to the seductive sweetness of romantic love? If so, then why not agree with Origen that the punishment of Hell is to have a purging effect, so that eventually all those in Hell can feel the pangs of conscience and then ask for, and receive, forgiveness? After all, doesn't the Catholic doctrine of Purgatory concede Origen's arguments for sinners being able to undergo purgation and thus open themselves to the love and forgiveness of God?

Dante's answer seems to be drawn from Aristotle's account of how habituation shapes character. If we choose often to engage in vicious behavior, then eventually we become habituated to it, it becomes our enduring character, and then it's hard, maybe impossible, to turn away from our vicious path. In much of the Divine Comedy, Dante seems to endorse the traditional Christian view that sin is so obsessive that at death the sinful will is unchangeable.

But how is this consistent with the idea of depravity as original sin? If we are so depraved that we have no choice but to sin, how is it just to punish us eternally? Aristotle thinks we have some choice in the matter, and therefore we can be held responsible for our choices. But if we are responsible for our conduct, why can't we choose to change, to reform, to turn back?

Is the idea of total depravity even intelligible? If we were totally depraved, how would we be able to recognize depravity as depravity? If we were totally depraved, how could be make the judgments of better and worse ways of acting that are reflected in the gradations of vicious and virtuous conduct in the Divine Comedy?

Such questions illustrate the freedom we have to criticize the cultural history of the afterlife.

INDIVIDUAL HISTORY
Within the limits set by his Christian culture, Dante had some freedom to exercise his individual judgment about how to depict the afterlife. He was able to use the Divine Comedy to criticize the Catholic Church in ways that prepared the way for the Protestant Reformation. He also implicitly endorsed a pre-Christian or pagan conception of politics and philosophy that some readers have seen as suggesting a secret teaching at odds with Christian culture (see Inferno, 9.61-63; Purgatorio, 8.19-21; Paradiso, 2.1-9, 13.121-23, 23.64-69).

Dante identifies himself with the "honorable men" and "great-hearted souls" in Limbo--pagan poets, politicians, scientists, and philosophers. At the center of the list of people in Limbo are Aristotle, Socrates, and Plato. He even includes three Muslims--Saladin, Avicenna, and Averroes. These people "did not sin," but since they "lived before Christianity" or "did not worship God in fitting ways," they cannot enter Heaven, but neither are they in the depths of Hell (Inferno, V).

As Dante finally ascends to the tenth heaven, the Empyrean, which is the highest realm of spiritual reality, the seat of imperial sovereignty is reserved for Henry VII, the Emperor, whom Dante had hoped would restore the Roman Empire. And it is prophesied that Henry's opponent--Pope Clement V--will be eternally condemned for simony (Paradiso, XXX).

As John Casey correctly notes, Dante depicts at the end of the Paradiso the supreme happiness of the eternal contemplation of God, which is the beatific vision. Dante sees the eternal light of God. "Within its depths, I saw ingathered, bound by love into one volume, the scattered leaves of all the universe" (XXXIII.85-87). Here then is the traditional idea of Christian culture that what human beings really want, what they most deeply desire, is to see God and thus to contemplate the whole order of things forever, which shows that a purely contemplative life is the only true good for human beings.

But what Casey does not notice is that even as Dante appears to endorse this, his placement of the Emperor Henry on a seat in the highest heaven suggests that political life is part of the supreme good, perhaps also suggesting that the contemplative life (whether religious or philosophic) cannot be separated from the political life, because human beings are political animals as well as rational animals.

Some posts on related themes can be found here, here, and here.

A Research Program for a Humean Science of Human Nature

In the Introduction to his Treatise of Human Nature, David Hume lays out his grand vision of a comprehensive science grounded on a science of human nature. All the sciences depend upon the knowledge of human nature, he argues. Even mathematics and the physical sciences depend on a science of human nature in so far as these sciences are objects of human study and are judged by human thought. He proposes that this science of human nature can be developed through the "experimental method of reasoning." And yet the "experiments" necessary for moral science cannot be controlled as they are in the physical sciences. "We must therefore glean up our experiments in this science from a cautious observation of human life, and take them as they appear in the common course of the world, by men's behaviour in company, in affairs, and in their pleasures. Where experiments of this kind are judiciously collected and compared, we may hope to establish on them a science, which will not be inferior in certainty, and will lie much superior in utility to any other of human comprehension."

Darwin's development of evolutionary science allowed for the fulfillment of this Humean project of a comprehensive science of human nature. Hume had already anticipated the theoretical possibility of Darwinian evolution. For example, Philo in Hume's Dialogues Concerning Natural Religion speaks of the possibility of the evolutionary natural selection of worlds and organisms. Darwin adopted this idea of evolution from Hume, Smith, and other Scottish philosophers and elaborated the scientific evidence and arguments for it.

Every topic that I have taken up on this blog could be understood as part of this Humean science of human nature rooted in Darwinian evolutionary science. This science could be sketched out according to at least ten broad themes running through my blog posts: (1) Darwinian liberal education, (2) deep history, (3) ethology, (4) behavioral game theory, (5) neuroscience, (6) social history, (7) moral psychology, (8) evolution of religion, (9) evolutionary aesthetics, and (10) the Darwinian future of human nature.

(1) DARWINIAN LIBERAL EDUCATION
Hume's comprehensive science of human nature corresponds to what Edward O. Wilson calls "consilience" and to what I call "Darwinian liberal education." The simple idea is the unification of knowledge through evolutionary reasoning, which would unify the natural sciences, the social sciences, and the humanities.

The evolutionary reasoning would have to move through at least three levels of evolutionary history--genetic evolution, behavioral evolution, and symbolic evolution. Like all forms of life, the human species has arisen from a history of genetic evolution. Like other animals with behavioral traditions of learning, the human species has a history of behavioral evolution. Although other animals can think, learn, and communicate, human beings are unique in their capacity for symbolic conceptualization, which supports a history of symbolic evolution. Consequently, a Humean and Darwinian science of human nature depends on a coevolutionary theory of the complex interaction of three systems of inheritance--genetic, behavioral, and symbolic.

Pertinent posts can be found here, here, here, and here.

(2) DEEP HISTORY
This science of human nature is a science of deep history in the sense that it views human history as part of the whole history of the universe and life. The proper study of history, Hume claims, should allow us "to see all human race, from the beginning of time, pass, as it were, in review before us; appearing in its true colours, without any of those disguises, which, during their life-time, so much perplexed the judgment of the beholders." And so, "a man acquainted with history may, in some respect, be said to have lived from the beginning of the world, and to have been making continual additions to his stock of knowledge in every century" ("Of the Study of History").

Like John Locke, Hume believed that "in the beginning, all the world was America." In other words, the reports from the New World about the American Indians who lived as hunter-gatherers could be interpreted as showing how human beings had lived in the earliest savage societies. Once they began to herd animals, human beings moved into pastoral societies. Then, with the emergence of agriculture, they entered into agrarian societies, which have dominated written human history since the invention of writing. Most recently, modern human beings have entered commercial societies that sustain larger populations and more progress in the arts and sciences than had hitherto been seen.

Although European historical thought was dominated by a Biblical history that assumed that human beings appeared first about 6,000 years ago, Hume suggested that human history was probably much older than that. But it was not until the 19th century that research in geology, archaeology, and paleoanthropology indicated that the earth and humanity were much older than suggested by the Biblical timescale. By the 1860s, John Lubbock and others were able to show the prehistoric ancestry of the human species, which supported Darwin's account of human evolution in deep time.

Most recently, historians like David Christian and Daniel Smail have argued that the very idea of "prehistory" is misconceived, because it reflects a legacy of Biblical history, and instead we should look to science as supporting a deep history that embraces the entire evolutionary history of humanity within the natural history of the earth and the universe.

Pertinent posts can be found here, here, here, and here.

(3) ETHOLOGY
In his study of human nature, Hume repeatedly relies on comparisons with other animals, because "the lives of men depend upon the same laws as the lives of all other animals," and "all animals are entrusted to their own prudence and skill for their conduct in the world" ("Of Suicide"). The study of human thinking, feeling, and learning is illuminated by comparisons with other animals that have similar mental and behavioral capacities. In Darwin's earliest notebooks, we can see his notes on these writings of Hume comparing human beings and other animals.

Though the scientific study of animal behavior goes back as far as Aristotle, the methodical study of animal behavior--ethology--has seen its greatest advances over the last 50 years. In particular, the work of primatologists like Jane Goodall and Frans de Waal makes it clear that many primate groups show cultural traditions of behavioral learning that constitute the unique cultural history of each group. This research also makes it clear that each animal has a unique personality that manifests the inborn temperament and individual history of the animal. Thus, the individuality and historicity of human life characterize animal life generally. Consequently, all animals must exercise something like judgment in deciding how to conduct themselves in navigating their way through the complex physical and social world in which they find themselves.

Pertinent posts can be found here, here, here, here, and here.

(4) BEHAVIORAL GAME THEORY
One of Hume's most insightful ideas is how cooperation evolves gradually through conventions without explicit design by which human beings become conditional cooperators. People see that it is mutually beneficial to cooperate as long as they are sure that others are cooperating and not cheating. As people interact over time, they develop trust in one another, which supports their continued cooperation. Languages evolve in this way. And so do institutional practices like property, contracts, and allegiance to government. Groups with cooperative members will tend to prevail over groups with lots of cheaters.

These traditions of cooperation are fragile, and they can fail as people collapse into a Hobbesian war of all against all, because human selfishness can always tempt people into cheating. This tense balance between cooperation and conflict manifests the mixed motives of human nature, in which there is, as Hume says, "some particle of the dove, kneaded into our frame, along with the elements of the wolf and the serpent."

Hume's story of the gradual evolution of cooperation was continued by Darwin and elaborated into a formal theoretical format in modern behavioral game theory. Experiments in game theory have shown that human beings are both selfish and social, and these experiments have shown the conditions required for the emergence of cooperation even among people with "elements of the wolf and the serpent." Evolutionary game theory has shown how this might happen through the evolutionary history of group selection. One advantage of such game theory is that experiments in game theory can be conducted with groups of people to test predictions, and in recent years such experiments have been conducted with groups of people around the world in diverse kinds of societies. So here is an example of the "experimental method" in the science of human nature.

A pertinent post can be found here.

(5) NEUROSCIENCE
In his essay "Of the Immortality of the Soul," Hume concluded from the evidence of natural experience: "Every thing is in common between soul and body. The organs of the one are all of them the organs of the other. The existence therefore of the one must be dependent on that of the other." From the seventeenth century, the work of the first neurologists such as Thomas Willis had indicated that the brain was the primary part of the body supporting the soul or mind. Darwin believed that the evolution of the human mind depended on the evolution of the brain.

Recent advances in the science of the brain make neuroscience crucial to any science of human nature. The conclusions of neuroscience support the claim that the unique intellectual and moral freedom of human beings arises from the emergent evolution of the soul in the brain. It still remains somewhat mysterious, however, as to whether the science of the brain can fully account for our introspective experience of self-conscious awareness in our thinking, feeling, and willing.

In any case, neuroscience does seem largely to confirm Hume's psychological observations. For example, Hume argues that in sympathy the human mind becomes a mirror to the thoughts and feelings of others. Adam Smith elaborated this idea of sympathy as sustaining moral sentiments. Now the research on "mirror neurons" suggests a neurological basis for this.

Pertinent posts can be found here, here, here, here, here, and here.

(6) SOCIAL HISTORY
Hume's science of human nature is a historical science, because he thought that the record of human history was "so many collections of experiments" from which the philosopher could infer a science of human nature in both its uniformity and diversity.

Hume's History of England is a massive 6-volume history of England from Julius Caesar's invasion in 55 BC to the Revolution of 1688. Although he emphasizes the history of government, Hume's history includes every aspect of English society, including morals, manners, economics, religion, technology, science, and the arts. Like Aristotle's study of regimes, Hume's history of England is a history of the whole way of life of the English people. Like Aristotle's study of the best regime, Hume's essay "The Idea of a Perfect Commonwealth" sketches a model of a perfect form of government that reflects the historical experience of England in balancing liberty and authority. Subsequently, as Douglas Adair has shown, this Humean conception of politics influenced James Madison, Alexander Hamilton, John Adams, and others at the American founding.

This idea of social history is so broad as to embrace many topics bearing on human history as an experimental record of human nature. For example, Hume was a moral critic of slavery, and he saw the history of the debate over slavery as manifesting the moral history of human nature.

Pertinent posts can be found here, here, here, here, here, here, here, here, here, here, and here.

(7) MORAL PSYCHOLOGY
Hume is famous--if not notorious--for insisting that morality is more a matter of passion than of reason, because reason by itself cannot move us to act. In the Treatise of Human Nature (II.iii.3), he writes: "We speak not strictly and philosophically when we talk of the combat of passion and reason. Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them. . . . Where a passion is neither founded on false suppositions, nor chooses means insufficient for the end, the understanding can neither justify nor condemn it. It is not contrary to reason to prefer the destruction of the whole world to the scratching of my finger. It is not contrary to reason for me to choose my total ruin, to prevent the least uneasiness of an Indian or person wholly unknown to me. It is as little contrary to reason prefer even my own acknowledged lesser good to my greater, and have a more ardent affection for the former than the latter. . . . In short, a passion must be accompanied with some false judgment in order to its being unreasonable; and even then it is not the passion, properly speaking, which is unreasonable, but the judgment."

One should notice, however, in this passage that reason is important to moral experience because the moral passions depend on judgments about the world that can be either reasonable or not. But Hume stresses the primacy of moral passions to counter the excessive rationalism of those moral philosophers who assume that morality arises as a deduction of pure reason. Determining what is right or wrong is not a matter of rational proof but of natural feeling.

In Darwin's account of the evolution of morality, he followed the lead of Hume, Smith, and other Scottish moral philosophers in emphasizing the importance of moral emotions in expressing the human moral sense as rooted in evolved human nature.

In recent years, there has been a growing movement among some philosophers, biologists, and psychologists that Hume and Darwin were right about the moral psychology of the emotions as founded in the moral instincts of human nature.

Pertinent posts can be found here, here, here, and here.

(8) EVOLUTION OF RELIGION
A science of human nature must include some explanation of the almost universal human inclination to religious belief and practice. One explanation might be that human beings have adopted religion through some some kind of rational proof--as in the testimony for the reality of miracles or in reasoning from the natural design of the universe to the existence of a divine designer. Hume lays out his reasoning for why one should be skeptical about each of these lines of reasoning, although in principle he is open to such reasoning from experience for the truth of religion.

Whether or not one is persuaded by the theological truth of religion, one must explain the psychological dispositions of human nature that incline human beings to religion. Does religion satisfy transcendent longings for immortality and redemption? Does it answer existential questions about the meaning of human anxiety before death and suffering? Does it explain the ultimate cause of the order of nature that eludes secular reason? Does it provide support for morality by organizing human beings into cooperative groups bound together by a common purpose?

Alfred Russel Wallace shared with Darwin the distinction of developing the idea of evolution by natural selection. But unlike Darwin, Wallace believed that religion (along with art and science) manifested a spiritual reality in human nature that could not be fully explained by natural evolution. Evolutionary theorists today continue this debate over how or whether evolutionary science can provide a naturalistic explanation for religion.

And, of course, much of the popular debate over Darwinian science turns on the question of whether it necessarily promotes atheism, or whether it is open to the intellectual and moral claims of religion.

Pertinent posts can be found at here, here, here, here, here, here, and here.

(9) EVOLUTIONARY AESTHETICS
One expression of human nature is the natural desire for aesthetic beauty. For Hume--as for many of the Scottish philosophers--the aesthetics of "taste" was closely related to the moral sense of beauty. The standards for both aesthetics and morality are "qualities of mind useful or agreeable to ourselves or others." Although both aesthetic taste and moral taste vary somewhat across individuals and societies, there is a universal standard of taste rooted in the "original structure of the internal fabric" of human nature.

The importance of aesthetic taste in human evolution is evident in the evolutionary theorizing about the emergence of human art. Because human beings are unique in their capacity for symbolism, and because art is one evident manifestation of that symbolism, the earliest evidence of art (as in prehistoric cave paintings and figurines) serves as signs of the transition to fully human minds.

Contemporary exponents of evolutionary psychology have begun to formulate Darwinian theories of art in general and literary art in particular.

Pertinent posts can be found here and here.

(10) THE DARWINIAN FUTURE OF HUMAN NATURE
The historical science of human nature as developed by Hume and Darwin allows us to see the emergence of human nature in the past and the present, because we have a record of human experience for the past and the present. But the future is undetermined, because we have no experience of the future. Consequently, we have no cosmic view of the whole of history, which would require a providential vision of the historical whole as a narrative of past, present, and future, which comes through the Biblical view of history and the Hegelian philosophy of history as dependent on the Biblical view.

It does seem, however, from the Humean and Darwinian view of past extinctions of species that the human species will eventually go extinct. For many people, this is deeply disturbing.

We can foresee that the conditions for life on earth are so fragile that no matter what human beings do, eventually changes in the climate of the earth or in the flow of energy from the sun will make human life impossible.

Some people foresee the emergence of transhuman life--perhaps robotic androids into which we might download our accumulated knowledge and self-awareness. But whether that would be either possible or desirable is open to question.

In any case, we must wonder whether a science of human nature that affirms the contingency of human life rather than its eternity can satisfy us.

Pertinent posts can be found here, here, and here.

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.

Does Evolution Explain Human Nature?

Does evolution explain human nature?

That's the question posed by the John Templeton Foundation to a dozen scholars of biology--Francisco Ayala, Francis Collins, Eva Jablonka, Lynn Margulis, Geoffrey Miller, Simon Conway Morris, Martin Nowak, Joan Roughgarden, Jeffrey Schloss, Frans de Waal, David Sloan Wilson, and Robert Wright. Each wrote a brief essay in response. All of the essays can be found on the Templeton website.

These essays challenge four of the major misconceptions about evolutionary explanations of human nature. It is often said that such evolutionary reasoning (1)promotes atheism, (2) assumes genetic determinism, (3) subverts morality, and (4)denies the wondrous mysteries of life.

All of these authors accept the truth of evolutionary reasoning in explaining--either partly or entirely--the complexity of human nature. Some of them are religious believers, and two of them (Collins and Nowak) write about their Christian beliefs. Some of the others speak about the importance of explaining the evolutionary origins of religiosity.

That genetic determinism is insufficient for explaining human nature is a major theme for nine of the authors (Ayala, Collins, Jablonka, Margulis, Miller, Morris, Nowak, Schloss, and Wilson). Margulis speaks about "symbiogenesis, the evolution of new species from the coming together of members of different species," and this goes beyond random genetic mutations. Many of these authors stress the importance of cultural evolution, particularly through the human capacity for symbolism. An evolutionary explanation of human nature requires an account of the complex interaction of genetic evolution and cultural evolution.

The most interesting essay on the multi-leveled character of human evolution is Jablonka's. She suggests that an evolutionary explanation of human nature would have to reformulate Aristotle's biological psychology in an evolutionary framework. Aristotle explained human nature--particularly in the De Anima--as manifesting three levels: goal-directed living systems, animal mentality, and human thought. She argues that evolutionary explanations must move through these levels. We need to define evolution as "the set of processes that lead to changes in the nature and frequency of heritable types in populations over time." These "heritable types" include "genotypes, types of transmissible epigenetic (that is, developmentally acquired) variations, types of socially learned animal behavior, and types of symbol-based transmitted information." Jablonka's reasoning is elaborated in her book Evolution in Four Dimensions, which supports the same complex view of human evolution that I will develop in my new book.

Against the claim that a Darwinian view of human nature denies the importance of morality, six of the authors (Ayala, Collins, Jablonka, Margulis, de Waal, Wilson, Wright) emphasize the importance of morality for human evolution. Although the capacity for human morality is rooted in human genetic evolution, the development of moral experience depends on human cultural evolution.

Here is where I would say that we need to move through three levels of analysis--natural order, customary order, and deliberate judgment. The evolutionary history of human morality shows the universality of moral sentiments, the variability of moral traditions, and the individuality of moral judgments.

Many people think that evolutionary explanations of human nature gloss over the deep mysteries of the human condition. But some of these authors (especially, Collins, Nowak, and Wright) insist that evolutionary science forces us to confront the inescapable mysteries of human life. Collins writes: "We see science as the way to understand the awesome nature of God's creation and as a powerful method for answering the 'how' questions about the universe. But we also see that science is powerless to answer the fundamental 'why' questions, such as 'Why is there something instead of nothing?,''Why am I here?,' and 'Why should good and evil matter?'" Nowak speaks of "the mystery and purpose of life, which cannot be answered by natural science alone." And Wright points to two "awe-inspiring mysteries"--the mystery of the cosmic First Cause and the mystery of consciousness.

Nowak thinks another kind of mystery is manifested in mathematics: "the great theorems of mathematics are statements of an eternal truth that comes from another world, a world that seems to be entirely independent of the particular trajectory that biological evolution has taken on earth." We must wonder how "evolution has led to a human brain that can gain access to a Platonic world of forms and ideas."

The conclusion I draw from all these essays is that an evolutionary explanation of human nature requires a combination of all the traditional intellectual disciplines--in the natural sciences, the social sciences, and the humanities--as we strive for a Darwinian liberal education in pondering the deepest questions of human existence.