The Dominant Animal

Human Evolution and the Environment

Protected: Chapter 4. Of Genes and Culture

Chapter Summary

Although the ability to develop stores of non-genetic information—culture—and transmit it between individuals can be found in many kinds of animals, especially chimpanzees and bonobos, human beings possess cultures of surpassing complexity and extent. Chimps and some other animals can make momentary use of natural objects, such as sticks or rocks, and learn how to use them by imitating others. But only humans make tools, often in a culturally prescribed manner, for special purposes. And, unlike other animals, human beings are able to transmit ideas as well as information. Also uniquely, humans have become dependent on their culture, probably since tool-making and -using became essential for providing food for human groups. Today’s extensive and highly complex culture, if ever lost, would require contributions from thousands of individuals and many generations to replace.

The vast extent of human culture has been facilitated by our large brains and, especially, by the evolution of speech and language. A culture is the shared knowledge, ideas, beliefs, laws, customs, and so on, of a social group that are passed from each generation to the next. As new ideas and information accumulate, a society’s behavior patterns change and its culture evolves, on a much shorter time scale than can genetic evolution.

The earliest records of culture are the stone tools made by early Homo species some 2.5 million years ago, tools that changed little for hundreds of thousands of years. A somewhat more complex version of these tools appeared later and persisted for another 1.5 million years. Sometime during this lengthy period, human beings, perhaps gradually, gained control of fire. In the past half million years or so, tool-making became progressively more complex and sophisticated, accompanying the second exodus from Africa. About 50,000 years ago (perhaps much earlier—the timing is controversial), technologies and other new activities suddenly began to proliferate—a transformation called the “great leap forward.” Along with new kinds of tools, including those made from bone, ivory, and shells, was the appearance of art forms such as body and cave painting, sculpture, and evidence of burials and other rituals. At the same time, other hominins, including Neanderthals, disappeared from Eurasia. Some scientists think that a significant change in human brains, including an advance in language skills, may have led to the great leap forward and the subsequent acceleration in the rate of cultural evolution.

Language, of course, is a critical element in the evolution of culture. The physical underpinnings of speech probably evolved in tandem with the enlarging brains of our forebears, which in turn enabled the development of language with syntax and the storage of vast amounts of information. While the human brain resembles those of chimps and other primates in its basic structure, it is far more complex and adaptable, differing mainly in size and the numbers of neurons and synapses. This complexity provides an ability to adapt to varying or changing environmental conditions. It also allowed an increasingly social animal to infer the thoughts of other individuals—to have a “theory of mind.” This obviously would be advantageous for social interactions, and it likely gave rise to empathy and eventually to the formation of ethics.

Some apes seem to have a rudimentary theory of mind as well; they do recognize themselves and understand mirrors. Indeed, human beings and other primates have elements in their brains called “mirror neurons,” which respond both to one’s own actions and to similar behavior by others. Some scientists think mirror neurons are involved in communication through gestures, which must have been a precursor to spoken language and, naturally, in the evolution of empathy.

The modern human brain requires a large amount of energy for its manifold functions and is capable of recovering from and/or compensating for many serious injuries. Among the brain’s many “programs” for different functions, those of the frontal cortex regulate many kinds of behaviors and mature late—in the mid-twenties. The brain also regulates the release of many hormones, some of which, in turn, regulate the brain and contribute to emotions. Thus a person’s brain can monitor the individual’s own feelings and those of others and guide thinking and decision making accordingly—making use of the theory of mind. The human brain also has a capacity for memory and problem solving far exceeding the abilities of other primates, in part because of the possession of consciousness and language. It also can create programs that bias perceptions and behaviors and develop beliefs that persist. The brain’s structure makes people smart and flexible and able to solve problems, unlike many animals whose behavior is at least partly genetically programmed.

A capacious memory is clearly essential for transmitting culture, and this must have been especially so before writing was invented. Even so, there are limits to capacity, and the brain has ways of compensating, such as storing “gist” and discarding details, stereotyping, and categorizing. Some disadvantages exist, such as fading memories, suggestibility, and the “fallacy of misplaced concreteness.” Consciousness is another trait of human beings that differs at least in degree from that of other animals. It is related to bodily sensations and awareness of one’s environment, but how it is generated remains mysterious. Our genes and our environments work together to shape each individual and are inseparable in producing cultures and cultural evolution. Even so, the amount of cultural information available dwarfs the amount of genetic information an individual possesses—some 20,000 to 25,000 genes. The information storage capacity of a human brain is hundreds of thousands of times that of our genes. Natural selection no doubt determined the shapes and physical attributes of human beings, but it seems unlikely that it primarily determines other aspects of our natures such as religions, languages, and temperaments.

Proponents of “evolutionary psychology,” citing studies of identical twins reared separately, claim that natural selection has programmed our genes to control at least 50 percent of our everyday behavior and our personalities—that the heritability of behavior is 50 percent or more. The idea that genetic and environmental contributions to traits are separable was developed in a context of selective breeding of livestock, which is conducted in a controlled environment. But controlling environments in studies of human beings is not possible, and the differences or similarities seen in such studies may or may not be genetically determined.

The lack of independence is known as gene-environment correlation. Some of the difficulty of sorting out the factors has been shown by twin studies, which compared sets of identical twins with fraternal twins, all reared apart, without accounting for differences or similarities in the children’s environments. Intelligence studies aimed at determining the heritability of IQ in different socioeconomic settings have found a higher heritability among individuals with advantaged backgrounds compared with those from disadvantaged backgrounds because better environments allow more variance in IQ to be expressed. The importance of the environment is shown by people with genetic anomalies such as Down syndrome, who may mature severely mentally handicapped or able to function in society, depending on how they are reared. Thus we cannot say whether genes or environment contributes more to expression of a particular trait, but we can say which contributes more to a given change in a trait or capability.

Genes of course do affect behavior by establishing the basic makeup of each person—the body and especially the brain, which as it develops interacts with pre- and postnatal environments. Some mental disorders may emerge in individuals with particular genes under certain environmental conditions. The famous nineteenth-century Siamese twins, who were genetically identical and obviously reared in the same environment, had completely different personalities. The Dionne quintuplets, also identical genetically, were also very different and led very different lives as adults. And, of course, children born in one culture but raised in another will speak the language and conform to the customs of the adoptive culture. Finally, culture commonly overrides the genetic imperative to outreproduce others; reproductive limitation in some form appears in virtually every culture.

The most important indication that genes don’t control most behavior is what we have called a “gene shortage”—the inability of 25,000 or so genes to code all of our separate behaviors into our genome, although there are enough to produce a complex body and guide the development of a complex, flexible brain. Because each gene can influence many characteristics, and there are innumerable gene-gene interactions, natural selection can’t operate on a single trait without changing something else—there may thus be no such thing as a genetic “free lunch.” It may explain why humans seem so different from our nearest relatives, chimpanzees, even though the genetic difference (established by comparing DNA sequences) is very small.

The subtlety of environmental influences also makes them hard to explain—why siblings raised in the same environment are frequently so different in personality and temperament or even why genetically similar laboratory mice behave differently. One possibility is environmental influences during the very complex process of prenatal development. Another is the effects of how a young infant is treated, as well as influences throughout childhood and youth. Genes influence variation in many traits, but their expression—especially of behavioral traits—is mediated through the environment.

Key Terms

  • Bonobos
  • Consciousness
  • Cultural evolution
  • Culture
  • Emergent property
  • Frontal cortex
  • Gene-culture coevolution
  • Gene shortage
  • Great leap forward
  • Identical twins
  • IQ
  • Language with syntax
  • Mirror neurons
  • Neuron
  • “Out of Africa” episode
  • Stereotyping
  • Theory of mind

Discussion Questions

  1. The relative suddenness of a “great leap forward” in human evolution is somewhat controversial. What kind of evidence do you think might be found to help clarify the issue?
  2. What is the most important evidence for and against the notion that the brain is organized into modules?
  3. What do you think are the most important features that separate the capabilities of human brains from those of other great apes?
  4. If genes don’t control our behavior, how does one explain the similarities that exist between identical twins who have been raised separately?
  5. In what ways are the problems of defining species, cultures, and religions similar?