What Does it Mean to be Human? An Evolutionist’s View

It is important to recognize these as separate questions before relating them to each other. We can begin by asking a third question—“What does it mean to be species X?”—where X is any biological species other than humans. What does it mean to be an E. coli, an oak tree, a monarch butterfly, or a polar bear? I am not referring to the interior experience of being such a species, which would be difficult or impossible for humans to apprehend.[1] I am referring to the measurable properties of such a species—what an evolutionary biologist would call its phenotype—which is fully amenable to scientific understanding.

Answering the question, “What does it mean to be species X?” is what evolutionary biologists do for a living. Each species is a product of evolution in relation to its environment. The process of natural selection adapts each species to survive and reproduce in its environment. E. coli has the properties required to survive and reproduce in the human gut. Monarch butterflies sequester the toxic compounds of milkweed plants for their own defense and undertake an amazing migration to survive the seasons. Polar bears are white to conceal themselves from their prey and have myriad other adaptations to survive the arctic environment.

No species is perfectly adapted to its environment, and all species bear vestiges of the far distant past, reflecting the fact that evolution is a path-dependent historical process. The first vertebrates were fish and all subsequent vertebrates, such as mammals, bear the marks of their fishy origin.[2] In addition, all organisms are a product of development over the period of their lifetimes. Single genes typically affect many phenotypic traits, and single phenotypic traits are typically influenced by many genes. Any given phenotypic trait therefore cannot be analyzed in isolation from other traits, but rather as part of a genetically influenced developmental system.[3] Nikko Tibergen, who shared the Nobel Prize in 1973 for helping to found the science of ethology, stressed that four questions must be addressed to fully understand any particular trait: (1) Why does the trait exist, compared to many other traits that could exist, often (but not always) based on the winnowing action of natural selection? (2) How does the trait exists in a physical sense? (3) How did the trait evolve over a period of generations? (4) How does the trait develop during the lifetime of the organism? Tinbergen posed these questions in an article titled “The Methods and Aims of Ethology,” but they also describe the methods and aims of evolutionary biology as a whole.[4]

In this fashion, evolutionary biologists provide answers to the question, “What does it mean to be species X?” Now imagine placing any given species in a different environment than its past range of environments. There is no guarantee that it will be well adapted to its new environment. Perhaps it is so maladapted that it goes extinct immediately—like a fish out of water. Perhaps it survives and reproduces well enough to remain in existence as a population, in which case natural selection will begin to change its properties so that it becomes better adapted to its new environment. When this happens, the question, “What does it mean to be species X?” will have a different answer than it did before.

This is why the question, “What does it mean to be species X?” must be recognized as different from the question, “How can species X cope with problems threatening its current existence?” The first question is inherently about the relationship between the organism and its past environments—what evolutionists call the “environment of evolutionary adaptedness,” or EEA. The second question is inherently about the relationship between the organism and its current environment and arises primarily when the current environment becomes different from the EEA.[5]

To provide a concrete example, imagine a lizard species adapted to live in the rainforest that is suddenly transported to the desert (alternatively, the lizard can stay where it is and the climate can change). The phenotypic properties of the lizard don’t change; they simply have different consequences in the new environment than they did in the old environment. The answer to the question, “How can the lizard cope with the problems threatening its current existence?” is “It can’t without human assistance. Otherwise, only natural selection will cause it to cope better, at which point it will have different properties as a species.”

Actually, this example needs to be complicated in two ways to be fully relevant to the human case. All species have evolved by natural selection to respond to environmental change during the course of their lifetimes, a property that evolutionists call phenotypic plasticity.[6] Continuing our example, rainforest habitats vary in their degree of wetness and dryness. Rainforest lizards are well adapted to seek out dry, sunny spots to bask, seek moisture when they become desiccated, and so on. Their phenotypic plasticity might serve them to a degree in the new desert environment, but there is no guarantee that the pattern of phenotypic plasticity that evolved in the rainforest environment will be equally well adapted to the desert environment. Thus, phenotypic plasticity by itself does not solve the problem of mismatch that arises when a species encounters an environment that differs from the EEA.

The second complication is based on the fact that the development of phenotypic traits typically involves an interaction between genes and the environment. Even traits that develop reliably in all individuals, such as the vertebrate eye, require environmental inputs over the course of development. Place a patch over the eye of a developing vertebrate, and the eye will fail to develop. This means that when a species is placed in a new environment, its phenotypic traits might immediately change, based on an altered gene-environment interaction. However, the new gene-environment interaction is no more likely to adapt the organism to its new environment than a new genetic mutation. Thus, the mere fact of gene-environment interactions does not solve the problem of mismatch any more than the mere fact of phenotypic plasticity does.

To summarize, the question, “What does it mean to be species X?”—which I take to mean “What are the phenotypic properties of species X?”—must be answered on the basis of the past environments inhabited by species X. The question, “How can species X cope with the problems threatening its existence?” must be answered on the basis of the current environment inhabited by species X. These questions are different to the extent that the current environment departs from past environments. The answer to the second question is, “Species X doesn’t cope, and only natural selection in the new environment will enable it to cope better.”

A Fourth Question

So far, my path to answering the question, “What does it mean to be human?” has involved a detour to answer the question, “What does it mean to be species X?” where X is any biological species other than humans. Before I can address the human case, I must pose another question that is not focused on humans: “What is an evolutionary process?”

The standard answer to this question is that evolution requires four ingredients: (1) A population of reproducing entities that (2) vary in their phenotypic properties with (3) corresponding variation in survival and reproduction and (4) heritability, which is defined as a phenotypic resemblance between parents and offspring. When all four ingredients are present, then phenotypic traits that increase survival and reproduction accumulate in the population, adapting the entities to their environments.[7]

The most important observation to make about this standard answer for the purpose of this essay is that the definition of an evolutionary process says nothing about genes. Heredity is required, and genes enter the picture only insofar as they provide a mechanism of inheritance. By the same token, if other mechanisms of inheritance exist, then genes are not required for a process to count as evolutionary.

Even though the answer provided above is standard, there is also a sense in which it is new and even revolutionary. Darwin knew nothing about genes, but once genes were discovered, they became the one and only mechanism of inheritance for most evolutionary biologists. There is a tradition of thinking about evolution as a substrate-neutral process, but it needs to become more central to evolutionary thought than it has been in the past.[8] An excellent recent account is provided by Eva Jablonka and Marion Lamb in their book Evolution in Four Dimensions.[9] They provide a concise history of why evolution became so gene-centric and describe three additional mechanisms that are capable of creating a resemblance between parents and offspring. The first is epigenetics, which involves changes in gene expression rather than gene frequency. The second is social learning. The third is symbolic thought. These mechanisms are capable of producing trans-generational inheritance. In addition, the vertebrate immune system includes an evolutionary process that involves the random formation of antibodies and their selection based on their success at binding to antigens. This is trans-generational as far as the antibodies are concerned but takes place within one individual vertebrate. Individual learning and other neural processes can also be regarded as variation-and-selection processes taking place within single organisms, regardless of whether they are transferred to other organisms.[10] I will have more to say about these mechanisms as we proceed. For now, the main point to keep in mind is that thinking in terms of heredity, rather than genes, vastly expands the domain of evolutionary theory.

Photograph of Charles Darwin, published by John G. Murdoch in 1874

The Human Case

Now I am in a position to address the question, “What does it mean to be human?” The answer is the same as for any other species, except that non-genetic evolutionary processes operate more strongly in humans than in other species. This is especially true for symbolic thought as an inheritance system, which comes close to being uniquely human.[11]

Symbolic thought is a network of mental associations that need not bear any relationship with the external environment. This is in contrast to associative learning, where the mental associations are closely linked to environmental associations. Associative learning enables a rat to associate food with a sound whenever these things are paired and to disassociate them when they become unpaired. A symbol, such as a word, remains permanently associated with its referent and can even refer to entities that don’t exist in the real world, such as a “ghost.”

Associative learning is clearly adaptive, but what is symbolic thought good for? Even though any particular network of associations need not correspond to the real world, it still motivates action in the real world. If we coin the new word “symbotype” to refer to a particular network of associations, there is a symbotype-phenotype relationship similar to a genotype-phenotype relationship. In addition, symbotypes exist in almost infinite variety. They have the same kind of combinatorial diversity as genotypes and antibodies, giving humans a behavioral flexibility unmatched in any other species. Symbotypes can also be transmitted across generations, thereby qualifying as a full-blown inheritance system.

We can appreciate the importance of symbolic thought as an inheritance system for humans by stepping back and observing the panorama of human cultural evolution. Our distant ancestors once had a geographical range comparable to other great ape species. Then something happened that enabled them to spread over the entire planet, inhabiting all climatic zones, occupying hundreds of ecological niches, and speaking thousands of languages.[12] For any given culture, survival and reproduction require an extensive physical and mental toolkit that must be learned and transmitted across generations. This is only possible thanks to language and other forms of symbolic thought. Emile Durkheim was prescient when he wrote: “In all its aspects and at every moment in history, human social life is only possible thanks to a vast symbolism.”[13]

Our capacity for symbolic thought is connected to another hallmark of our species—our ability to cooperate in groups of individuals that need not be genetically related to each other. Our cooperative nature is easy to appreciate in the context of physical activities such as childcare, hunting, and warfare. In addition, we need to appreciate the cooperative nature of the most distinctively human mental activities such as symbolic thought. This has led a number of evolutionists to adopt a “cooperation came first” hypothesis for human evolution.[14] The first event in the path to becoming human was the ability to suppress disruptive competition within groups, making succeeding as a group the primary evolutionary force.[15] This watershed event led to a package of physical and mental cooperative activities, including the sharing of symbolically encoded acquired information.

If this hypothesis is correct, then the answer to the question, “What does it mean to be human?” is that cooperation is the signature adaptation of our species. One manifestation of cooperation is a symbolic inheritance system that makes adaptation an extremely fast process, at least compared to genetic evolution.

Addressing the Second Question

Like the other authors commissioned by the Center of Humans and Nature to write essays on what it means to be human, I am drawn to the second question: “How can we cope with the enormous problems threatening humans and the natural world, caused largely by the impact of humans on the natural world?” I believe that my answer to the first question provides an exceptionally powerful toolkit for addressing the second question.

To begin, the fact that we are capable of rapid adaptation means that we can potentially solve problems on timescales that matter, as opposed to the slow timescale of genetic evolution. An evolutionist is not required to point out that people are capable of rapid change, a fact that is established by every decision that is made, everything that is learned subliminally, and every social movement recorded by history. What’s new is to study these processes more explicitly as evolutionary processes, in ways that are currently largely restricted to the study of genetic evolution and the vertebrate immune system.

I and my colleagues have developed this thesis in more detail elsewhere.[16] Here I will provide three vignettes to illustrate what it means to approach real-world problems from an evolutionary perspective.

What Does It Mean to Be Human Skin?

Our skin provides a lesson in genetically evolved phenotypic plasticity and the concept of mismatch.[17] Sunlight is important for the skin to manufacture vitamin D but also causes cancer. In sunny equatorial regions, constant exposure to the sun led to the genetic evolution of dark skin as a fixed trait. In the temperate zones, seasonal variation in exposure to the sun led to the evolution of skin pigmentation as a phenotypically plastic trait—sun-tanning. Dark-skinned people who move to temperate regions suffer from an inability to manufacture enough vitamin D, a clear case of mismatch that luckily can be corrected with dietary supplements. People capable of sun-tanning can also experience mismatch in a variety of ways because their phenotypically plastic adaptation is calibrated to the particular pattern of variation that existed in their ancestral environment. A person from England who moves to Australia will never become as dark as the aborigines who have inhabited Australia for forty thousand years. A person capable of tanning who spends a lot of time indoors or covered with clothing will experience sunburn when his or her skin is suddenly exposed to the sun, a pattern of variation that seldom, if ever, occurred in his or her ancestral environment. This example illustrates that phenotypically plastic traits are as vulnerable to mismatch as phenotypically fixed traits whenever the patterns of environmental variation in the new environment depart from those of the EEA. It also illustrates that many aspects of our phenotype are a product of genetic evolution, just like any other species. We are capable of rapid adaptation in some respects, but not in all respects. The properties of our skin will not change except over a period of many generations. If we want to solve problems associated with our skin in relation to our current environment, we will need to create workarounds, such as clothing, sunscreen, vitamin D supplements, and so on. Many other examples of genetic mismatches could be provided involving our immune systems, diet, and exercise regimes.[18]

What Does It Mean to Be Mary (or Any Other Person)?

The question, “What does it mean to be human?” obscures the fact that individuals are very different from each other. If we restrict our thinking to human universals, we ignore the phenomenon of human diversity. Everyone acknowledges that people are a product of both their genes and their environment, but this generality lacks traction without more detail. One way to provide more detail is by noting that individuals are evolving systems in their own right, responding to the contingencies of their particular environments. This was the fundamental insight of B.F. Skinner, who termed it “selection by consequences,” which remains valid, however flawed the tradition of behaviorism was in other respects.[19] Another way to provide more detail is by understanding the gene-environment interaction in more mechanistic detail, especially in relation to physiology and development. Mechanistic understanding was one of the main shortcomings of behaviorism, but it is richly provided by other branches of psychology and neurobiology.[20]

To appreciate the relevance of these ideas, many problem behaviors in individuals are associated with harsh environments during development. One interpretation is that children develop optimally in benign environments and abnormally in harsh environments, resulting in dysfunctions that need to be fixed, in the same way that a broken automobile needs to be repaired. Another interpretation is that benign and harsh environments have been part of the EEA for all species, including humans. When the going gets tough, the tough don’t fall apart—they behave adaptively in the context of the tough environments. That’s what adaptive phenotypic plasticity is all about.[21]

The idea that what counts as a problem from the standpoint of human and environmental welfare can be adaptive in the evolutionary sense of the word is a new concept for many people, including many formulators of public policy. Yet it is central to any evolutionary process, no matter what the mechanism of inheritance. Adaptations frequently benefit some individuals at the expense of others or promote short-term welfare at the expense of long-term welfare. Adaptations are also path dependent. As evolving systems, people can find themselves on tiny adaptive hills, unable to scale taller peaks because going up first requires going down.

It’s not as if all problem behaviors are adaptive in the evolutionary sense of the word; some are genuine malfunctions, as we have seen in the case of mismatch. However, it is essential to correctly diagnose any particular case to devise successful solutions. When a problem behavior is adaptive, the best way to change it is by finding less problematic strategies that are more adaptive and especially by changing the environment in a way that selects for less problematic behaviors. This is becoming the basis for enlightened intervention strategies for problem behaviors in a variety of contexts, such as risky adolescent behavior and psychotherapy for adults.[22]

What Does It Mean to Be Norway (or Any Other Group)?

Individuals are not the only human entities that differ in their phenotypic properties. Groups differ at all scales, from neighborhoods to corporations to nations. Selection processes also operate at all scales, sometimes resulting in groups that function well as corporate units, but at other times resulting in dysfunctional groups, especially based on conflicts of interest within the groups.

I have already described how genetic evolution endowed us with a propensity to cooperate in small-scale groups, thanks largely to mechanisms that suppress self-serving behaviors within groups. The same dynamic has operated for cultural evolution throughout human history.[23] A book titled The Spirit Level: Why Greater Equality Makes Societies Stronger, by Kate Pickett and Richard Wilkinson, documents variation in the functional organization of developed nations in impressive detail.[24] The degree of equality within a nation emerges as a critical variable for just about every social welfare outcome that can be measured. Developed nations vary enormously in their degree of equality, from nations such as Norway, Switzerland, and Japan on the egalitarian end to the United States, Great Britain, and Portugal on the inegalitarian end. The same patterns of variation that exist among developed nations also exist among the fifty states of the United States.

Another book titled Why Nations Fail: The Origins of Power, Prosperity, and Poverty, by Daron Acemoglu and James Robinson, adds a historical dimension to the analysis of contemporary nations offered by Pickett and Wilkinson.[25] Acemoglu and Robinson contrast extractive cultures, where most members are exploited by a small group of elites, with inclusive cultures, where most members can profit from their own initiative. Inclusive cultures are much more innovative than extractive cultures, which results in large differences in how well they function as polities. According to Acemoglu and Robinson, England was the birthplace of the industrial revolution because it had become a more inclusive society than other European nations. The technological innovations that were implemented in England gave it an advantage in economic and military competition out of all proportion to its size. The first European colonies in Central and South America were extractive in nature and led to nations that still function poorly today. The very first British colonies in North America attempted to be extractive, first by attempting to exploit the Native Americans and then by attempting to import cheap British labor, but the laborers had too many other options in the New World, forcing their bosses to adopt more egalitarian relations. The relative equality that existed in the United States and Canada caused them to develop much faster as nations than the Central and South American nations. Differences in equality within the United States gave the North a competitive edge over the South during the Civil War.[26]

Notice that Britain and the United States were on the egalitarian end of the spectrum earlier in their histories, even though they are at the inegalitarian end of the spectrum today. Nations change during their histories, and these changes are consequential for how well they function as corporate units. Equality is vulnerable to subversion from within and must be supported by norms enforced by rewards and punishments. The suppression of dysfunctional forms of selection within groups was essential for our genetic evolution as a cooperative species, and it is just as essential for the cultural evolution of modern-day groups at all scales.

Conclusions

My three vignettes barely scratch the surface of real-world problems that can be addressed from an evolutionary perspective. I chose them to be different from each other: The first highlights the fact that some aspects of our phenotype are a product of genetic evolution and will not change over short timescales. If they pose a problem in our current environments, then workaround solutions comparable to sunscreen and dietary supplements are required. The second highlights the other end of the spectrum. Individual people are evolving systems in their own right, capable of adapting to their environments during the course of their lifetimes, but also capable of exhibiting the dysfunctional outcomes of evolutionary processes. The third highlights the fact that groups can evolve into corporate units capable of adapting to their environments, but only when certain conditions are met. Remarkably, these conditions apply to groups of all sizes, from neighborhoods to nations, and even to the future formation of a global village.

Every major real-world problem has received the attention of many smart people representing dozens of academic disciplines. Most of them accept the theory of evolution and endorse the ideal of consilience, or unity of knowledge, which requires every branch of knowledge to be consistent with every other branch. Yet the human-related disciplines are famously isolated from each other, and many of them developed without reference to evolutionary theory for most of the twentieth century. Evolutionary theory was itself too gene-centric to accommodate the human-related disciplines, but once we expand the theory to include all mechanisms of inheritance, then it becomes possible to understand our species entirely from an evolutionary perspective. The integration that is currently underway for the human sciences will be comparable to the integration that took place for the biological sciences during the twentieth century (and continuing).

In this short essay, I have tried to convey these trends by supplementing the two questions, “What does it mean to be human?” and “How can we cope with our current problems?” with two additional questions: “What does it mean to be any species?” and “What is an evolutionary process?” My conclusion is that addressing the latter two questions provides a novel answer to the first question, which in turn can provide novel solutions to our problems. I hope that I have piqued the interest of the reader enough to join the integration that is in progress.