Biocultural evolution, often framed by the Dual Inheritance Theory (DIT), posits that human evolution results from two interacting processes: genetic evolution and cultural evolution. This framework moves beyond the traditional view of natural selection acting solely on genes. It recognizes that human behaviors, technologies, and traditions—our culture—also evolve and shape our biology. Human development cannot be fully understood by isolating genes or culture, but only by studying the dynamic feedback loop between them. Culture is a distinct system of inheritance that has profoundly influenced the physical and psychological traits of our species.
Defining the Dual Inheritance System
The Dual Inheritance System outlines that humans possess two streams of inherited information that guide adaptation and behavior: genetic and cultural. Genetic inheritance involves the transmission of DNA from parents to offspring. This process is relatively stable and slow, changing over many generations through biological mechanisms like mutation and natural selection. This stream governs our physiology, anatomy, and neurological structures, providing the basic biological foundation of the human organism.
Cultural inheritance involves the transmission of socially learned information, including knowledge, skills, beliefs, and practices, passed on through imitation, teaching, and language. This system evolves much more rapidly than the genetic stream, with changes capable of sweeping through a population in a single generation or less. This form of inheritance is not physically replicated like DNA, but actively reconstructed and selectively imitated by the learner. The two systems are interdependent, with cultural traits influencing which genetic traits are favored, and genetic traits influencing the capacity for cultural learning.
Mechanisms of Biocultural Interaction
The interaction between genes and culture is driven by two interrelated mechanisms: gene-culture coevolution and niche construction. Gene-culture coevolution describes a specific feedback loop where a cultural practice creates a new selection pressure on the human genome. The resulting genetic change then affects the subsequent evolution of the cultural trait. This process leads to rapid adaptation because cultural change often occurs faster than genetic mutation, creating novel environments to which genes must adapt.
Cultural niche construction is the process by which humans actively modify their local environment through cultural activities, such as building shelters, developing tools, or domesticating plants and animals. By altering their surroundings, humans change the selective pressures that act back on them and other organisms, creating an “ecological inheritance” for future generations. For example, the cultural innovation of agriculture radically reshaped human diets and population densities, generating new selective pressures related to disease resistance and nutrient metabolism.
Real-World Examples of Coevolution
The most frequently cited example of coevolution is the relationship between dairy farming and lactase persistence in adults. In most human populations, the ability to digest the milk sugar lactose declines after infancy due to the down-regulation of the LCT gene. However, the cultural practice of domesticating livestock for milk production, which began approximately 10,000 years ago, provided a reliable food source. This cultural shift created a strong selective advantage for individuals who possessed a genetic variant allowing them to continue producing the lactase enzyme into adulthood (lactase persistence). The frequency of the associated allele, such as the $-13,910\T$ variant in European populations, increased rapidly under this selection pressure. Similarly, the adoption of cereal agriculture, which is high in starch, has been linked to an increase in the copy number of the AMY1 gene, which codes for salivary amylase.
Another documented instance involves the relationship between slash-and-burn agriculture and resistance to malaria in certain regions. The cultural practice of clearing forests for farming creates sunlit, stagnant pools of water, which are ideal breeding grounds for Anopheles mosquitoes that transmit malaria. This cultural change increased the local prevalence of the disease, creating a selective pressure for genetic traits that confer malaria resistance. The classic example is the sickle cell trait, where carrying one copy of the hemoglobin S allele (HbS) provides protection against severe malaria, even though carrying two copies causes sickle cell disease. The agricultural practice, a form of niche construction, changed the disease environment, which then drove the genetic evolution of the human population.
The Impact on Human Adaptation
The Dual Inheritance System is responsible for the speed and flexibility of human adaptation across diverse global environments. The capacity for cumulative cultural evolution allows human groups to rapidly develop and transmit complex solutions to environmental challenges without waiting for slow genetic changes. For instance, humans can inhabit arctic environments not through biological insulation like thick fur, but through culturally-transmitted innovations such as tailored clothing, specialized hunting techniques, and insulated shelters. The primary mechanism for human survival in a new environment is often the acquisition of new cultural knowledge. The feedback loop between genes and culture enhances this adaptability, as genetic traits that improve social learning, communication, or cognitive capacity for culture are favored. Biocultural evolution explains how Homo sapiens successfully colonized nearly every terrestrial environment on Earth, leveraging both the stability of genetic inheritance and the plasticity of cultural inheritance.