Not too long ago, many scientists argued that genetics were the key to all biological processes. The genotype, or genetic code, wrote the phenotype, an organism’s traits — physical characteristics, development, and even behavior. As the genetic code evolved over thousands of years, it determined all phenotypic change. It was a strict reading of Darwinian evolution that saw genes as the sole factor in human development and evolution. However, molecular biology and gene sequencing during that late twentieth century showed that our genetics were only part of the biological equation. It became apparent that genes cannot account for all of human variation. In fact, only in rare instances do individual genes map to specific traits anyway. It turns out that what the genetic code does is provide a number of possible biological outcomes. Potential traits manifest themselves only after interacting with external influences, known as epigenetic factors. Take a stem cell, for example. It has the potential to become any of a range of cell types. However, differentiation occurs only after chemical markers known as morphogens decide their purpose. They determine which cells become livers, which become hair, and which become toes. Morphogens are just one variety of molecules at the embryonic level that interact with our genetic code, but there are many epigenetic influences that allow some traits to be suppressed and others to be expressed.
The environment plays an important epigenetic role in our development. If, for example, we are malnourished as we grow, we may never express the trait for height that is coded in our genes. Certain toxins in the environment can be passed down from mothers to children, affecting the children’s development. And, chemicals in our air, water, and food can accumulate in our bodies over the course of our lifetime, changing which of our genetic traits are expressed. Even identical twins might develop differently, both physically and behaviorally, because of epigenetic influences. For example, one may develop cancer while the other does not. In all, while our genetics shape us, they do not completely determine our development.
Chemical compounds are only one environmental factor that can alter our genetic development and influence inheritance across generations. Other interactions with the physical world shape us as well. Take, for example, our brains. In no other part of our bodies are epigenetic variables more apparent. Our brains develop and change in response to the world around us. In fact, much of the brain’s structure is determined by our physical and cultural experiences. It relies on them. This is because our genetics code for a highly adaptable brain structure that uses external inputs to guide its development. Throughout our lives, but especially when we are young, our brains grow and adapt to external stimuli. Early on, our brains develop the necessary structures that we will need throughout our lives. But, they are not hardwired like a computer. Instead, they change to match world around us. On occasion they create new neural pathways, but they also “prune” unused structures. While genetics and epigenetic molecular interactions code for the basic anatomical structures of our brains, our experiences shape our neural architecture. This ability is commonly referred to as neuroplasticity, or the “plastic brain.”
Due to evolutionary adaptations, brains seem to be genetically programmed to expect certain environmental stimuli as they develop, what scientists call experience-expectant development. If a brain does not experience them, it does not build mechanisms for responding to them. Take sight as an example. Our brains develop visual-spatial reasoning quite early in our infancy. However, if we do not experience visual phenomena by a certain point in our growth, we will find it difficult to adjust later in life. While our brains have the capacity to understand visual phenomena, we rely on experience to shape the necessary neural pathways.
Experience-dependent development, on the other hand, is the ability of the brain to change in response to unexpected stimuli. It may grow neural pathways to respond to cues from the physical world. For example, experiments with rats show that those experiencing more complex situations — especially those that use a broader range of motor skills, perhaps through constant practice in an obstacle course — develop more synapses in their brains. Similar findings in human brains reveal that experience-dependent neural plasticity begins in newborns and lasts throughout our lives to greater and lesser degrees. In a recent study, for example, patients who had lost vision developed the capacity for echolocation — the ability to tell space by using sound. fMRIs of the patients’ brains showed that the areas of the brain formerly associated with sight rearranged themselves to focus on echolocation.
Social worlds play an important role in experience-dependent development in animals, including humans. A telling experiment with rats shows that social interaction helps develop their upper visual cortex. After weaning, baby rats were separated into two groups. One group was isolated from other rats. A second group was allowed social interaction. The rats that engaged with others developed 20-25% more synapses in their upper visual cortex than the isolated rats. Studies of humans show analogous patterns: social experiences can sculpt our neural anatomy. Psychological trauma and stress – even personal attachments, or lack thereof – shape and reshape our pre-frontal cortex, a highly plastic area of the brain. This area of the brain is central to processing social behaviors, which means that our relationships can condition our brains to respond in particular ways.
Cultural variables can shape the brain over the course of someone’s lifetime; they can also transform the genetics of entire populations over longer time scales. By culture, anthropologists generally mean the entire range of human undertakings mediated by symbolic activity. Language, for example, is culture in that it uses symbols – words and word groupings – to stand in for ideas and things. Language is an essential feature of most human cultural activity – from agriculture to religion. That said, not all cultural activities are immediately linked to spoken language. For example, the person who owns an expensive car does not need to explain that they are wealthy. The car itself stands in for the idea of personal wealth. Symbolic forms change over time and have different functions in different contexts.
Scientists often use lactose intolerance as an example of how culture affects evolution. Humans are the only mammals to drink milk into adulthood, but most of us cannot process lactose beyond childhood. On the other hand, adults who have ancestry in areas where dairy herding was common – northern Europe for example – are more likely to produce lactase, the enzyme that breaks down lactose. The cultural practice of dairy farming favored genetic mutations that allowed some populations to produce lactase into adulthood. What this means is that genetics and culture can evolve together. Just as genetic transformations allowed for the evolution of the human brain and its capacity for culture, culture created selection pressures that drove genetics. This process is called gene-culture coevolution.
Co-evolution is one of the most exciting discoveries in recent decades because it has forced us to rethink much of what we had assumed about humanity’s history. Until recently, many scholars believed that the genetic composition of humans had stabilized by the end of the Upper Paleolithic – about 12,000 years ago. This perspective had several consequences. Among some researchers, this meant that all human evolution had stopped. What we are today – biologically and psychologically — could be read back thousands and thousands of years. Our brains, for example, had hardly changed. From this perspective neuroscientists had little to learn from anthropologists, historians, and archaeologists. The human genetic code and, by association, human nature were static, unchanging phenomena. For their part, social scientists tended to ignore advances in science. If human evolution had stalled, then culture was the key to explaining behavioral variations among humans. It turns out, however, that human evolution has not been static. The ability of European adults to produce lactase developed only in the last 7500 years. And, in fact, up to 10 percent of the entire human genome has changed in the last 50,000 years. It appears that rather than slowing human evolution, the socio-cultural processes of agriculture, domestication, and urbanization may have actually increased selective pressures.
When it comes to our brain, the notion of gene-culture co-evolution suggests that to fully understand humanity, we need to study both biology and culture. More importantly, we have to study them together. They are not separate from each other. They are inextricably linked.
Culture shapes our brains. They are, as Steven Mithen and Lawrence Parsons have argued, “cultural artifacts.” And, the way cultural processes shape them can be hereditary. On the one hand, culture can create selective pressures that favor genetic variations. The co-evolutionary feedback loop that favored a larger, more plastic brain in homo sapiens is a good example. The hominin brain was not on a predetermined evolutionary path when homo erectus began fabricating Acheulian handaxes 1.4 million years ago. In fact, for another 1.2 million years, hominin tool construction styles remained fairly static. Something happened around 200,000 years ago that then accelerated in the Upper Paleolithic. That “thing” was probably not a single item. Rather, it was a complex series of interactions between genetics, brains, bodies, and environment.
On the other hand, cultural contexts shape our neural architecture. And, they do so in profound ways, especially considering that culture influences nearly all of our environmental experiences. To name just a few examples, we learn how to communicate, manipulate material objects, empathize, and read as we grow up. Genetics give us the capacity to do all of these things, but only through practice do we develop the neural pathways necessary to be proficient in them. As we form our brains in response to our cultural contexts, we have the tendency to reproduce the cultural ideas that we were taught. In this way, culture can be hereditary, pass down through generations, and shape our brains across generations. However, as with genetic replication, culture is quite malleable. Humans are constantly experimenting with new ways of doing things. So, while we may have been taught something, it does not mean that we will do it the same way every time. We may even abandon older cultural forms and embrace new ones.
Neuroarchaeologists have been adept at demonstrating how material culture may have helped drive evolutionary change. Arguing that cognition is not something that happens solely within the brain, they see the material world as part of humanity’s cognitive structure. In other words, the things that we manipulate in the world are part of our mental apparatuses. This is especially true of the things that we create – our cultural artifacts. A simple demonstration of this concept can be found in mathematics. While we are capable of running equations in our head, we have developed pencils, paper, and symbols to work out complex equations. The written visual symbols and tools of mathematics – its material culture – are part and parcel of our intellectual process. Or, consider the construction of nearly any handcrafted item, from a tool to a piece of furniture to a work of art. What we make is not necessarily a representation of an idea that we have in our heads. Every single piece is different. This is not because we want them to be different. Rather, circumstances transform outcomes. Perhaps our paintbrush slips while we are painting. Or, when we are knapping flakes off of a rock, sparks start a fire. Things like this happen every time that we participate in material-based activities. Sometimes, we will consider the unintended outcomes as improvements. Sometimes, they will change the way that we think about and interact with the world. Sometimes, they will change how our communities do things, establishing new norms. And, sometimes, they will change our cognitive structure in more fundamental ways. As the historian Daniel Lord Smail points out, various cultural practices, such as music or dancing, can alter our brain chemistry. Likewise, early tool use may have driven the expansion of neural areas that, in turn, became centers for higher cognitive processes, including language. The things we make and the things we do have the ability not only to represent the world as we imagine it, but they also have the ability to shape our imaginations in the first place.
It would be a mistake to try to identify a single causative factor that drove the genetic, neural, linguistic, and socio-cultural changes among hominins. Likely, there was web of interactions working on different time scales, each influencing and driving the other. Better tools meant more calories and the ability to support a larger brain. Improved tools meant new ways of engaging with the environment, and the restructuring of neural pathways. Larger brains allowed for language, and it, in turn, helped homo sapiens cooperate more effectively. Language facilitated social learning, allowing humans to respond to environmental pressures and build more efficient tools. It also gave humans the capacity to store knowledge in new ways. Across the community, the brains of individuals were connected through language, and their individual knowledge became part of a common pool of information that could be shared. To make this possible, cooperative social behaviors were necessary – things such as empathy, an adaptation that was coded partly through the genetic structure and partly through sociocultural practices. Combining these traits, humans became effective at shaping new environmental niches, such as agricultural communities and cities. In turn, these cultural products produced new selection pressures that continue to influence human development.
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