Although the impact of natural selection with regard to biological features is well understood, its influence on human behavior is a topic of controversy. The debate concerning the genetic basis of human behavior has raged for half a century, spawning the new field of sociobiology and, more recently, evolutionary psychology. While the level of genetic contribution to human behavior is debated, what is evident is that behavior is part of the human phenotype and thus susceptible to natural selection. Beginning in the 1970s, some researchers, archaeologists in particular, began to argue that not only is human behavior subject to natural selection, but also the products of that behavior, in particular, components of material culture (e.g., Dunnell 1978a, b). As humans have evolved, we have lost many of our biological mechanisms for protection and display, such as body hair, claws, and long canines, the functions of which are now performed by our technology. By extension, then, technology must contribute to our fitness, and thus the argument has been made by evolutionary archaeologists that the material products of behavior should be considered part of the human phenotype (e.g., Leonard and Jones 1987). Because natural selection operates on the phenotype, by definition, then, it operates on material culture.
From this perspective, Endler’s argument concerning natural selection might be rephrased to include material culture: (a) that there is variation in the technology humans make and use, (b) that some variants affect fitness, and (c) that a form of inheritance occurs when we learn how to make and use technology (information is passed from the teacher to learner). Let’s consider each of these in more detail using an archaeological example: the transition from the atlatl and dart technology to the bow and arrow in North America. We begin with some background on this transition.
Prior to about 9,000 years ago, the spear or javelin, propelled by hand, was replaced by a new propulsion system, the spear thrower, or atlatl (see, for example, Hughes 1998). The atlatl, which acts as an extension of the arm, significantly increased the distance a spear could be thrown (Fig. 1). This new technology comprised three basic components: the atlatl, the spear (termed a dart), and the stone projectile tip, or dart point. Archaeological examples illustrate that the morphologies of these components varied across geographic regions as well as over the 7,000 years during which this technology was in use. About 2,000 years ago, the bow and arrow appeared, replacing the atlatl and dart almost entirely. Because this replacement was relatively rapid, it is believed that this new technology offered considerable advantage to hunters. So, how do we portray this transition within Endler’s model as it has been recast for material culture? We begin with phenotypic variation.
Phenotypic Variation
For natural selection to operate, there must be several alternatives that can complete the same task. This variation may exist within or between technologies. For example, within the atlatl and dart technology, there are varied forms of atlatls, some comprising a single piece and others of several parts. Some incorporate weights for balance while others do not, some have handles, others grips, and so forth. The dart points vary in size, form, and raw material from which they are made.
At a more inclusive scale, the variation is between the two technologies, which differ in propulsive force, accuracy, and so forth. Selection here is operating on the totality of components comprising the technology rather than on variations within individual components. This is not to say that variation did not exist within components of the bow and arrow; it most certainly did, and changes in that technology after it was introduced (such as the bow size and form, arrow shaft form, point size and form, e.g., Fig. 2) probably resulted from the influence of natural selection. The advantages of the bow and arrow over the atlatl and dart, however, are undoubtedly what led to the replacement, which brings us to fitness variation.
Fitness Variation
As is the case for biological traits, cultural traits can be either functional or neutral, the latter designated as stylistic, and their status at any one time is context-specific (Dunnell 1978a, b). Also like functional biological traits, not all functional cultural traits enhance fitness. For example, some traits may be carried along from generation to generation because they “piggyback” onto one or more traits that are under strong, positive selection. The criterion of fitness variation requires only that some variants affect fitness. Our two propulsion technologies, on the whole, represent functional traits, although there are characteristics in the individual components of each that could be stylistic. But at least some of functional traits of these technologies affect fitness. Although it is difficult to point specifically to those that do so, we can examine what fitness advantages the bow and arrow may have conferred on its users over the atlatl and dart. First, the bow and arrow had the same advantage over the atlatl and dart that the latter had over the javelin: greater propulsion distance. This would have allowed a hunter to launch farther away from his prey, lessening the possibility that he would have been discovered. In addition, it also had the advantages of greater missile speed (because of the greater force propelling the arrow) and greater precision and accuracy, in part due to the greater speed. These differences would have increased a hunter’s efficiency, allowing the capture of more game.
Inheritance
This component has been one of the most contentious among those who reject the application of Darwinian principles to explain artifact variation; artifacts don’t reproduce like biological organisms and thus cannot inherit anything. Evolutionary archaeologists argue, however, that the learning process, in which information is passed from one individual to another, what archaeologists call cultural transmission (Boyd and Richerson 1985), is analogous to biological inheritance. Cultural transmission is learned and can occur between parent and offspring (vertical transmission), between learners and relatives or other more experienced members of society (oblique transmission), or from peer-to-peer interaction (horizontal transmission). Cultural transmission can also vary by how much instruction is given in learning the craft (of say making projectile points) and/or differ in the extent to which there is a fitness benefit related to reproductive success; it could also relate to some potential prestige a person may obtain based on some behavioral variant. However, not all traits that are conveyed through cultural transmission are functional; stylistic traits are also conveyed in this manner.
Archaeologists have utilized the concept of cultural transmission on an intuitive basis for over a century in the form of “culture contact” and diffusion, but only through evolutionary archaeology has it been the focus of intense study. Similarity in form, especially for style, indicates some degree of contact and transmission, but this has been difficult to measure. Behavioral scientists do not agree upon what constitutes a “package” of transmitted cultural information; we have no direct unit equivalent to the gene. Further, we know that the packages are stored in different ways and reconstituted in complex ways of generating manifold opportunities for innovation and consequent selection, making the measurement of cultural transmission even more difficult. One recent approach in physics (based on statistical inference) has proved quite useful in graphing how similar/different one object is from another through network analysis (see Farrah et al. 2009). Network analysis reveals how a technological tradition, a set of rules or templates for properly making a particular item by members of a social group, is translated into a set of physical objects. Goodale et al. (2011) applied this approach to a group of projectile points (both dart and arrow points), from the northern Plains of North America. First, a set of morphometric measurements were taken from a set of landmarks on the points to describe their morphology, and then comparisons of each point to all others were made statistically, resulting in quantitative measures of similarity. The results are represented in a network diagram where each point is designated by a circle (or node, Fig. 3). Lines (degrees) connect nodes that are most similar in morphology. As we can see, the network is partitioned into two subgroups, which happen to correspond to cultural units (called types) that archaeologists previously have labeled Besant and Avonlea traditions of projectile point manufacture. The blue nodes represent points belonging to the Besant dart point tradition, while the green nodes correspond to the Avonlea arrow point tradition. These results indicate that the Besant and Avonlea point types are fairly distinct from one another, suggesting two sub-networks in a larger network of making projectile points. That is, there are different templates for making these two types of points and these templates were transmitted to members of the social group in which each is made (cladistical approaches have similarly been applied to identifying material cultural lineages, e.g., O’Brien and Lyman 2001).
Once natural selection is in operation on material culture, the question is, how do we judge fitness, which for biological organisms is measured by reproductive success? As stated above, components of material culture do not genetically reproduce so logically they cannot have reproductive success. They may enhance the reproductive success of their users, although this can be difficult for archaeologists to measure directly. But in exactly the same way, it is difficult for paleontologists to measure the fitness benefits of morphological variation. Evolutionary archaeologists instead view material goods manufactured by humans to have replicative success or differential persistence through time and across space (Leonard and Jones 1987). What this means is that given a set of alternative forms, some will be chosen by users more often than others, and those chosen forms will be made known to other users through vertical, horizontal, or oblique transmission at the expense of the less desired forms. Replicative success is a useful concept because the relative abundance of technological forms, the amount of variation within a technology, and the causes for that variation allow us to operationalize natural selection in archaeology.
Summarizing this discussion, it is evident that the transition from atlatl and dart to bow and arrow fulfills all three of Endler’s criteria: (a) there is variation between these two technologies in the maximum effective distance between user and target, speed of the launched missile, and the precision and accuracy in hitting the target; (b) there is fitness variation between these two technologies, such as the proximity of the user to the target and the degree of success in the user hitting the target; and (c) there is cultural transmission resulting in substantial replicative success, indicated by the speed and degree of replacement.
Recall, however, that in Endler’s model, if fitness variation (that is, traits that enhance fitness) is not present, then the change in a particular trait will be due to drift. Thus, both functional and stylistic traits can have replicative success, the former due to natural selection and the latter due to drift. In most cases, artifacts comprised both functional and stylistic (adaptively neutral) traits. However, determining which traits are functional and which are stylistic can be a difficult task. One means that we have of determining function from style is through experimental archaeology where replication and use of a technology is analyzed through practice. In experimental archaeology, an analyst may replicate projectile points with the same dimensions as dart points and arrow points and then test their relative efficiencies in performance. In the case of a pottery vessel, we may use instruments to analyze the thermal properties in cooking and their performance. However, a more nuanced approach in terms of looking at the time depth in the archaeological record is to examine how differential attributes behave over time to identify them as either stylistic or functional.
In general, functional traits tend to spread rapidly once they are invented or introduced if they offer an immediate advantage to the recipients. As our example shows, the bow and arrow conveyed several advantages to its users over the atlatl and dart and thus was adopted relatively quickly. On the other hand, pottery, which was present among semi-sedentary horticulturalists in the southwestern U.S. more than 1,500 years ago, was only adopted very late in prehistory by the mobile hunter-gatherers in the adjacent Great Basin, and even then was used sparingly. Pottery provides a much better cooking vessel than basketry and boiling stones, but it is heavy and it breaks, two negative factors for people who move their camps on a regular basis.
A functional trait will remain in use until something better comes along, which, in some cases, can be a long period of time. Functional traits, however, may come and go over time, as needs change. A classic functional example is the stone endscraper, used for scraping animal hides. The earliest of these tools appear hundreds of thousands of years ago, and although their frequencies have fluctuated through time (making their temporal frequency distributions multimodal), they persisted in roughly the same form until the appearance of metal tools. In contrast, the temporal frequency distributions of stylistic traits tend to be unimodal through time. After inception, they gain popularity, rise to a peak, then fade and eventually disappear. Styles rarely return in the same form. This unimodal distribution through time and the fact that they rarely return make stylistic traits especially useful for chronology. Once a time span can be identified with the unimodal distribution of a particular style, the presence of this style at different locations indicates the temporal period during which it was deposited.
From this discussion, it may seem fairly simple to distinguish functional from stylistic traits, but this process is not so straightforward. Where the temporal distribution of functional traits is more often multimodal, it can also be unimodal, mimicking that of a stylistic trait. On the other hand, a stylistic trait can spread relatively quickly where the population is highly mobile and there are few social boundaries, thus mimicking a functional trait under strong selection. To complicate the situation further, the status of a trait is context-specific and thus can change as conditions and selection pressures change. In some cases, this distinction is not an important one, such as the technological replacement example discussed earlier where the advantages conferred by one variant over the other are easily identified. In others, however, the distinction may be necessary because the variants compared may be quite similar, making it difficult to figure out which traits enhance fitness. Therefore, multiple forms of evidence are necessary in an attempt to distinguish one from the other (e.g., performance characteristics, analogous changes in comparable contexts, temporal and spatial patterns).